lib/modinfolib.php in Moodle 2.0.x before 2.0.10, 2.1.x before 2.1.7, 2.2.x before 2.2.4, and 2.3.x before 2.3.1 does not check for a group-membership requirement when determining whether an activity is unavailable or hidden, which allows remote authenticated users to bypass intended access restrictions by selecting an activity that is configured for a group of other users.
https://exchange.xforce.ibmcloud.com/vulnerabilities/76963
http://www.securityfocus.com/bid/54481
http://secunia.com/advisories/49890
http://openwall.com/lists/oss-security/2012/07/17/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-33466