It has been reported that the H.323 filter used by Microsoft ISA Server 2000 is prone to a remote buffer overflow vulnerability. The condition presents itself due to insufficient boundary checks performed by the Microsoft Firewall Service on specially crafted H.323 traffic. Successful exploitation of this vulnerability may allow a remote attacker to execute arbitrary code in the context of Microsoft Firewall Service running on ISA Server 2000. This may lead to complete control of the vulnerable system.
Block external access at the network boundary, unless external parties require service.
Block TCP port 1720 at network perimeter or gateway. The H.323 filter listens on TCP port 1720 for incoming traffic, therefore blocking this port will protect against a remote attack.
Deploy network intrusion detection systems to monitor network traffic for malicious activity.
Use intrusion detection systems to monitor networks for anomalous activity and report attempted attacks.
Implement multiple redundant layers of security.
An attacker's ability to exploit this vulnerability to execute arbitrary code may be hindered through the use of various memory protection schemes. Where possible, implement the use of non-executable and randomly mapped memory segments.
Microsoft has released security advisory MS04-001 to address this issue. Users are strongly advised to obtain fixes.