Cross Site Tracing
Description
A Cross-Site Tracing (XST) attack involves the use of Cross-site Scripting (XSS) and the TRACE or TRACK HTTP methods. According to RFC 2616, “TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information.”, the TRACK method works in the same way but is specific to Microsoft’s IIS web server. XST could be used as a method to steal user’s cookies via Cross-site Scripting (XSS) even if the cookie has the “HttpOnly” flag set or exposes the user’s Authorization header.
The TRACE method, while apparently harmless, can be successfully leveraged in some scenarios to steal legitimate users’ credentials. This attack technique was discovered by Jeremiah Grossman in 2003, in an attempt to bypass the HttpOnly tag that Microsoft introduced in Internet Explorer 6 sp1 to protect cookies from being accessed by JavaScript. As a matter of fact, one of the most recurring attack patterns in Cross Site Scripting is to access the document.cookie object and send it to a web server controlled by the attacker so that they can hijack the victim’s session. Tagging a cookie as HttpOnly forbids JavaScript to access it, protecting it from being sent to a third party. However, the TRACE method can be used to bypass this protection and access the cookie even in this scenario.
Modern browsers now prevent TRACE requests being made via JavaScript, however, other ways of sending TRACE requests with browsers have been discovered, such as using Java.
Examples
An example using cURL from the command line to send a TRACE request to a web server on the localhost with TRACE enabled. Notice how the web server responds with the request that was sent to it.
$ curl -X TRACE 127.0.0.1
TRACE / HTTP/1.1
User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
Host: 127.0.0.1
Accept: */*
In this example notice how we send a Cookie header with the request and it is also in the web server’s response.
$ curl -X TRACE -H "Cookie: name=value" 127.0.0.1
TRACE / HTTP/1.1
User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
Host: 127.0.0.1
Accept: */*
Cookie: name=value
In this example the TRACE method is disabled, notice how we get an error instead of the request we sent.
$ curl -X TRACE 127.0.0.1
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>405 Method Not Allowed</title>
</head><body>
<h1>Method Not Allowed</h1>
<p>The requested method TRACE is not allowed for the URL /.</p>
</body></html>
Example JavaScript XMLHttpRequest TRACE request. In Firefox 19.0.2 it will not work and return a “Illegal Value” error. In Google Chrome 25.0.1364.172 it will not work and return a “Uncaught Error: SecurityError: DOM Exception 18” error. This is because modern browsers now block the TRACE method in XMLHttpRequest to help mitigate XST.
<script>
var xmlhttp = new XMLHttpRequest();
var url = 'http://127.0.0.1/';
xmlhttp.withCredentials = true; // send cookie header
xmlhttp.open('TRACE', url, false);
xmlhttp.send();
</script>
Remediation
Apache
In Apache versions 1.3.34, 2.0.55 and later, set the TraceEnable directive to “off” in the main configuration file and then restart Apache. See TraceEnable for further information.
TraceEnable off