UBB.classic is not a vector for this worm - it only infects phpBB boards.
The worm, once attached to the board, proceeds to scour the server for writable files with certain extensions, which then proceed to get overwritten with the worm's message.
Your server has a phpBB running on it somewhere that was infected, and proceeded to jump boundaries into your account (and surely everyone else's on that server) and do its work.
Again, there is no way for UBB.classic (or UBB.threads
) to be a vector for this worm.