CVE-2026-21251: Windows Server Use-After-Free Flaw

CVE-2026-21251 Overview

CVE-2026-21251 is a Use After Free (UAF) vulnerability in the Windows Cluster Client Failover component that allows an authorized attacker to elevate privileges locally. This memory corruption flaw occurs when freed memory is improperly accessed, potentially enabling attackers with local access to gain elevated system privileges on affected Windows Server systems.

Critical Impact

Local privilege escalation via memory corruption in Windows Cluster Client Failover service could allow attackers to gain SYSTEM-level access on Windows Server infrastructure.

Affected Products

• Microsoft Windows Server 2016
• Microsoft Windows Server 2019
• Microsoft Windows Server 2022
• Microsoft Windows Server 2022 23H2
• Microsoft Windows Server 2025

Discovery Timeline

• 2026-02-10 - CVE-2026-21251 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-21251

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to use a pointer after the memory it references has been deallocated. In the context of Windows Cluster Client Failover, this flaw can be exploited by an authenticated local attacker to manipulate memory in a way that leads to arbitrary code execution with elevated privileges.

The attack requires local access and low-privilege authentication, making it particularly dangerous in enterprise environments where multiple users have access to server infrastructure. Once exploited, the attacker can achieve complete compromise of confidentiality, integrity, and availability on the affected system.

Root Cause

The root cause lies in improper memory management within the Windows Cluster Client Failover component. When certain cluster failover operations are performed, the code fails to properly track memory lifecycle, resulting in a dangling pointer that references freed memory. This freed memory can be reallocated and populated with attacker-controlled data, which is then used by the vulnerable code path.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated access to the target Windows Server system. The exploitation flow involves:

1. An attacker with low-level privileges triggers specific cluster failover operations
2. The vulnerable component frees memory but retains a reference to it
3. The attacker manipulates heap allocations to control the contents of the freed memory region
4. When the dangling pointer is dereferenced, the attacker-controlled data is used
5. This allows hijacking of program execution flow to achieve privilege escalation

The vulnerability does not require user interaction and can be exploited with low attack complexity once local access is obtained.

Detection Methods for CVE-2026-21251

Indicators of Compromise

• Unexpected crashes or restarts of the Windows Cluster Service (clussvc.exe)
• Anomalous memory access patterns in Windows Cluster Client Failover related processes
• Privilege escalation events in Windows Security logs from low-privileged accounts

Detection Strategies

• Monitor Windows Event Log for Event ID 4688 (process creation) showing suspicious process spawning from cluster services
• Implement memory integrity monitoring for cluster-related components using EDR solutions
• Track failed and successful privilege escalation attempts through Windows Security audit logs

Monitoring Recommendations

• Enable advanced process auditing on all Windows Server systems running cluster services
• Deploy SentinelOne agents configured to detect memory corruption exploitation techniques
• Review cluster service logs regularly for abnormal failover operations or service behavior

How to Mitigate CVE-2026-21251

Immediate Actions Required

• Apply the latest Windows security updates from Microsoft immediately
• Restrict local access to Windows Server systems running cluster services to essential personnel only
• Implement least-privilege principles for all accounts with server access
• Enable Windows Defender Credential Guard on affected systems

Patch Information

Microsoft has released security updates to address this vulnerability. Administrators should apply the appropriate patches for their Windows Server version immediately. Detailed patch information and download links are available in the Microsoft Security Response Center advisory.

Workarounds

• Limit local access to affected servers through group policy restrictions
• Disable Windows Cluster Client Failover service on systems where clustering is not required
• Implement network segmentation to isolate cluster nodes from general user access
• Monitor and alert on any use of cluster-related administrative tools

# Disable Cluster Client Failover service if not required
sc.exe config ClusSvc start= disabled
sc.exe stop ClusSvc