The send-private-message functionality (send-private-message.asp) in PD9 Software MegaBBS 2.1 allows remote attackers to read private messages of other users via a modified replyid parameter.
https://exchange.xforce.ibmcloud.com/vulnerabilities/24050
http://www.vupen.com/english/advisories/2006/0095
http://www.securityfocus.com/bid/16168
http://www.pd9soft.com/megabbs/forums/thread-view.asp?tid=4924