CommuniGate Pro 3.1 through 4.0.6 sends the session ID in the referer field for an HTTP request for an image, which allows remote attackers to hijack mail sessions via an e-mail with an IMG tag that references a malicious URL that captures the referer.
https://exchange.xforce.ibmcloud.com/vulnerabilities/11932
https://euvd.enisa.europa.eu/vulnerability/EUVD-2003-1471
http://www.securityfocus.com/bid/7501