The Data Access component in Apple iOS before 5 does not properly handle the existence of multiple user accounts on the same mail server, which allows local users to bypass intended access restrictions in opportunistic circumstances by leveraging a different account's cookie.
https://exchange.xforce.ibmcloud.com/vulnerabilities/70553
http://support.apple.com/kb/HT4999
http://lists.apple.com/archives/Security-announce/2011//Oct/msg00001.html