OpenX is a tool used by hosting providers and webpage developers to provide ads on webpages. Rotating banner ads have been an attack vector that has been quite popular and effective in the recent past. This is probably one reason why.
An announcement this week from the OpenX ad server team noted that a backdoor had recently been discovered in their official source code distributions, that has been present since November 2012. This vulnerability only applies to the free downloadable open source product, OpenX Source.
Exploitation is occurring in the wild, with attacks consisting of simple POST requests to a specific file that allows for remote code execution on the affected server. Users are urged to follow instructions being provided by the community for checking their servers, and rebuilding any that are impacted immediately.
References:
MadMark's Blog 