The NSURL implementation in the CFNetwork SSL component in Apple iOS before 9 does not properly verify X.509 certificates from SSL servers after a certificate change, which allows man-in-the-middle attackers to spoof servers and obtain sensitive information via a crafted certificate.
http://lists.apple.com/archives/security-announce/2015/Sep/msg00001.html
http://lists.apple.com/archives/security-announce/2015/Sep/msg00005.html
http://lists.apple.com/archives/security-announce/2015/Sep/msg00008.html
https://support.apple.com/HT205212
https://support.apple.com/HT205213