CVE 2011-4587

lib/moodlelib.php in Moodle 1.9.x before 1.9.15, 2.0.x before 2.0.6, and 2.1.x before 2.1.3 does not properly handle certain zero values in the password policy, which makes it easier for remote attackers to obtain access by leveraging the possible existence of user accounts that have unchangeable blank passwords.

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