lib/formslib.php in Moodle 2.2.x before 2.2.6 and 2.3.x before 2.3.3 allows remote authenticated users to bypass intended access restrictions via a modified value of a frozen form field.
https://moodle.org/mod/forum/discuss.php?d=216156
http://www.securityfocus.com/bid/56505
http://openwall.com/lists/oss-security/2012/11/19/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-32785