calendar/externallib.php in Moodle through 2.6.11, 2.7.x before 2.7.13, 2.8.x before 2.8.11, 2.9.x before 2.9.5, and 3.0.x before 3.0.3 provides calendar-event data without considering whether an activity is hidden, which allows remote authenticated users to obtain sensitive information via a web-service request.
https://moodle.org/mod/forum/discuss.php?d=330178
http://www.securitytracker.com/id/1035333
http://www.openwall.com/lists/oss-security/2016/03/21/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-52808