phpBB 2.0.17 and earlier, when register_globals is enabled and the session_start function has not been called to handle a session, allows remote attackers to bypass security checks by setting the $_SESSION and $HTTP_SESSION_VARS variables to strings instead of arrays, which causes an array_merge function call to fail.
http://www.securityfocus.com/bid/15243
http://www.hardened-php.net/advisory_172005.75.html
http://www.debian.org/security/2005/dsa-925
http://securitytracker.com/id?1015121
http://securityreason.com/securityalert/130
http://secunia.com/advisories/18098