CFNetwork in Apple iOS before 9 relies on the hardware UID for its cache encryption key, which makes it easier for physically proximate attackers to obtain sensitive information by obtaining this UID.
http://lists.apple.com/archives/security-announce/2015/Sep/msg00001.html
http://lists.apple.com/archives/security-announce/2015/Sep/msg00005.html
https://support.apple.com/HT205212