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Windows CVE-2026-21251

HIGH
Use After Free (CWE-416)
2026-02-10 secure@microsoft.com
Windows Use After Free Microsoft Windows Server 2016 Windows Server 2019 Windows Server 2022 Windows Server 2022 23h2 Windows Server 2025
7.8
CVSS 3.1 · NVD
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Authenticate to Windows system with local user
  2. Exploit
    Trigger use-after-free in Cluster Client Failover
  3. Impact
    Execute arbitrary code with elevated privileges

Vulnerability Assessment

Weakness
CWE
CWE-416
OWASP Top 10:2025
A06:2021 - Vulnerable and Outdated Components
ASVS 5.0
V15 - Secure Coding and Architecture Memory lifecycle management
MITRE ATT&CK

2 techniques across 2 tactics: Execution, Privilege Escalation

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

Local authenticated user account required. Windows Cluster Client Failover service must be active. Attacker must have PR:L (low privilege) local user credentials on the target system.
Impact

CVSS 7.8 (HIGH). Attack vector: Local. Complexity: Low. Privileges required: Low. User interaction: None. Exploitable via local access.
Exploit ScenarioDerived

An attacker could exploit this vulnerability to compromise the affected system.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 12, 2026

Privilege escalation in Windows Cluster Client Failover exploits a use-after-free memory vulnerability, enabling authenticated local users to gain elevated system privileges. The flaw affects Windows Server 2016, 2019, and 2025 installations where an attacker with existing local access can trigger the vulnerability through the failover clustering component. No patch is currently available for this high-severity vulnerability.

Earlier versions of this analysis (1)
v1 Mar 12, 2026 - 22:02 CVSS 7.8

Use after free in Windows Cluster Client Failover allows an authorized attacker to elevate privileges locally. [CVSS 7.8 HIGH]

RemediationAI

Monitor vendor advisories for a patch.

Affected Packages

microsoft windows_server_2016_10_0_14393_8868 fix: Windows Server 2016 10.0.14393.8868, Windows Server 2016 (Server Core installation) 10.0.14393.8868
microsoft windows_server_2019_10_0_17763_8389 fix: Windows Server 2019 10.0.17763.8389, Windows Server 2019 (Server Core installation) 10.0.17763.8389
microsoft windows_server_2022_10_0_20348_4773 fix: Windows Server 2022 10.0.20348.4773, Windows Server 2022 (Server Core installation) 10.0.20348.4773
microsoft windows_server_2022_23h2_edition_10_0_25398_2149 fix: Windows Server 2022, 23H2 Edition (Server Core installation) 10.0.25398.2149
microsoft windows_server_2025_10_0_26100_32370 fix: Windows Server 2025 10.0.26100.32370, Windows Server 2025 (Server Core installation) 10.0.26100.32370

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 5 packages in microsoft

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

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Technical ContextAI · show details

Classified as CWE-416 (Use After Free). Affects Windows Server 2016. Use after free in Windows Cluster Client Failover allows an authorized attacker to elevate privileges locally.

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  • Exploit and PoC references, threat intelligence from 22 sources
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Severity by source

NVD PRIMARY 7.8 HIGH
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

DescriptionCVE.org

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

Executive ViewAI

Why It Matters

CVE-2026-21251 is a privilege escalation vulnerability in Windows Cluster Client Failover that allows authorized users to gain elevated system access. This poses a significant risk to organizations running Windows failover clusters, as compromised accounts could lead to full cluster compromise and potential service disruption.

Business Impact: High
Compensating Controls

Restrict cluster administrative credentials to essential personnel only; disable Windows Cluster Client Failover service if not actively required; implement network segmentation to limit access to cluster nodes to trusted networks only; enhance logging and monitoring for privilege escalation attempts; enforce multi-factor authentication for cluster administration accounts; regularly audit user permissions and access logs.

Threat Intelligence Overview

EPSS Score
0.0%
Top 94% exploitable
Data Quality
3 sources
Confidence: Low
NVD CVE.org EPSS

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.0%
Patch available Yes
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Scheduled
Exploitation· derived
None
System Exposure· derived
Small
Automatable· derived
No
Human Impact
High

Priority Score

39
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +39
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Use After Free HIGH

Use-after-free vulnerabilities occur when a program continues to access memory through a pointer after that memory has been deallocated.

Affected Products (CPE)CVE.org

cpe:2.3:o:microsoft:windows_server_2016:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2019:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2022:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2022_23h2:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2025:*:*:*:*:*:*:*:*

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References

Advisories & patches
ADVISORY https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21251

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