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.
http://secunia.com/advisories/35230
http://secunia.com/advisories/43314
http://www.mandriva.com/security/advisories?name=MDVSA-2010:054
http://www.openwall.com/lists/oss-security/2009/05/27/1
http://www.securityfocus.com/archive/1/516397/100/0/threaded
http://www.securityfocus.com/bid/35112
http://www.vmware.com/security/advisories/VMSA-2011-0003.html
http://www.vupen.com/english/advisories/2009/1448
https://bugzilla.redhat.com/show_bug.cgi?id=502602
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