Microsoft ISA and Proxy Server Web Site Spoofing Vulnerability



Date Discovered

November 4, 2004


Microsoft ISA and Proxy Server are reportedly prone to a Web site spoofing vulnerability. Successful exploitation of this issue could allow a remote attacker to spoof a trusted Web site. If a connection were made to the spoofed Web site using SSL, the malicious site would not be able to spoof the certificate of the legitimate site. This would likely cause a warning message to appear to the user. Microsoft Small Business Server 2000 and 2003 include the affected software.

Technologies Affected

  • Microsoft ISA Server 2000
  • Microsoft ISA Server 2000 FP1
  • Microsoft ISA Server 2000 SP1
  • Microsoft ISA Server 2000 SP2
  • Microsoft Proxy Server 2.0
  • Microsoft Proxy Server 2.0 SP1
  • Microsoft Small Business Server 2000
  • Microsoft Small Business Server 2003


Permit local access for trusted individuals only. Where possible, use restricted environments and restricted shells.
Do not permit untrusted individuals to have access to computers. This may limit exposure to vulnerabilities that may be locally exploitable.

Block external access at the network boundary, unless external parties require service.
Network access controls should be deployed to restrict all unauthorized incoming and outgoing network traffic. This will provide a general security measure against vulnerabilities in remotely accessible services.

Deploy network intrusion detection systems to monitor network traffic for malicious activity.
Network intrusion detection systems should be deployed to monitor all network traffic for signs of suspicious or anomalous activity. This may provide a means of detecting malicious activity related to exploitation of latent vulnerabilities.

Do not follow links provided by unknown or untrusted sources.
To exploit this vulnerability, an attacker would first have to entice a user to visit a malicious Web site. Do not follow links to unfamiliar or untrusted Web sites.

Manually inspect certificate properties.
When establishing an SSL connection to a Web site, always inspect the certificate properties to ensure that they are valid.

Microsoft has released a security bulletin and fixes to address this issue for supported versions. Update (11/16/2004): Microsoft has revised MS04-039 to include updated fixes for ISA Server 2000 SP1 and for users who are running the vulnerable software on Microsoft Windows 2000 SP3 platforms. These updated fixes address installation problems. Please see Microsoft Security Bulletin MS04-039 for further information.



Discovery is credited to Martijn de Vries and Thomas de Klerk of of Info Support.

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