CVE-2026-21255
Local bypass of a Hyper-V security feature due to improper access control.
Is CVE-2026-21255 being exploited?
Not confirmed. CVE-2026-21255 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.37% probability of exploitation in the next 30 days.
How severe is CVE-2026-21255?
CVE-2026-21255 is rated High with a CVSS score of 8.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-21255?
Yes. A fix has been recorded for CVE-2026-21255. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-21255 affect?
CVE-2026-21255 affects Windows Hyper-V. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-21255?
Apply updates when released; restrict local admin access
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Local attacker with valid access can bypass Hyper-V security controls, risking host isolation.
Patch when possible
- affected>= 10.0.14393.0 and < 10.0.14393.8868
- affected>= 10.0.17763.0 and < 10.0.17763.8389
- affected>= 10.0.19044.0 and < 10.0.19044.6937
- affected>= 10.0.19045.0 and < 10.0.19045.6937
- affected>= 10.0.22631.0 and < 10.0.22631.6649
- affected>= 10.0.22631.0 and < 10.0.22631.6649
- affected>= 10.0.26100.0 and < 10.0.26100.7840
- affected>= 10.0.26200.0 and < 10.0.26200.7840
- affected>= 10.0.28000.0 and < 10.0.28000.1575
- affected>= 10.0.28000.0 and < 10.0.28000.1575
- affected>= 10.0.14393.0 and < 10.0.14393.8868
- affected>= 10.0.14393.0 and < 10.0.14393.8868
- affected>= 10.0.17763.0 and < 10.0.17763.8389
- affected>= 10.0.17763.0 and < 10.0.17763.8389
- affected>= 10.0.20348.0 and < 10.0.20348.4773
- affected>= 10.0.25398.0 and < 10.0.25398.2149
- affected>= 10.0.26100.0 and < 10.0.26100.32370
- affected>= 10.0.26100.0 and < 10.0.26100.32370
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.