Clang in LLVM, as used in Apple Xcode before 6.3, performs incorrect register allocation in a way that triggers stack storage for stack cookie pointers, which might allow context-dependent attackers to bypass a stack-guard protection mechanism via crafted input to an affected C program.
https://support.apple.com/HT204663
http://www.securitytracker.com/id/1032081
http://www.securityfocus.com/bid/73987
http://lists.apple.com/archives/security-announce/2015/Apr/msg00004.html