Certain forms in phpBB before 2.0.22 lack session checks, which has unknown impact and remote attack vectors.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2006-6824
http://www.securityfocus.com/bid/21806
http://www.phpbb.com/phpBB/viewtopic.php?f=14&t=489624
http://www.debian.org/security/2008/dsa-1488