At Halloween 2005 I had disclosed a number of bugs in phpBB, including a remote code execution exploit through the signature_bbcode_uid variable. You will not find this vulnerability in the phpBB security tracker because it is phpBB project's practise to blame their bugs on PHP and/or otherwise downplay them or hide them. Unfortunately there is now a public exploit for this vulnerability, which was released yesterday while most of us were celebrating christmas.
The exploit allows for any kind of remote command to be executed - a very dangerous problem. It's for versions 2..0.17 and below, but they've released version 2.0.18 already - so, if you're running an older codeset, head over and upgrade today.