mod/forum/rsslib.php in Moodle 2.1.x before 2.1.7 and 2.2.x before 2.2.4 does not properly implement the requirement for posting before reading a Q&A forum, which allows remote authenticated users to bypass intended access restrictions by leveraging the student role and reading the RSS feed for a forum.
https://exchange.xforce.ibmcloud.com/vulnerabilities/76957
http://www.securityfocus.com/bid/54481
http://secunia.com/advisories/49890