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XML internal entity expansion

ID: py/xml-bomb
Kind: path-problem
Security severity: 7.5
Severity: warning
Precision: high
Tags:
   - security
   - external/cwe/cwe-776
   - external/cwe/cwe-400
Query suites:
   - python-code-scanning.qls
   - python-security-extended.qls
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

Parsing untrusted XML files with a weakly configured XML parser may be vulnerable to denial-of-service (DoS) attacks exploiting uncontrolled internal entity expansion.

In XML, so-called internal entities are a mechanism for introducing an abbreviation for a piece of text or part of a document. When a parser that has been configured to expand entities encounters a reference to an internal entity, it replaces the entity by the data it represents. The replacement text may itself contain other entity references, which are expanded recursively. This means that entity expansion can increase document size dramatically.

If untrusted XML is parsed with entity expansion enabled, a malicious attacker could submit a document that contains very deeply nested entity definitions, causing the parser to take a very long time or use large amounts of memory. This is sometimes called an XML bomb attack.

Recommendation

The safest way to prevent XML bomb attacks is to disable entity expansion when parsing untrusted data. Whether this can be done depends on the library being used. Note that some libraries, such as lxml, have measures enabled by default to prevent such DoS XML attacks, so unless you have explicitly set huge_tree to True, no further action is needed.

We recommend using the defusedxml PyPI package, which has been created to prevent XML attacks (both XXE and XML bombs).

Example

The following example uses the xml.etree XML parser provided by the Python standard library to parse a string xml_src. That string is from an untrusted source, so this code is vulnerable to a DoS attack, since the xml.etree XML parser expands internal entities by default:

from flask import Flask, request
import xml.etree.ElementTree as ET

app = Flask(__name__)

@app.post("/upload")
def upload():
    xml_src = request.get_data()
    doc = ET.fromstring(xml_src)
    return ET.tostring(doc)

It is not possible to guard against internal entity expansion with xml.etree, so to guard against these attacks, the following example uses the defusedxml PyPI package instead, which is not exposed to such internal entity expansion attacks.

from flask import Flask, request
import defusedxml.ElementTree as ET

app = Flask(__name__)

@app.post("/upload")
def upload():
    xml_src = request.get_data()
    doc = ET.fromstring(xml_src)
    return ET.tostring(doc)

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