The MNET authentication functionality in Moodle 1.9.x before 1.9.15, 2.0.x before 2.0.6, and 2.1.x before 2.1.3 allows remote authenticated users to impersonate other user accounts by using the Login As feature in conjunction with a remote MNET single sign-on capability, as demonstrated by a Mahara site.
https://bugzilla.redhat.com/show_bug.cgi?id=761248
http://www.debian.org/security/2012/dsa-2421
http://moodle.org/mod/forum/discuss.php?d=191751
http://git.moodle.org/gw?p=moodle.git%3Ba=commit%3Bh=10df8657c1c138c0d0ab1d4796c552fcec0c299b