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

Executive Summary

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

Overview

7.8

CVSS HIGH

Important

MS Severity

Not Exploited

MS Exploit Status

Exploitation Unlikely

MS Exploit Likelihood

Category Elevation of Privilege

Released Feb 10 2026

Last Updated Feb 10 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.00417 — 0.33194 percentile

CVSS Vector

ATTACK VECTOR

Local

ATTACK COMPLEXITY

Low

PRIVILEGES REQUIRED

Low

USER INTERACTION

None

SCOPE

Unchanged

Temporal Score: 6.8

EPSS Score

0.00417

probability of exploitation in the next 30 days

0.33194 percentile - updated 2026-06-20

View on FIRST.org

Affected Products

9 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows Server 2016	`5075999 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2016 (Server Core installation)	`5075999 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019	`5075904 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019 (Server Core installation)	`5075904 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022	`5075906 (Security Update) 5075943 (Security Hotpatch Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022 (Server Core installation)	`5075906 (Security Update) 5075943 (Security Hotpatch Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022, 23H2 Edition (Server Core installation)	`5075897 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025	`5075899 (Security Update) 5075942 (Security Hotpatch Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025 (Server Core installation)	`5075899 (Security Update) 5075942 (Security Hotpatch Update)`	Important	Elevation of Privilege	Yes

Patches

5 patches

Article	Type	Restart
`5075999`	Security Update	Yes
`5075904`	Security Update	Yes
`5075906 (Security Update) 5075943`	Security Update	Yes
`5075897`	Security Update	Yes
`5075899 (Security Update) 5075942`	Security Update	Yes

Known Exploits

No known exploits have been linked for this CVE yet. When available, exploit references will be sourced from public repositories and may be unverified, incomplete, or non-functional. Always review code carefully before use in any environment.

Acknowledgments

ChenJian with Sea Security Orca Team

References

MSRC Advisory MITRE NVD