Shame on us for not catching this a month ago when it was first reported, but it seems that two of the biggest caching plugins in WordPress have what we would classify a very serious vulnerability – remote code execution (RCE), a.k.a., arbitrary code execution:
…arbitrary code execution is used to describe an attacker’s ability to execute any commands of the attacker’s choice on a target machine or in a target process. – Wikipedia
It appears that a user by the name of kisscsaby first disclosed the issue a month ago via the WordPress forums. As of 5 days ago both plugin authors have pushed new versions of their plugins disabling the vulnerable functions by default. The real concern however is the seriousness of the vulnerability and the shear volume of users between both plugins.
There are a few posts, released within the past few hours that do a great job of explaining what the issue was and what was being exploited. You can find some good after action thoughts on Frank Goosens’ blog and on Acunetix’s blog as well.