The filtering implementation in AppleEvents in Apple OS X before 10.11 mishandles attempts to send events to a different user, which allows attackers to bypass intended access restrictions by leveraging a screen-sharing connection.
https://support.apple.com/HT205267
http://www.securitytracker.com/id/1033703
http://lists.apple.com/archives/security-announce/2015/Sep/msg00008.html