The X.509 certificate-trust implementation in Apple iOS before 9.1 does not recognize that the kSecRevocationRequirePositiveResponse flag implies a revocation-checking requirement, which makes it easier for man-in-the-middle attackers to spoof endpoints by leveraging access to a revoked certificate.
https://support.apple.com/HT205641
https://support.apple.com/HT205370
http://www.securitytracker.com/id/1033929
http://www.securityfocus.com/bid/77268
http://lists.apple.com/archives/security-announce/2015/Oct/msg00002.html
http://lists.apple.com/archives/security-announce/2015/Dec/msg00002.html