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.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.003
EPSS Ranking 50.8%