Patch tracker / CVE reference / CVE-2026-21240

CVE-2026-21240

CVE-2026-21240 is a Microsoft vulnerability that Microsoft rates Important, with a CVSS score of 7.8 (High). Its EPSS score puts the probability of exploitation in the next 30 days at 25%. Microsoft has shipped the fix: deploy one of the 11 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Risk summary

High

7.8

CVSS v3

25%

EPSS, 30 days

High

Severity

Not in the CISA KEV catalog. EPSS exploit probability 25%. CVSS 7.8 (High).

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

Senserva AI Opinion for CVE-2026-21240

This HTTP.sys elevation of privilege flaw targets the older end of the fleet, Windows 10 1809 and Windows Server 2019, at High Severity, CVSS 8.8. EPSS rates exploitation probability at 26 percent. Server 2019 still fronts a great deal of production IIS, and kernel-mode HTTP flaws there sit directly in the request path.

Do this: Patch Windows Server 2019 web servers now and review what else binds to HTTP.sys on those hosts.

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

Senserva AI Opinion and rich prompt for CVE-2026-21240

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-2026-21240 (source: https://senserva.com/cve/CVE-2026-21240.html, data from MSRC, CISA KEV, FIRST.org EPSS, NVD, refreshed daily (early morning US Central)):
- CVE-2026-21240: Microsoft vulnerability, Microsoft rating Important, CVSS 7.8 (High)
- EPSS 30-day exploitation probability: 25%
- Actively exploited (CISA KEV): no
- Fixed by: KB5075897, KB5075899, KB5075904, KB5075906, KB5075912, KB5075941, KB5075942, KB5075943, KB5077179, KB5077181, KB5077212
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 25%).

Fixed by KB5075897 and 10 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-2026-21240

Elevated priority: high severity or a meaningful exploitation probability, sooner if an affected product is internet-facing. In Intune or Defender, confirm whether the fixing updates KB5075897, KB5075899, KB5075904, KB5075906, KB5075912, KB5075941, KB5075942, KB5075943, KB5077179, KB5077181, KB5077212 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-2026-21240. 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.

KB5075897 KB5075899 KB5075904 KB5075906 KB5075912 KB5075941 KB5075942 KB5075943 KB5077179 KB5077181 KB5077212

Scope and sources

Affected products

Fixed by the same updates

CVE-2026-21510 CVE-2026-21513 CVE-2026-21519 CVE-2026-21533 CVE-2026-21525 CVE-2026-21255 CVE-2026-21231 CVE-2026-21232

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.