FUDForum 2.6.15 with "Tree View" enabled, as used in other products such as phpgroupware and egroupware, allows remote attackers to read private posts via a modified mid parameter.
http://www.securityfocus.com/bid/14556
http://www.debian.org/security/2005/dsa-899
http://www.debian.org/security/2005/dsa-798
http://secunia.com/advisories/17643
http://secunia.com/advisories/16414
http://archives.neohapsis.com/archives/fulldisclosure/2005-08/0383.html