CVE 2011-4584

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.

See the CVE page on Mitre.org for more details.