source: https://www.securityfocus.com/bid/11671/info

Private Message System is reported prone to multiple vulnerabilities that can allow remote attackers to carry out cross-site scripting attacks and disclose arbitrary private messages.

Private Message System 1.1.3 is reported vulnerable to these issue, however, it is possible that other version are affected as well.

http://www.example.com/message_send.php?quote=[ID]