Don't freak out. SANS rates this one as "low risk", mainly because of the construction of Linux permission structure.
A while back, a buffer overflow flaw was found in XML-RPC for PHP. Most of the affected applications have already been patched. Check the link above for a partial list of affected apps and a link to the SANS report. The cure is to patch your apps and the PHP scripting engine.
As the prominence of Linux increases, there will be more and more of this sort of malware floating around. And by the way; ClamAV detects it.