The is_enrolled function in lib/accesslib.php in Moodle 2.2.x before 2.2.4 and 2.3.x before 2.3.1 does not properly interact with the caching feature, which might allow remote authenticated users to bypass an intended capability check via unspecified vectors that trigger caching of a user record.
https://exchange.xforce.ibmcloud.com/vulnerabilities/76955
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-33916