The System Integrity Protection feature in Apple OS X before 10.11.2 mishandles union mounts, which allows attackers to execute arbitrary code in a privileged context via a crafted app with root privileges.
https://support.apple.com/HT205637
http://www.securitytracker.com/id/1034344
http://lists.apple.com/archives/security-announce/2015/Dec/msg00005.html