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Windows Server 2016Operating system · Microsoft

CVE-2026-21251

HIGH · 7.8 CVSS v3.1 Published 2026-02-10
Fix available
A fix is available. Upgrade to 10.0.14393.8868 / 10.0.17763.8389 or later.
See remediation →
80/100
Remediation priority · High
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

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

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

A use-after-free vulnerability in the Windows Cluster Client Failover component allows a locally authorized attacker to escalate privileges by manipulating memory after it has been freed, potentially executing code at a higher privilege level.

MitigationApply the relevant Microsoft security patch for this vulnerability to all affected Windows systems running Cluster Client Failover components. Prioritize systems where untrusted users have local access.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Windows Server 2016Operating system
Affected:< 10.0.14393.8868
Windows Server 2019Operating system
Affected:< 10.0.17763.8389
Windows Server 2022Operating system
Affected:< 10.0.20348.4711
Windows Server 2022 23h2Operating system
Affected:< 10.0.25398.2149
Windows Server 2025Operating system
Affected:< 10.0.26100.32313

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Windows Server version
    Run 'winver' or 'systeminfo' in command prompt, or use PowerShell: Get-ComputerInfo | Select-Object OsName, OsVersion, OsBuildNumber
    Affected if The OS build number is less than 14393.8868 (Server 2016), 17763.8389 (Server 2019), 20348.4711 (Server 2022), 25398.2149 (Server 2022 23h2), or 26100.32313 (Server 2025)
  2. Verify Cluster Client Failover component is installed
    Run PowerShell command: Get-WindowsOptionalFeature -Online | Where-Object {$_.FeatureName -like '*FailoverCluster*' -or $_.FeatureName -like '*Clustering*'} | Select-Object FeatureName, State
    Affected if Any Cluster Client Failover or Failover Clustering feature shows a State of 'Enabled'
  3. Confirm Cluster service is running (if feature is enabled)
    Run 'sc query ClusSvc' in command prompt or Get-Service -Name 'ClusSvc' in PowerShell
    Affected if The Cluster service (ClusSvc) is in a Running state

The system is affected if it runs an unpatched Windows Server version within the affected ranges AND has the Cluster Client Failover or Failover Clustering component installed and enabled.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 10.0.14393.8868 / 10.0.17763.8389 / 10.0.20348.4711 or later
Fixed in 10.0.14393.886810.0.17763.838910.0.20348.4711
Interim mitigation

Apply the relevant Microsoft security patch for this vulnerability to all affected Windows systems running Cluster Client Failover components. Prioritize systems where untrusted users have local access.

Recommended fix High confidence

Upgrade to Windows Server 2016 build 10.0.14393.8868 or later | Windows Server 2019 build 10.0.17763.8389 or later | Windows Server 2022 build 10.0.20348.4711 or later | Windows Server 2022 23h2 build 10.0.25398.2149 or later | Windows Server 2025 build 10.0.26100.32313 or later

  1. Identify the current Windows Server version by running 'winver' or 'systeminfo' in Command Prompt
  2. Navigate to the Microsoft Security Response Center (msrc.microsoft.com) and search for CVE-2026-21251
  3. Download and install the appropriate security update for your Windows Server version from Microsoft's update catalog
  4. Restart the server to complete the update installation
  5. Verify the update was successfully installed by checking the installed updates via 'Control Panel > Programs > View installed updates'
  6. Confirm the build number matches or exceeds the fixed version for your Windows Server edition
Caveat Standard Windows Server update risks apply - ensure backups are current and schedule maintenance window for reboot

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Windows Server 2016 Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,280
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Scan for this in your stack

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dbcve dependency scanner

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21251 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data