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://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A9652
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A7081
https://bugzilla.redhat.com/show_bug.cgi?id=502602
http://www.vupen.com/english/advisories/2009/1448
http://www.vmware.com/security/advisories/VMSA-2011-0003.html
http://www.securityfocus.com/bid/35112
http://www.securityfocus.com/archive/1/516397/100/0/threaded
http://www.openwall.com/lists/oss-security/2009/05/27/1
http://www.mandriva.com/security/advisories?name=MDVSA-2010:054
http://secunia.com/advisories/43314