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

Source: https://junglewise.ai/threats/cve-2026-20927-microsoft-windows-smb-server-race-condition-denial-of-service  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-01-13. Updated: 2026-07-30  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/windows-11), [Microsoft Windows Server 2008 R2](https://junglewise.ai/threats/technologies/windows-server-2008-r2), [Microsoft Windows 10](https://junglewise.ai/threats/technologies/windows-10). Vendors: [Microsoft](https://junglewise.ai/threats/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

- [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20927](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20927)

## Related threats

- [Microsoft Windows Recovery Environment UEFI/BIOS password bypass](https://junglewise.ai/threats/cve-2026-116638-the-windows-recovery-environment-winre-in-windows-10-and-11-may)
- [Use after free in Windows Ancillary Function Driver for WinSock allows an authorized attacker to elevate privileges locally.](https://junglewise.ai/threats/cve-2026-68820-microsoft-windows-ancillary-function-driver-for-winsock-use-after)
- [Microsoft Windows Backup Engine privilege escalation](https://junglewise.ai/threats/cve-2026-58598-microsoft-windows-backup-engine-privilege-escalation)
- [Microsoft Windows Boot Loader security feature bypass](https://junglewise.ai/threats/cve-2026-58638-microsoft-windows-boot-loader-security-feature-bypass)
- [Microsoft Windows Client-Side Caching Service privilege escalation](https://junglewise.ai/threats/cve-2026-58637-microsoft-windows-client-side-caching-service-privilege)