When I got to work and viewed this blog I noticed that Sidebar Widgets was disabled. I thought "That’s weird!"
When I tried to login to the administration interface I was told that my WordPress database needed upgrading. I thought "That’s weird!"
Some further investigation revealed that someone managed to upload a PHP script called ro8kfbsmag.txt (MD5 sum df3b74cd38c717d9d7bbf0cd1910baa1) to my /tmp directory. It starts like this:
/*Magic Include Shell by Mag icq 884888*/
//TODO: ñëèòü ôàéëî íà ñâîé ôòï (!)
This gave me enough information too start googling. A must-read is Detailed Post-Mortem of a Website Hack Through WordPress & How To Protect Your WordPress Blog From Hacking, as it describes a very similar attack. There is also a support thread at wordpress.org: Weird and Dangerous : ro8kfbsmag.txt.
The attack vector on my server looked like this, originating from 18.104.22.168 with HTTP/1.0 as protocol version and "Opera" as User-Agent. I wish I logged POST data!
Needless to say, I have restored a backup and taken certain precautions to prevent this from happening again.