This is the result of malicious code that was injected into the knoppix.net server, exploiting some new vulnerability in vBulletin (and possibly also Apache). Possibly the best discussion of this problem, including suggestions on how to eliminate it, is at Vbulletin myfilestore hack - Find the traces and remove them. It's very technical, but an experienced vBulletin administrator should find some useful info there.

And, just for the record, it happened to me again while I was looking up this info.