Bugzilla before 2.14 allows Bugzilla users to bypass group security checks by marking a bug as the duplicate of a restricted bug, which adds the user to the CC list of the restricted bug and allows the user to view the bug.
http://www.redhat.com/support/errata/RHSA-2001-107.html
http://www.iss.net/security_center/static/10479.php