The (1) setreuid and (2) setregid system-call implementations in the kernel in Apple iOS before 8.3, Apple OS X before 10.10.3, and Apple TV before 7.2 do not properly perform privilege drops, which makes it easier for attackers to execute code with unintended user or group privileges via a crafted app.
http://lists.apple.com/archives/security-announce/2015/Apr/msg00001.html
http://lists.apple.com/archives/security-announce/2015/Apr/msg00002.html
http://lists.apple.com/archives/security-announce/2015/Apr/msg00003.html
http://www.securitytracker.com/id/1032048
https://support.apple.com/HT204659
https://support.apple.com/HT204661