The Forum module in Moodle 2.7.x before 2.7.10 allows remote authenticated users to post to arbitrary groups by leveraging the teacher role, as demonstrated by a post directed to "all participants."
https://moodle.org/mod/forum/discuss.php?d=320288
http://www.securitytracker.com/id/1033619
http://www.openwall.com/lists/oss-security/2015/09/21/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-50576