iBooks in Apple iOS before 9.2 and OS X before 10.11.2 allows remote attackers to read arbitrary files via an iBooks file containing an XML external entity declaration in conjunction with an entity reference, related to an XML External Entity (XXE) issue.
https://support.apple.com/HT205637
https://support.apple.com/HT205635
http://www.securitytracker.com/id/1034344
http://lists.apple.com/archives/security-announce/2015/Dec/msg00005.html
http://lists.apple.com/archives/security-announce/2015/Dec/msg00000.html