http://community.mybboard.net/thread-52474.html
Revert index template to default.
Delete any non-default files from ./cache/themes/
Delete any new admins.
Also update to the latest release of MyBB and try to keep more up to date in future, there's been updates from the version you're on for a few weeks now... you're on 1.4.6, or a previous version to that, yeah?? There was an exploit in 1.4.6 but it was patched around a month ago.
looks like im not the only one either
google search - Unknown_3rr0r - Th3_Analyz3r - Su_root
looks like this little fu*kers got a few forums
All I get are php errors not hackings.
Here what you do
1: Panic completely, this is a huge issue.
2: Assemble a team of geniuses, I'd recommend, Barrack Obama for the team leader
3: Track down a cure for this
If all else fails, post a thread here
http://community.mybboard.net/forum-81.html