CVE-2026-21251: Cluster Client Failover (CCF) Elevation of Privilege Vulnerability

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-10 · updated 2026-08-19
Weakness: CWE-416

Description

Use after free in Windows Cluster Client Failover allows an authorized attacker to elevate privileges locally.

Affected products

Microsoft Windows Server 2016: before 10.0.14393.8868 — fixed in 10.0.14393.8868
Microsoft Windows Server 2019: before 10.0.17763.8389 — fixed in 10.0.17763.8389
Microsoft Windows Server 2022: before 10.0.20348.4711 — fixed in 10.0.20348.4711
Microsoft Windows Server 2022, 23H2 Edition (Server Core installation): before 10.0.25398.2149 — fixed in 10.0.25398.2149
Microsoft Windows Server 2025: before 10.0.26100.32313 — fixed in 10.0.26100.32313
Microsoft Windows Server 2016: from 10.0.14393.0 before 10.0.14393.8868 — fixed in 10.0.14393.8868
Microsoft Windows Server 2019: from 10.0.17763.0 before 10.0.17763.8389 — fixed in 10.0.17763.8389
Microsoft Windows Server 2022: from 10.0.20348.0 before 10.0.20348.4773 — fixed in 10.0.20348.4773
Microsoft Windows Server 2022, 23H2 Edition (Server Core installation): from 10.0.25398.0 before 10.0.25398.2149 — fixed in 10.0.25398.2149
Microsoft Windows Server 2025: from 10.0.26100.0 before 10.0.26100.32370 — fixed in 10.0.26100.32370

How to fix

Upgrade to a fixed version: Microsoft Windows Server 2016 10.0.14393.8868; Microsoft Windows Server 2019 10.0.17763.8389; Microsoft Windows Server 2022 10.0.20348.4711; Microsoft Windows Server 2022, 23H2 Edition (Server Core installation) 10.0.25398.2149; Microsoft Windows Server 2025 10.0.26100.32313; Microsoft Windows Server 2016 10.0.14393.8868; Microsoft Windows Server 2019 10.0.17763.8389; Microsoft Windows Server 2022 10.0.20348.4773; Microsoft Windows Server 2022, 23H2 Edition (Server Core installation) 10.0.25398.2149; Microsoft Windows Server 2025 10.0.26100.32370. Check the vendor advisory for mitigations.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21251

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