calendar/managesubscriptions.php in the Manage Subscriptions implementation in Moodle 2.4.x before 2.4.1 omits a capability check, which allows remote authenticated users to remove course-level calendar subscriptions by leveraging the student role and sending an iCalendar object.
https://moodle.org/mod/forum/discuss.php?d=220167
http://openwall.com/lists/oss-security/2013/01/21/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-37106