Microsoft Security Response CenterCVE-2026-56179

Windows Network Address Translation (NAT) Spoofing Vulnerability

Origin validation error in Windows Network Address Translation (NAT) allows an unauthorized attacker to perform spoofing over an adjacent network.

Read the vendor bulletin Get alerts for Windows Server 2025 (Server Core installation)

8.3CVSS out of 10Medium severity

Scope

What the vendor says is affected

Windows Server 2025 (Server Core installation)
Windows 11 Version 25H2 for ARM64-based Systems
Windows 11 Version 25H2 for x64-based Systems
Windows 11 Version 24H2 for ARM64-based Systems
Windows 11 Version 24H2 for x64-based Systems
Windows Server 2025
Windows 11 version 26H1 for x64-based Systems
Windows 11 Version 26H1 for ARM64-based Systems

Versions the vendor lists as fixed

10.0.26100.33296
10.0.26100.33222
10.0.26200.9168
10.0.26200.9106
10.0.26100.9168
10.0.26100.9106
10.0.28000.2704

Next step

What the vendor recommends

The mitigation is disabled by default. To enable it, apply the following registry keys and reboot the machine. reg add HKLM\SYSTEM\CurrentControlSet\Services\Winnat /v TcpSeqValidation /t REG_DWORD /d 1 /f reg add HKLM\SYSTEM\CurrentControlSet\Services\Winnat /v TcpSeqRandomization /t REG_DWORD /d 1 /f Install KB5120233. Install KB5120228. Install KB5121003. Install KB5120994. Install KB5121000.

Review the complete instructions on the vendor's site

Update history

What changed in later vendor updates

SecurityAlert records field-level changes from the point we begin following a bulletin. Earlier vendor changes may not have a field-by-field record.

Aug 20, 2026 at 7:00 PM UTC
- The vendor changed its remediation guidance.

Timeline

When this advisory changed

Aug 11, 2026 at 7:00 AM UTC
Published by Microsoft Security Response Center
The publication date reported by the vendor.
Aug 20, 2026 at 6:50 PM UTC
Added to SecurityAlert
We collected the advisory from the official source.
Sep 6, 2026 at 6:25 AM UTC
Confirmed at the source
Our collector saw this advisory during a later source check.

Vulnerabilities

CVEs named in this advisory

CVE-2026-56179View CVSS, KEV, EPSS, and threat context

Source

Start with the vendor's bulletin

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Vendor: Microsoft Security Response Center
Advisory ID: CVE-2026-56179
Last source check: Sep 13, 2026 at 2:40 AM UTC

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Your environment

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Test the update or mitigation through your normal change process.

Watch Windows Server 2025 (Server Core installation)

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What the vendor says is affected

Versions the vendor lists as fixed

What the vendor recommends

What changed in later vendor updates

When this advisory changed

CVEs named in this advisory

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