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.
osvdb.org/54791
secunia.com/advisories/35230
secunia.com/advisories/43314
www.mandriva.com/security/advisories?name=MDVSA-2010:054
www.openwall.com/lists/oss-security/2009/05/27/1
www.securityfocus.com/archive/1/516397/100/0/threaded
www.securityfocus.com/bid/35112
www.vmware.com/security/advisories/VMSA-2011-0003.html
www.vupen.com/english/advisories/2009/1448
bugzilla.redhat.com/show_bug.cgi?id=502602
oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A7081
oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A9652