Windows TCP/IP Denial of Service Vulnerability

Null pointer dereference in Windows TCP/IP allows an unauthorized attacker to deny service over a network.

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

7.5CVSS out of 10High 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

10.0.26100.32860
10.0.26100.32772
10.0.26200.8457
10.0.26200.8390
10.0.26100.8457
10.0.26100.8390
10.0.28000.2113

Next step

What the vendor recommends

Install KB5087539. Install KB5087423. Install KB5089549. Install KB5089466. Install KB5089548.

Review the complete instructions on the vendor's site

Timeline

When this advisory changed

May 12, 2026 at 7:00 AM UTC
Published by Microsoft Security Response Center
The publication date reported by the vendor.
Sep 27, 2026 at 11:11 PM UTC
Added to SecurityAlert
We collected the advisory from the official source.

Vulnerabilities

CVEs named in this advisory

CVE-2026-40405View 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-2026-40405
Last source check: Sep 27, 2026 at 11:11 PM UTC

Open the official bulletin How SecurityAlert builds this record

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

When this advisory changed

CVEs named in this advisory

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