CVE-2026-21251 Cluster Client Failover (CCF) Elevation of Privilege Vulnerability
An authorized local attacker can exploit a use-after-free flaw in Windows Cluster Client Failover to elevate privileges.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-21251?
CVE-2026-21251 is a high-severity use after free vulnerability in Microsoft Windows Server 2016, published on 10 Feb 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
Use after free in Windows Cluster Client Failover allows an authorized attacker to elevate privileges locally.
Weakness: CWE-416
Which versions are affected by CVE-2026-21251?
It affects 5 products.
Products include Microsoft Windows Server 2016, Microsoft Windows Server 2019, Microsoft Windows Server 2022, Microsoft Windows Server 2022 23h2, Microsoft Windows Server 2025.
How do I fix CVE-2026-21251?
No fix has been published for CVE-2026-21251 yet. Limit exposure of the affected software until one is available.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-21251 being exploited?
There is no known exploitation of CVE-2026-21251 in the wild and no public exploit code.
Other vulnerabilities in Microsoft Windows Server 2016
- CVE-2021-34458 Windows Kernel Remote Code Execution Vulnerability
- CVE-2024-38076 Windows Remote Desktop Licensing Service Remote Code Execution Vulnerability
- CVE-2026-27928 Windows Hello Security Feature Bypass Vulnerability
- CVE-2025-27482 Windows Remote Desktop Services Remote Code Execution Vulnerability
- CVE-2024-49106 Windows Remote Desktop Services Remote Code Execution Vulnerability
Sources
Also known as GHSA-6q2h-rrfg-fm75.