Those malicious files hid on the openx-2.8.10.zip, the tgz and the bz2 archives and appears to be there since November 2012.
In another investigation, it stated that the backdoor is hidden inside the following file: /plugins/deliveryLog /vastServeVideoPlayer /flowplayer/3.1.1/ flowplayer-3.1.1.min.js.
Back in March 2012, the website is also compromised that then served malware to users.