CVE-2026-40405: Null pointer dereference in Windows TCP/IP allows an unauthorized attacker to deny service over a network.

Q: How do I fix CVE-2026-40405?

windows 11 24h2: upgrade to 10.0.26100.8390; windows 11 25h2: upgrade to 10.0.26200.8390; windows 11 26h1: upgrade to 10.0.28000.2113. Exact commands for each OS are on this page.

Q: Is CVE-2026-40405 being exploited?

CVE-2026-40405 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-40405 affect?

CVE-2026-40405 affects windows 11 24h2, windows 11 25h2, windows 11 26h1. Severity is high (CVSS 7.5). Null pointer dereference in Windows TCP/IP allows an unauthorized attacker to deny service over a network.

Q: How urgent is CVE-2026-40405?

Priority 52/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

This month

HIGH 7.5REMOTEWATCHLISTUPGRADEUPGRADEUPGRADE

CVE-2026-40405 is a high-severity flaw (CVSS 7.5) affecting windows 11 24h2, windows 11 25h2, windows 11 26h1. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.1% (top 35%)
CISA exploited list	No
Weakness	CWE-476
Published	2026-05-12
Last modified	2026-06-17

Publisher (CNA): Microsoft Corporation · Vendor: microsoft

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-12 18:17 UTCPublished CVE record published
2026-06-17 10:45 UTCUpdated Record last modified

microsoft windows 11 24h2 UPGRADE

Affected: < 10.0.26100.8390

Fixed in: 10.0.26100.8390

Safe to downgrade to: no unaffected older release

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

microsoft windows 11 25h2 UPGRADE

Affected: < 10.0.26200.8390

Fixed in: 10.0.26200.8390

Safe to downgrade to: no unaffected older release

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

microsoft windows 11 26h1 UPGRADE

Affected: < 10.0.28000.2113

Fixed in: 10.0.28000.2113

Safe to downgrade to: no unaffected older release

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

microsoft windows server 2025 UPGRADE

Affected: < 10.0.26100.32772

Fixed in: 10.0.26100.32772

Safe to downgrade to: no unaffected older release

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-40405?

windows 11 24h2: upgrade to 10.0.26100.8390; windows 11 25h2: upgrade to 10.0.26200.8390; windows 11 26h1: upgrade to 10.0.28000.2113. Exact commands for each OS are on this page.

Is CVE-2026-40405 being exploited?

CVE-2026-40405 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-40405 affect?

CVE-2026-40405 affects windows 11 24h2, windows 11 25h2, windows 11 26h1. Severity is high (CVSS 7.5). Null pointer dereference in Windows TCP/IP allows an unauthorized attacker to deny service over a network.

How urgent is CVE-2026-40405?

Priority 52/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Windows vulnerabilities · Windows Server vulnerabilities

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References

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