MEDIUM
Bugzilla 2.23.3 through 2.22.7, 3.0.x through 3.3.x, 3.4.x before 3.4.12, 3.5.x, 3.6.x before 3.6.6, 3.7.x, 4.0.x before 4.0.2, and 4.1.x before 4.1.3 allows remote attackers to determine the existence of private group names via a crafted parameter during (1) bug creation or (2) bug editing.
http://secunia.com/advisories/45501
http://www.bugzilla.org/security/3.4.11/
http://www.debian.org/security/2011/dsa-2322
http://www.securityfocus.com/bid/49042