The private-browsing implementation in Apple Safari before 6.2.5, 7.x before 7.1.5, and 8.x before 8.0.5 allows attackers to obtain sensitive browsing-history information via vectors involving push-notification requests.
https://support.apple.com/HT204658
http://www.securitytracker.com/id/1032047
http://lists.apple.com/archives/security-announce/2015/Apr/msg00000.html