Moodle 2.8.x before 2.8.6 does not consider the tool/monitor:subscribe capability before entering subscriptions to site-wide event-monitor rules, which allows remote authenticated users to obtain sensitive information via a subscription request.
https://moodle.org/mod/forum/discuss.php?d=313684
http://www.securitytracker.com/id/1032358
http://www.securityfocus.com/bid/74721
http://openwall.com/lists/oss-security/2015/05/18/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-50039