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What tools are available to verify a patch's validity?

Do any patch management tools verify the content of the patches downloaded from Microsoft before they are tested and deployed? Can they be identified as true Microsoft files and not malicious files?

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Before installing any patch, verify its source and integrity. This is typically done using a digital signature or some form of checksum. These verification techniques ensure the patch hasn't been modified since the signature was applied or since the checksum count was calculated. Signed patches also validate the patch's creator. Unfortunately, Microsoft occasionally releases unsigned code updates, and you may run into problems when using the automatic download and installation services of WSUS (Windows Server Update Services), Microsoft's patch management system for Windows Server 2000, Windows Server 2003 and Windows XP operating systems. If you set the "Warn but allow installation" policy object for unsigned code, Windows will prompt with a warning and halt the update procedure, waiting for confirmation that it's OK to install the update.

While Microsoft's documentation doesn't clarify whether WSUS performs digital signature or checksum counts prior to installation, its Baseline Security Analyzer (MBSA) examines file versions and checksums to verify the present files match with those released by Microsoft. If any of these files do not pass the test, MBSA will identify that the software update is not installed, or will flag the software update with a warning. It's important to note that this process can only be done after the patch has been installed. Interestingly enough, this product is licensed from Shavlik Technologies LLC, the makers of HFNetChkPro.

HFNetChkPro is a patch management program that validates file versions and checksums prior to deploying both Microsoft and non-Microsoft security patches. Its Basic Edition is designed for smaller organizations that don't require advanced patch management functionality like scheduled scans and email support.

PatchQuest is another automated patch management program that can distribute and manage security patches, hotfixes and updates across heterogeneous networks comprising Windows and Linux systems. It is a Web-based service that downloads patches, assesses patch authenticity and tests for functional correctness. The tool scans your network, identifies missing patches and software updates, distributes patches to vulnerable systems and keeps your systems up-to-date and free from vulnerabilities.

Should you receive a warning message that questions a patch's validity, you should fully investigate the digital certificate or checksum. Why you ask? In 2001, VeriSign Inc. issued two VeriSign Class 3 code-signing digital certificates to an individual who fraudulently claimed to be a Microsoft employee. The common name assigned to both certificates was "Microsoft Corporation," allowing the individual to sign executable content using keys that supposedly belonged to Microsoft. Thankfully, trust is defined on a certificate-by-certificate basis, rather than on the basis of the common name. Therefore, if a similar event occurred, a warning dialogue would be displayed before any of the signed content could be executed, even if the user had previously agreed to trust other certificates with the common name "Microsoft Corporation." The danger, of course, is that even a security-conscious user might agree to trust the bogus certificates and execute the content.

More information:

  • Use our step-by-step guide to successfully deploy a patch.
  • Get recommendations on proper patch management techniques.

    • This was first published in October 2006

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