Mail in Apple iOS before 6 does not properly handle reuse of Content-ID header values, which allows remote attackers to spoof attachments via a header value that was also used in a previous e-mail message, as demonstrated by a message from a different sender.
https://exchange.xforce.ibmcloud.com/vulnerabilities/78717
http://support.apple.com/kb/HT5503
http://lists.apple.com/archives/security-announce/2012/Sep/msg00003.html