CoreDisplay in Apple OS X before 10.12 allows attackers to view arbitrary users' screens by leveraging screen-sharing access.
https://support.apple.com/HT207170
http://www.securitytracker.com/id/1036858
http://www.securityfocus.com/bid/93055
http://lists.apple.com/archives/security-announce/2016/Sep/msg00006.html