Patch tracker / CVE reference / CVE-2026-21255

CVE-2026-21255

CVE-2026-21255 is a Microsoft vulnerability that Microsoft rates Important, with a CVSS score of 8.8 (High). Microsoft has shipped the fix: deploy one of the 12 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Our data comes from Microsoft, CISA and other public sources, and it can be wrong or out of date. If you are unsure, check the linked source before you act, and tell us when we are wrong so we can fix it.

Microsoft rates it Important. CVSS 8.8 (High). Fixed by 12 Microsoft security updates.

Senserva AI read on CVE-2026-21255Official patches for CVE-2026-21255Change historyAdmin questions

Risk summary

High
8.8
CVSS v3
<1%
EPSS, 30 days
High
Severity

Not in the CISA KEV catalog. EPSS exploit probability <1%. CVSS 8.8 (High).

This CVE is among the lower half of tracked Microsoft CVEs by EPSS exploit probability.

Senserva AI Opinion for CVE-2026-21255

CVE-2026-21255 is a Hyper-V security feature bypass in Windows that lets an attacker sidestep isolation protections meant to separate guest virtual machines from the host. It hits both the newer Windows 11 version 26H1 (ARM64 and x64) and the older Windows 10 version 1809, so mixed fleets running virtualization are exposed. With a CVSS of 8.8 and a 26 percent EPSS score, exploitation is plausible even though it is not yet in CISA KEV or tied to ransomware.

Do this: Deploy the Microsoft patch to all Hyper-V hosts running Windows 11 26H1 and Windows 10 1809, prioritizing machines that host untrusted or multi-tenant guest VMs.

AI-generated from public data, 2026-07-21. Verify against the vendor advisory before acting.

Schedule normally
No exploitation observed as of 2026-11-10: not in the CISA KEV catalog, EPSS under 1%.
Fixed by KB5075897 and 11 more (by build, listed below). Download from the Microsoft Update Catalog, or deploy via Windows Update, Intune, or WSUS.
Do this now: Deploy with your normal update cycle. This page refreshes daily; the verdict changes the day the data does.
Senserva AI Opinion and rich prompt for CVE-2026-21255

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

Official patch(es) for CVE-2026-21255

Moderate priority: schedule it in your normal patch cycle. In Intune or Defender, confirm whether the fixing updates KB5075897, KB5075899, KB5075904, KB5075906, KB5075912, KB5075941, KB5075942, KB5075943, KB5075999, 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-2026-21255. 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.

KB5075897KB5075899KB5075904KB5075906KB5075912KB5075941KB5075942KB5075943KB5075999KB5077179KB5077181KB5077212

Scope and sources

Affected products
Fixed by the same updates
CVE-2026-21510CVE-2026-21513CVE-2026-21519CVE-2026-21533CVE-2026-21525CVE-2026-21231CVE-2026-21232CVE-2026-21236
Authoritative references
Official CVE record (cve.org)NVD detailMicrosoft MSRCFIRST.org EPSS

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

New to a term on this page? CVE, KB, KEV, CVSS, and EPSS are each explained once, on the Senserva security lexicon. Reference: Microsoft CVE and vulnerability management and the Microsoft patching guide.
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