The Messages component in Apple OS X before 10.11.5 mishandles roster changes, which allows remote attackers to modify contact lists via unspecified vectors.
https://support.apple.com/HT206567
http://www.securitytracker.com/id/1035895
http://www.securityfocus.com/bid/90696
http://lists.apple.com/archives/security-announce/2016/May/msg00004.html