pam_krb5 2.2.14 through 2.3.4, as used in Red Hat Enterprise Linux (RHEL) 5, generates different password prompts depending on whether the user account exists, which allows remote attackers to enumerate valid usernames.
https://bugzilla.redhat.com/show_bug.cgi?id=502602
http://secunia.com/advisories/35230
http://secunia.com/advisories/43314
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A7081
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A9652
http://www.mandriva.com/security/advisories?name=MDVSA-2010:054
http://www.openwall.com/lists/oss-security/2009/05/27/1
http://www.vmware.com/security/advisories/VMSA-2011-0003.html