Microsoft ISA Server 2000 H.323 Filter Remote Buffer Overflow Vulnerability



Date Discovered

January 13, 2004


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.

Technologies Affected

  • Microsoft ISA Server 2000
  • Microsoft ISA Server 2000 FP1
  • Microsoft ISA Server 2000 SP1
  • Microsoft Small Business Server 2000
  • Microsoft Small Business Server 2003


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.



The disclosure of this issue has been credited to the University of Oulu Security Programming Group.

© 1995- Symantec Corporation

Permission to redistribute this alert electronically is granted as long as it is not edited in any way unless authorized by Symantec Security Response. Reprinting the whole or part of this alert in any medium other than electronically requires permission from


The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.

Symantec, Symantec products, Symantec Security Response, and are registered trademarks of Symantec Corp. and/or affiliated companies in the United States and other countries. All other registered and unregistered trademarks represented in this document are the sole property of their respective companies/owners.