CFNetwork in Apple iOS before 9.1 and OS X before 10.11.1 does not properly consider the uppercase-versus-lowercase distinction during cookie parsing, which allows remote web servers to overwrite cookies via unspecified vectors.
http://lists.apple.com/archives/security-announce/2015/Oct/msg00002.html
http://lists.apple.com/archives/security-announce/2015/Oct/msg00005.html
https://support.apple.com/HT205370