CVE-2026-20927: Microsoft Windows SMB Server race condition denial of service

CVE-2026-20927 · Severity: medium · CVSS 5.3 · Published 2026-01-13

A vulnerability in the Windows SMB Server, which is used for sharing files and printers across a network, could allow an authorized user to crash the service. This would result in a denial of service, preventing other users from accessing shared files or network resources. While the attack requires an account on the network, it can be performed remotely.

Technologies: Microsoft Windows 11, Microsoft Windows Server 2008 R2, Microsoft Windows 10. Vendors: Microsoft.

Quick answers

Is CVE-2026-20927 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-20927 being exploited in the wild.
How severe is CVE-2026-20927?
Medium severity, with a CVSS score of 5.3, published 13 January 2026.
Which products and versions are affected?
Microsoft Windows 11 22H3, 23H2, 24H2, 25H2; Microsoft Windows Server 2008 R2 All versions; Microsoft Windows 10 1607, 1809, 21H2, 22H2.
Is there a fix for CVE-2026-20927?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-20927 attacked?
It is attacked over the network, in an operating system.
Am I affected by CVE-2026-20927?
Check whether you run Microsoft Windows 11, Microsoft Windows Server 2008 R2 or Microsoft Windows 10 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A race condition (CWE-362) exists in the Windows SMB Server due to improper synchronization when multiple threads access shared resources concurrently. An attacker with low-privileged network access can exploit this flaw by sending specially crafted requests that trigger the synchronization error. Successful exploitation results in a denial of service (DoS) condition on the affected server. The attack has high complexity as it requires winning a race condition, but it can be executed remotely by an authenticated user. Microsoft has released security updates to address this issue across various Windows and Windows Server versions.

Affected products

  • Microsoft Windows 10 1607, 1809, 21H2, 22H2
  • Microsoft Windows 11 22H3, 23H2, 24H2, 25H2
  • Microsoft Windows Server 2008 R2 Service Pack 1 All versions

Timeline

  • 2026-01-13: advisory: Initial advisory published by Microsoft and NVD.

References

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