phpCOIN 1.2.3 and earlier stores messages based upon e-mail addresses, which allows remote authenticated users to read messages for other users by adding the sender's e-mail address as an "additional contact".
https://exchange.xforce.ibmcloud.com/vulnerabilities/26499
http://www.vupen.com/english/advisories/2006/1788
http://www.securityfocus.com/bid/17959