CloudKit in Apple iOS before 8.4.1 and OS X before 10.10.5 allows attackers to access an iCloud user record associated with a previous user's login session via a crafted app.
http://lists.apple.com/archives/security-announce/2015/Aug/msg00001.html
http://lists.apple.com/archives/security-announce/2015/Aug/msg00002.html
https://support.apple.com/kb/HT205030