pam_ssh 1.92 and possibly other versions, as used when PAM is compiled with USE=ssh, generates different error messages depending on whether the username is valid or invalid, which makes it easier for remote attackers to enumerate usernames.
https://www.redhat.com/archives/fedora-package-announce/2009-May/msg00145.html
https://www.redhat.com/archives/fedora-package-announce/2009-May/msg00116.html
http://secunia.com/advisories/34986