The Home & Lock Screen subsystem in Apple iOS before 8 does not properly restrict the private API for app prominence, which allows attackers to determine the frontmost app by leveraging access to a crafted background app.
https://exchange.xforce.ibmcloud.com/vulnerabilities/96094
http://www.securitytracker.com/id/1030866
http://www.securityfocus.com/bid/69949
http://www.securityfocus.com/bid/69882
http://support.apple.com/kb/HT6441
http://archives.neohapsis.com/archives/bugtraq/2014-09/0106.html