Patch tracker / CVE reference / CVE-2026-40405

CVE-2026-40405

CVE-2026-40405 is a critical severity Microsoft vulnerability with a CVSS score of 9.8. Its EPSS score puts the probability of exploitation in the next 30 days at 80%. Microsoft has shipped the fix: deploy one of the 5 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Risk summary

Critical

9.8

CVSS

80%

EPSS, 30 days

Critical

Severity

Not in the CISA KEV catalog. EPSS exploit probability 80%. CVSS 9.8 (Critical).

This CVE is among the top 25% of tracked Microsoft CVEs by EPSS exploit probability.

Senserva AI Opinion for CVE-2026-40405

This is a Windows TCP/IP flaw with an unusually high CVSS of 9.8 for something classified as denial of service, meaning a network-triggered crash rather than code execution, yet the 80 percent EPSS score signals attackers are likely to weaponize it soon. Exposed systems include Windows Server 2025 Server Core and Windows 11 25H2 on both ARM64 and x64, so any of these reachable over the network could be knocked offline. It is not yet in CISA KEV and has no known ransomware tie, but the network attack vector on core server infrastructure makes it worth prioritizing.

Do this: Apply Microsoft's patch for CVE-2026-40405 to your Windows Server 2025 and Windows 11 25H2 systems, and restrict inbound TCP/IP exposure on any that cannot be patched immediately.

AI-generated from public data, 2026-07-26. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2026-40405

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I am an IT admin assessing a vulnerability. Facts for CVE-2026-40405 (source: https://senserva.com/cve/CVE-2026-40405.html, data from MSRC, CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2026-40405: Critical severity Microsoft vulnerability, CVSS 9.8
- EPSS 30-day exploitation probability: 80%
- Actively exploited (CISA KEV): no
- Fixed by: KB5087423, KB5087539, KB5089466, KB5089548, KB5089549
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) What to patch or mitigate first. 4) How to check whether my devices are exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch this cycle

Not confirmed exploited yet, but the 30-day exploit probability is high (EPSS 80%).

Fixed by KB5087423 and 4 more (by build, listed below). Download from the Microsoft Update Catalog, or deploy via Windows Update, Intune, or WSUS.

Do this now: Patch in this maintenance window; do not let it slip to the next one.

Official patch(es) for CVE-2026-40405

High priority: EPSS puts the odds of near-term exploitation high. In Intune or Defender, confirm whether the fixing updates KB5087423, KB5087539, KB5089466, KB5089548, KB5089549 are installed across the affected products. Senserva does this automatically across your tenant and flags every missing patch.

Apply the Microsoft security updates below to fix CVE-2026-40405. Each KB page carries the Microsoft Update Catalog download, Microsoft's known issues for the update, a copy-paste PowerShell check for whether it is installed, and the Senserva AI read.

KB5087423 KB5087539 KB5089466 KB5089548 KB5089549

Scope and sources

Affected products

Fixed by the same updates

CVE-2026-34348 CVE-2025-54518 CVE-2026-21530 CVE-2026-32161 CVE-2026-32170 CVE-2026-32209 CVE-2026-33834 CVE-2026-33835

Authoritative references

Official CVE record (cve.org)NVD detail Microsoft MSRC FIRST.org EPSS

Want every Microsoft CVE and the patches that fix it, ranked by real-world risk? See the Microsoft Patch Tracker.