CVE-2026-40405: Windows TCP/IP Denial of Service Vulnerability

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-12 · updated 2026-08-10
Weakness: CWE-476

Description

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

Affected products

Microsoft Windows 11: before 10.0.26100.8390 — fixed in 10.0.26100.8390
Microsoft Windows 11 25H2: before 10.0.26200.8390 — fixed in 10.0.26200.8390
Microsoft Windows 11 version 26H1: before 10.0.28000.2113 — fixed in 10.0.28000.2113
Microsoft Windows Server 2025: before 10.0.26100.32772 — fixed in 10.0.26100.32772
Microsoft Windows 11: from 10.0.26100.0 before 10.0.26100.8457 — fixed in 10.0.26100.8457
Microsoft Windows 11 25H2: from 10.0.26200.0 before 10.0.26200.8457 — fixed in 10.0.26200.8457
Microsoft Windows 11 version 26H1: from 10.0.28000.0 before 10.0.28000.2113 — fixed in 10.0.28000.2113
Microsoft Windows Server 2025: from 10.0.26100.0 before 10.0.26100.32860 — fixed in 10.0.26100.32860

How to fix

Upgrade to a fixed version: Microsoft Windows 11 10.0.26100.8390; Microsoft Windows 11 25H2 10.0.26200.8390; Microsoft Windows 11 version 26H1 10.0.28000.2113; Microsoft Windows Server 2025 10.0.26100.32772; Microsoft Windows 11 10.0.26100.8457; Microsoft Windows 11 25H2 10.0.26200.8457; Microsoft Windows 11 version 26H1 10.0.28000.2113; Microsoft Windows Server 2025 10.0.26100.32860. Check the vendor advisory for mitigations.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40405

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