The Kerberos 5 (aka krb5) PAM module in Apple OS X before 10.12 does not use constant-time operations for determining username validity, which makes it easier for remote attackers to enumerate user accounts via a timing side-channel attack.
https://support.apple.com/HT207170
http://www.securitytracker.com/id/1036858
http://www.securityfocus.com/bid/93055
http://lists.apple.com/archives/security-announce/2016/Sep/msg00006.html