tag/tag_autocomplete.php in Moodle through 2.4.11, 2.5.x before 2.5.9, 2.6.x before 2.6.6, and 2.7.x before 2.7.3 does not consider the moodle/tag:edit capability before adding a tag, which allows remote authenticated users to bypass intended access restrictions via an AJAX request.
https://moodle.org/mod/forum/discuss.php?d=275157
http://www.securitytracker.com/id/1031215
http://openwall.com/lists/oss-security/2014/11/17/11
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-47965