Safari in Apple iOS before 8.4.1 does not limit the rate of JavaScript alert messages, which allows remote attackers to cause a denial of service (apparent browser locking) via a crafted web site.
https://support.apple.com/kb/HT205030
http://www.securitytracker.com/id/1033275
http://www.securityfocus.com/bid/76337
http://lists.apple.com/archives/security-announce/2015/Aug/msg00002.html