course/loginas.php in Moodle through 2.2.11, 2.3.x before 2.3.11, 2.4.x before 2.4.8, 2.5.x before 2.5.4, and 2.6.x before 2.6.1 does not enforce the moodle/site:accessallgroups capability requirement for outside-group users in a SEPARATEGROUPS configuration, which allows remote authenticated users to perform "login as" actions via a direct request.
https://moodle.org/mod/forum/discuss.php?d=252415
http://www.securitytracker.com/id/1029648
http://openwall.com/lists/oss-security/2014/01/20/1
http://lists.fedoraproject.org/pipermail/package-announce/2014-January/127533.html
http://lists.fedoraproject.org/pipermail/package-announce/2014-January/127510.html
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-42643