An issue was discovered in certain Apple products. iOS before 11 is affected. Safari before 11 is affected. The issue involves the "WebKit" component. It allows remote attackers to track Safari Private Browsing users by leveraging cookie mishandling.
https://support.apple.com/HT208116
https://support.apple.com/HT208112
http://www.securitytracker.com/id/1039427