Apple iOS before 9 allows attackers to discover the e-mail address of a player via a crafted Game Center app.
https://support.apple.com/HT205267
https://support.apple.com/HT205212
http://www.securitytracker.com/id/1033609
http://www.securityfocus.com/bid/76764
http://lists.apple.com/archives/security-announce/2015/Sep/msg00008.html
http://lists.apple.com/archives/security-announce/2015/Sep/msg00001.html