Keychain Access in Apple OS X before 10.11.2 and tvOS before 9.1 improperly interacts with Keychain Agent, which allows attackers to spoof the Keychain Server via unspecified vectors.
http://lists.apple.com/archives/security-announce/2015/Dec/msg00001.html
http://lists.apple.com/archives/security-announce/2015/Dec/msg00005.html
https://support.apple.com/HT205637