Nortel UNIStim protocol, as used in Communication Server 1000 and other products, uses predictable sequence numbers, which allows remote attackers to hijack sessions via sniffing or brute force attacks.
https://exchange.xforce.ibmcloud.com/vulnerabilities/41801
http://www.securitytracker.com/id?1019847
http://www.securityfocus.com/bid/28691
http://support.nortel.com/go/main.jsp?cscat=BLTNDETAIL&id=713455