As posted by Walter here:

As I know that the OpenX advertising script (formerly known as phpAds) is pretty popular amongst forum admins here are some bad news: There is a new exploit out there and there is currently no patch available from OpenX. There are first reports of compromises.

The exploit is done via the Open Flash Chart 2 module (you can upload pretty anything via ofc_upload_image.php). The only solution to prevent attacks is to delete admin/plugins/videoReport/lib/ofc2/ofc_upload_image.php if you don't use the module (99% don't use it).

There are many ways to use this exploit but one sign is if you have a file in admin/plugins/videoReport/lib/tmp-upload-images - e.g. a small shell code php script.

The OpenX web site is currently down (probably flooded by admins).
If you use OpenX as an advertising solution, and are not using their Hosted option, you may want to delete that file mentioned above ASAP.