Microsoft Communicator, and Communicator in Microsoft Office 2010 beta, allows remote attackers to cause a denial of service (memory consumption) via a large number of SIP INVITE requests, which trigger the creation of many sessions.
secunia.com/advisories/32940
www.exploit-db.com/exploits/12079
www.securityfocus.com/bid/39221
www.securitytracker.com/id?1021294
www.voipshield.com/research-details.php?id=133
exchange.xforce.ibmcloud.com/vulnerabilities/46673
exchange.xforce.ibmcloud.com/vulnerabilities/57581
www.exploit-db.com/exploits/7262