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Windows 10 21h2Operating system · Microsoft

CVE-2026-42904

CRITICAL · 9.6 CVSS v3.1 Published 2026-06-09
Fix available
A fix is available. Upgrade to 10.0.19044.7417 / 10.0.19045.7417 or later.
See remediation →
100/100
Remediation priority · Urgent
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
Heap-based buffer overflow in Windows TCP/IP allows an unauthorized attacker to elevate privileges over an adjacent 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

Heap-based buffer overflow in Windows TCP/IP allows an unauthenticated attacker on an adjacent network to elevate privileges. This is a memory corruption vulnerability in the Windows networking stack that enables local privilege escalation from an adjacent network position.

MitigationApply Microsoft security updates for Windows TCP/IP once released; limit adjacent network access through network segmentation and firewall rules to reduce exposure.

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 10 21h2Operating system
Affected:< 10.0.19044.7417
Windows 10 22h2Operating system
Affected:< 10.0.19045.7417
Windows 11 23h2Operating system
Affected:< 10.0.22631.7219
Windows 11 24h2Operating system
Affected:< 10.0.26100.8655
Windows 11 25h2Operating system
Affected:< 10.0.26200.8655
Windows 11 26h1Operating system
Affected:< 10.0.28000.2269
Windows Server 2022Operating system
Affected:< 10.0.20348.5256
Windows Server 2025Operating system
Affected:< 10.0.26100.32995

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/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 version and build number
    Open Settings > System > About, or run 'winver' command, or run 'systeminfo | findstr /B /C:"OS Name" /C:"OS Version"' in Command Prompt
    Affected if The build number is lower than the threshold for your Windows version (10.0.19044.7417 for 21h2, 10.0.19045.7417 for 22h2, 10.0.22631.7219 for 11 23h2, 10.0.26100.8655 for 11 24h2, 10.0.26200.8655 for 11 25h2, 10.0.28000.2269 for 11 26h1, 10.0.20348.5256 for Server 2022, 10.0.26100.32995 for Server
  2. Confirm TCP/IP stack is enabled
    Open Network Connections, right-click your active network adapter, select Properties, and verify 'Internet Protocol Version 4 (TCP/IPv4)' or 'Internet Protocol Version 6 (TCP/IPv6)' is checked
    Affected if Either TCP/IPv4 or TCP/IPv6 is enabled (vulnerability affects the TCP/IP stack, which is enabled by default on most Windows installations)
  3. Verify network adapter is active
    Run 'ipconfig /all' and confirm you have an active network interface with an IP address assigned
    Affected if The system has an active network adapter with IP connectivity (vulnerability is exploitable from an adjacent network)
  4. Check for applied security updates
    Run 'powershell Get-HotFix | Sort-Object InstalledOn' to list installed hotfixes, or check Windows Update history for recent security updates
    Affected if No recent TCP/IP-related security updates have been installed after the vulnerability disclosure date

Your system is likely affected if it runs an unpatched Windows version within the affected build ranges AND has TCP/IP enabled with network connectivity, making it reachable from an adjacent network segment.

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 10.0.19044.7417 / 10.0.19045.7417 / 10.0.20348.5256 or later
Fixed in 10.0.19044.741710.0.19045.741710.0.20348.5256
Interim mitigation

Apply Microsoft security updates for Windows TCP/IP once released; limit adjacent network access through network segmentation and firewall rules to reduce exposure.

Recommended fix High confidence

Windows 10 21h2: build 10.0.19044.7417+ | Windows 10 22h2: build 10.0.19045.7417+ | Windows 11 23h2: build 10.0.22631.7219+ | Windows 11 24h2: build 10.0.26100.8655+ | Windows 11 25h2: build 10.0.26200.8655+ | Windows 11 26h1: build 10.0.28000.2269+ | Windows Server 2022: build 10.0.20348.5256+ | Wi

  1. Identify the current Windows version and build number by running 'winver' or 'systeminfo'
  2. Determine which affected version category applies to your system (Windows 10 21h2, 10 22h2, Windows 11 23h2/24h2/25h2/26h1, or Windows Server 2022/2025)
  3. Apply the Microsoft security update for CVE-2026-42904 via Windows Update, or manually download the corresponding KB from the Microsoft Update Catalog
  4. For Windows Server systems, apply the update via Windows Update, WSUS, or manual KB installation
  5. Reboot the system after applying the update
  6. Verify the build number matches or exceeds the fixed version for your product (e.g., Windows 10 22h2 should be 10.0.19045.7417 or later)
Caveat Standard Windows update risks apply - ensure backups and test in staging if possible; no major functionality changes expected from this security patch

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

Fix this in Windows 10 21h2 Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing4.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,900
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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-2026-42904 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