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Windows Server 2019Operating system · Microsoft

CVE-2026-21243

HIGH · 7.5 CVSS v3.1 Published 2026-02-10
Fix available
A fix is available. Upgrade to 10.0.17763.8389 / 10.0.20348.4711 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges 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
Null pointer dereference in Windows LDAP - Lightweight Directory Access Protocol allows an unauthorized attacker to deny service over a network.

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 null pointer dereference vulnerability exists in Windows LDAP (Lightweight Directory Access Protocol) implementation. An unauthenticated remote attacker can send specially crafted LDAP requests to cause the LDAP service to crash, resulting in denial of service. This affects Windows systems running LDAP services.

MitigationApply the relevant Windows security update to patch the LDAP vulnerability. Test LDAP authentication and directory services after patching to ensure continued functionality.

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 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
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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 and build number
    Open Command Prompt or PowerShell and run 'winver' or 'systeminfo | findstr /B /C:"OS Name" /C:"OS Version"' to display the installed Windows version and build number
    Affected if The build number is less than 17763.8389 (Server 2019), 20348.4711 (Server 2022), 25398.2149 (Server 2022 23h2), or 26100.32313 (Server 2025)
  2. Identify if LDAP service is running
    Open Services console (services.msc) and locate 'LDAP Service' or 'Active Directory Domain Services', or run 'Get-Service | Where-Object {$_.DisplayName -like "*LDAP*"}' in PowerShell
    Affected if An LDAP service is enabled and running on the system
  3. Confirm LDAP listener is active
    Run 'netstat -an | findstr ":389"' or 'Get-NetTCPConnection -LocalPort 389 -State Listen' in PowerShell to check if port 389 (LDAP) or port 636 (LDAPS) is listening
    Affected if The system is listening on LDAP ports 389 or 636
  4. Check LDAP implementation version
    Query the LDAP service version through registry key 'HKLM\SYSTEM\CurrentControlSet\Services\LDAP' or use 'ldapsearch' utility with version flag to identify the LDAP daemon version
    Affected if The LDAP implementation version cannot be determined or shows as unpatched

A user is affected if they are running any of the listed Windows Server versions with a build number below the specified thresholds AND have an LDAP service enabled or listening on port 389/636.

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 10.0.17763.8389 / 10.0.20348.4711 / 10.0.25398.2149 or later
Fixed in 10.0.17763.838910.0.20348.471110.0.25398.2149
Interim mitigation

Apply the relevant Windows security update to patch the LDAP vulnerability. Test LDAP authentication and directory services after patching to ensure continued functionality.

Recommended fix High confidence

Windows Server 2019: 10.0.17763.8389+ | Windows Server 2022: 10.0.20348.4711+ | Windows Server 2022 23h2: 10.0.25398.2149+ | Windows Server 2025: 10.0.26100.32313+

  1. Identify the current Windows Server version by running 'winver' or 'systeminfo' in Command Prompt
  2. Determine which Windows Server version (2019, 2022, 2022 23h2, or 2025) is running
  3. Apply the appropriate Windows cumulative update that includes the security fix for this vulnerability
  4. For Windows Server 2019: Update to version 10.0.17763.8389 or later via Windows Update or download from Microsoft Update Catalog
  5. For Windows Server 2022: Update to version 10.0.20348.4711 or later via Windows Update or download from Microsoft Update Catalog
  6. For Windows Server 2022 23h2: Update to version 10.0.25398.2149 or later via Windows Update or download from Microsoft Update Catalog
  7. For Windows Server 2025: Update to version 10.0.26100.32313 or later via Windows Update or download from Microsoft Update Catalog
  8. After updating, verify the new version matches or exceeds the minimum fixed version for your server edition
Caveat Standard Windows update risks apply - test in staging environment before production deployment; some cumulative updates may require restarts

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

Fix this in Windows Server 2019 Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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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-2026-21243 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