mod/forum/post.php in Moodle 2.9.x before 2.9.1 does not consider the mod/forum:canposttomygroups capability before authorizing "Post a copy to all groups" actions, which allows remote authenticated users to bypass intended access restrictions by leveraging per-group authorization.
https://moodle.org/mod/forum/discuss.php?d=316663
http://www.securitytracker.com/id/1032877
http://openwall.com/lists/oss-security/2015/07/13/2
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-50220