CVE-2026-20921: Microsoft Windows SMB Server privilege escalation via race condition

CVE-2026-20921 · Severity: high · CVSS 7.5 · Published 2026-01-13

A security vulnerability exists in the Windows SMB Server, which is the component responsible for file and printer sharing across a network. An authorized user with low-level access could exploit a timing flaw to gain higher-level administrative privileges. This could allow an attacker to take full control of affected servers or workstations, potentially leading to data theft or service disruption.

Technologies: Microsoft Windows 11, Microsoft Windows 10, Microsoft Windows Server 2008, Microsoft Windows Server 2012, Microsoft Windows Server 2016, Microsoft Windows Server 2019, Microsoft Windows Server 2022, Microsoft Windows Server 2025. Vendors: Microsoft.

Quick answers

Is CVE-2026-20921 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-20921 being exploited in the wild.
How severe is CVE-2026-20921?
High severity, with a CVSS score of 7.5, published 13 January 2026.
Which products and versions are affected?
Microsoft Windows 11 23H2, 24H2, 25H2; Microsoft Windows 10 1607, 1809, 21H2, 22H2; Microsoft Windows Server 2008 SP2, R2 SP1; Microsoft Windows Server 2012 R2; and 4 more.
Is there a fix for CVE-2026-20921?
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-2026-20921 attacked?
It is attacked over the network, in an operating system.
Am I affected by CVE-2026-20921?
Check whether you run Microsoft Windows 11, Microsoft Windows 10, Microsoft Windows Server 2008 or one of the others listed 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 during concurrent execution using shared resources. The vulnerability is reachable over the network but requires the attacker to have at least low-level authenticated access (PR:L) and involves high attack complexity (AC:H) due to the timing requirements of a race condition. Successful exploitation allows for a local privilege escalation, granting the attacker high confidentiality, integrity, and availability impacts. Microsoft has released security updates to address this issue across multiple versions of Windows and Windows Server.

Affected products

  • Microsoft Windows Server 2008 SP2, R2 SP1
  • Microsoft Windows Server 2012 R2
  • Microsoft Windows 10 1607, 1809, 21H2, 22H2
  • Microsoft Windows 11 23H2, 24H2, 25H2
  • Microsoft Windows Server 2016 All versions
  • Microsoft Windows Server 2019 All versions
  • Microsoft Windows Server 2022 All versions, 23H2
  • Microsoft Windows Server 2025 All versions

Timeline

  • 2026-01-13: disclosed: Initial disclosure by Microsoft
  • 2026-01-13: advisory: NVD entry published

References

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