Microsoft Windows up to Server 2025 Container Isolation FS Filter Driver unionfs.sys race condition

Summaryinfo

A vulnerability labeled as very critical has been found in Microsoft Windows up to Server 2025. This impacts an unknown function of the file unionfs.sys of the component Container Isolation FS Filter Driver. Executing a manipulation can lead to race condition. This vulnerability appears as CVE-2026-97787. The attack requires local access. There is no available exploit. It is best practice to apply a patch to resolve this issue.

Detailsinfo

A vulnerability was found in Microsoft Windows up to Server 2025. It has been declared as very critical. This vulnerability affects an unknown part of the file unionfs.sys of the component Container Isolation FS Filter Driver. The manipulation with an unknown input leads to a race condition vulnerability. The CWE definition for the vulnerability is CWE-362. The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

Concurrent execution using shared resource with improper synchronization ('race condition') in Windows Container Isolation FS Filter Driver (unionfs.sys) allows an authorized attacker to elevate privileges locally.

The advisory is available at msrc.microsoft.com. This vulnerability was named CVE-2026-97787 since 09/25/2026. The exploitation appears to be easy. Local access is required to approach this attack. Technical details are known, but there is no available exploit. The structure of the vulnerability defines a possible price range of USD $5k-$25k at the moment (estimation calculated on 10/14/2026).

Applying a patch is able to eliminate this problem.

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Productinfo

Type

• Operating System

Vendor

• Microsoft

Name

• Windows

Version

• 11 23H2
• 11 24H2
• 11 25H1
• 11 26H2
• Server 2022
• Server 2025

License

• commercial

Website

• Vendor: https://www.microsoft.com/
• Product: https://www.microsoft.com/en-us/windows

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