mod/forum/classes/post_form.php in Moodle through 2.3.11, 2.4.x before 2.4.11, 2.5.x before 2.5.7, 2.6.x before 2.6.4, and 2.7.x before 2.7.1 does not enforce the moodle/site:accessallgroups capability requirement before proceeding with a post to all groups, which allows remote authenticated users to bypass intended access restrictions by leveraging two or more group memberships.