admin/admin_disallow.php in phpBB 2.0.18 allows remote attackers to obtain the installation path via a direct request with a non-empty setmodules parameter, which causes an invalid append_sid function call that leaks the path in an error message.
http://www.vupen.com/english/advisories/2006/0010
http://www.vupen.com/english/advisories/2005/2991
http://www.securityfocus.com/archive/1/420537/100/0/threaded
http://www.phpbb.com/phpBB/viewtopic.php?f=14&t=352966
http://securityreason.com/securityalert/269
http://securityreason.com/achievement_securityalert/29
http://secunia.com/advisories/18252