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Date: Sat, 29 Apr 2017 16:44:13 +0200
From: "Securify B.V." <lists@...urify.nl>
To: oss-security@...ts.openwall.com
Subject: SyntaxHighlight MediaWiki extension allows injection of arbitrary
 Pygments options

------------------------------------------------------------------------
SyntaxHighlight MediaWiki extension allows injection of arbitrary
Pygments options
------------------------------------------------------------------------
Yorick Koster, February 2017

------------------------------------------------------------------------
Abstract
------------------------------------------------------------------------
A vulnerability was found in the SyntaxHighlight MediaWiki extension.
Using this vulnerability it is possible for an anonymous attacker to
pass arbitrary options to the Pygments library. By specifying specially
crafted options, it is possible for an attacker to trigger a (stored)
Cross-Site Scripting condition. In addition, it allows the creating of
arbitrary files containing user-controllable data. Depending on the
server configuration, this can be used by an anonymous attacker to
execute arbitrary PHP code.

------------------------------------------------------------------------
See also
------------------------------------------------------------------------
- CVE-2017-0372 [2]
- T158689 [3]: Parameters injection in SyntaxHighlight results in
multiple vulnerabilities
- Fix REL1_28 [4]: SECURITY: Escape start argument before passing to
pygments
- Fix REL1_27 [5]: SECURITY: Escape start argument before passing to
pygments
- MediaWiki-announce [6]: Security Release: 1.28.1 / 1.27.2 / 1.23.16
(fix not included in this release)

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Tested versions
------------------------------------------------------------------------
This issue was tested on SyntaxHighlight version 2.0 as bundled with
MediaWiki version 1.28.0.

------------------------------------------------------------------------
Fix
------------------------------------------------------------------------
This issue was supposed to be fixed in MediaWiki version 1.28.1 and
version 1.27.2. It appears that the fix was pushed to the git
repository, but for some reason it was not included in the release
packages. It is advised to apply the patch committed to Github.

https://github.com/wikimedia/mediawiki-extensions-SyntaxHighlight_GeSHi/commit/2d5a60a89fb3995b73e17df5901d6f023e41df3d
https://github.com/wikimedia/mediawiki-extensions-SyntaxHighlight_GeSHi/commit/a88c5e1dcbdb3e9940c6f55a6744c62a6d62710f

------------------------------------------------------------------------
Introduction
------------------------------------------------------------------------
The SyntaxHighlight [7] extension for MediaWiki [8] allows formatting of
source code using the <syntaxhighlight> tag. Version 2.0 uses the Python
Pygments [9] library to format the code. SyntaxHighlight is bundled with
MediaWiki version 1.21 and later. Version 2.0 is bundled with MediaWiki
1.26.0 and later (other versions may or may not include this version as
well).

The <syntaxhighlight> tag supports various parameters. It was found that
the start parameter is not validated and/or sanitized. This allows an
attacker to pass arbitrary options to the Lexer and/or Formatter that is
used when Pygments is invoked. By specifying specially crafted options,
it is possible for an attacker to trigger a (stored) Cross-Site
Scripting condition. In addition, the HTML formatter allows the creating
of arbitrary files containing user-controllable data. Depending on the
server configuration, this can be used by an attacker to execute
arbitrary PHP code.

------------------------------------------------------------------------
Details
------------------------------------------------------------------------
The SyntaxHighlight extension utilizes Pygments to format source code.
Pygments is a Python library, a copy is provided with the extension. In
order to use Pygments, the extension invokes it using Symfony's [10]
ProcessBuilder [11] component. This component performs escaping of
command line arguments to prevent command injection.

SyntaxHighlight_GeSHi.class.php:

$optionPairs = array();
foreach ( $options as $k => $v ) {
	$optionPairs[] = "{$k}={$v}";
}
$builder = new ProcessBuilder();
$builder->setPrefix( $wgPygmentizePath );
$process = $builder
	->add( '-l' )->add( $lexer )
	->add( '-f' )->add( 'html' )
	->add( '-O' )->add( implode( ',', $optionPairs ) )
	->getProcess();
	
$process->setInput( $code );
$process->run();

The used Lexer is specified through the lang parameter, the Formatter is
always set to the HtmlFormatter. Additional options for the Lexer and/or
Formatter are provided using the -O command line argument. These options
can be controlled by the parameters that are supported by the
<syntaxhighlight> tag. Each option is a key value pair, the options are
comma separated.

It was found that no input validation and/or sanitization is done on the
start parameter. This parameter is used to define the first line number
of a code block. If line numbers are enabled, the numbering will start
with the value provided in the start parameter. Normally, this value
should only contain numbers. Due to the lack of validation/sanitization,
it can be set to any value.

SyntaxHighlight_GeSHi.class.php:

// Starting line number
if ( isset( $args['start'] ) ) {
	$options['linenostart'] = $args['start'];
}

Since Lexer/Formatter options are comma separated, it is possible for an
attacker to provide arbitrary options when invoking Pygments. Depending
on the options supported by the Lexer or Formatter, this allows the
attacker to perform various types of attacks. For example it is possible
for an attacker to trigger a (stored) Cross-Site Scripting condition by
passing a specially crafted prestyles option to the HTML Formatter.

<syntaxhighlight lang="java" 
start='0,prestyles="&gt;&lt;script&gt;alert(document.cookie)&lt;/script&gt;'>
	string foo="bar";
</syntaxhighlight>


When the option full is passed to the HTML Formatter, it is possible to
specify a local CSS file using the cssfile option. If the CSS file does
not exist it will be created - provided that Pygments has write
privileges on the provided path. This CSS file contains the styles that
are used for formatting the source code. Providing additional options,
it is possible to control parts of the CSS. One such option is the
classprefix option.

Combining these options can result in execution of arbitrary PHP code,
provided that a writeable folder exists within the webserver's document
root that allows the execution of PHP files. The proof of concept below
will try to create a PHP file name foo.php in the images folder located
within the document root.

<syntaxhighlight lang='java' 
start='0,full=1,title=,cssfile=images/foo.php,classprefix=&lt;?php
phpinfo();exit; ?&gt;'>
</syntaxhighlight>

Unless the Wiki is configured as private, it is possible to exploit this
issue without logging into the Wiki. If the Wiki is set to private, an
account with read access is required to exploit this vulnerability.
------------------------------------------------------------------------
References
------------------------------------------------------------------------
[1] 
https://www.securify.nl/advisory/SFY20170201/syntaxhighlight_mediawiki_extension_allows_injection_of_arbitrary_pygments_options.html
[2] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-0372
[3] https://phabricator.wikimedia.org/T158689
[4] 
https://github.com/wikimedia/mediawiki-extensions-SyntaxHighlight_GeSHi/commit/2d5a60a89fb3995b73e17df5901d6f023e41df3d
[5] 
https://github.com/wikimedia/mediawiki-extensions-SyntaxHighlight_GeSHi/commit/a88c5e1dcbdb3e9940c6f55a6744c62a6d62710f
[6] 
https://lists.wikimedia.org/pipermail/mediawiki-announce/2017-April/000207.html
[7] https://www.mediawiki.org/wiki/Extension:SyntaxHighlight
[8] https://www.mediawiki.org
[9] http://pygments.org/
[10] https://symfony.com/
[11] 
http://api.symfony.com/3.2/Symfony/Component/Process/ProcessBuilder.html

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