sessions.php in phpBB 2.0.12 and earlier allows remote attackers to gain administrator privileges via the autologinid value in a cookie.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2005-0615
http://www.phpbb.com/phpBB/viewtopic.php?t=267563
http://secunia.com/advisories/14413