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Windows 11 24h2Operating system · Microsoft

CVE-2026-40405

HIGH · 7.5 CVSS v3.1 Published 2026-05-12
Fix available
A fix is available. Upgrade to 10.0.26100.8390 / 10.0.26100.32772 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 TCP/IP 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 the Windows TCP/IP network stack that allows remote, unauthenticated attackers to cause a denial of service by crashing the affected system's network stack through specially crafted network packets.

MitigationApply the appropriate Microsoft security patch for this vulnerability through Windows Update or manual patch deployment, and verify network connectivity and TCP/IP functionality post-patch.

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 11 24h2Operating system
Affected:< 10.0.26100.8390
Windows 11 25h2Operating system
Affected:< 10.0.26200.8390
Windows 11 26h1Operating system
Affected:< 10.0.28000.2113
Windows Server 2025Operating system
Affected:< 10.0.26100.32772

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. Identify Windows version and build number
    Run 'winver' from command prompt or check System Properties to view the OS version and build. Alternatively, run 'systeminfo | findstr /B /C:"OS Name" /C:"OS Version"' to get detailed version info.
    Affected if The system is running Windows 11 24h2, Windows 11 25h2, Windows 11 26h1, or Windows Server 2025 with a build number lower than the thresholds in the affected versions list.
  2. Confirm exact build number via PowerShell
    Run '[System.Environment]::OSVersion.Version' or '$PSVersionTable.OS' in PowerShell to get the precise version string in 10.0.xxxx.yyyy format.
    Affected if The version string starts with 10.0.26100 and is below 10.0.26100.8390, or 10.0.26200 below 10.0.26200.8390, or 10.0.28000 below 10.0.28000.2113, or 10.0.26100 below 10.0.26100.32772 depending on the Windows edition.
  3. Verify TCP/IP stack is in use
    Confirm the system has active network connectivity by running 'ipconfig' or checking that network adapters are enabled. The vulnerability exists in the Windows TCP/IP implementation (tcpip.sys).
    Affected if The system has network adapters with TCP/IP enabled, as the vulnerability is triggered by receiving specially crafted network packets.
  4. Check for pending security updates
    Open Windows Settings > Windows Update > Update history, or run 'powershell Get-HotFix' to list installed security updates.
    Affected if The system has not received the security update addressing CVE-2026-40405, as indicated by the absence of a corresponding KB patch in the update history.

The environment is affected if the system runs Windows 11 24h2, 25h2, 26h1, or Windows Server 2025 with a build number lower than the specified thresholds AND has TCP/IP networking 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 10.0.26100.8390 / 10.0.26100.32772 / 10.0.26200.8390 or later
Fixed in 10.0.26100.839010.0.26100.3277210.0.26200.8390
Interim mitigation

Apply the appropriate Microsoft security patch for this vulnerability through Windows Update or manual patch deployment, and verify network connectivity and TCP/IP functionality post-patch.

Recommended fix Moderate confidence

Windows 11 24h2: build 10.0.26100.8390 or later | Windows 11 25h2: build 10.0.26200.8390 or later | Windows 11 26h1: build 10.0.28000.2113 or later | Windows Server 2025: build 10.0.26100.32772 or later

  1. Check current Windows build version by running 'winver' or 'systeminfo | findstr /B /C:"OS Build"'
  2. Open Settings > Windows Update and check for updates
  3. Apply all available Windows security updates, specifically looking for updates that address TCP/IP vulnerabilities
  4. Alternatively, run 'Check for updates' from Windows Update or use Windows Server Update Services (WSUS) for managed environments
  5. After updates are installed, restart the system as prompted
  6. Verify the fix by checking the build version matches or exceeds: 10.0.26100.8390 for Windows 11 24h2, 10.0.26200.8390 for Windows 11 25h2, 10.0.28000.2113 for Windows 11 26h1, or 10.0.26100.32772 for Windows Server 2025
Caveat Standard Windows update - minimal risk; patch Tuesday cumulative updates recommended

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

Fix this in Windows 11 24h2 Scoped from the published advisory
  • Consultation6.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
16.0 hours of engineering $2,880
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Scan for this in your stack

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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-2026-40405 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