Affects Linux kernel KVM with AMD SEV used broadly in cloud/virtualization; could enable host compromise or wide outages.
CVE-2026-63940
KVM SEV: zero-length Port I/O can underflow kernel, enabling crash or guest-to-host compromise.
Is CVE-2026-63940 being exploited?
Not confirmed. CVE-2026-63940 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 0.18% probability of exploitation in the next 30 days.
How severe is CVE-2026-63940?
CVE-2026-63940 is rated Critical with a CVSS score of 9.3. 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-2026-63940?
Yes. A fix has been recorded for CVE-2026-63940. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-63940 affect?
CVE-2026-63940 affects Linux kernel (KVM with AMD SEV), Hosts/hypervisors running KVM/SEV, Cloud providers using SEV-enabled VMs. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-63940?
Update Linux kernels to patched versions; isolate or restrict untrusted SEV guests.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Malicious VM can send zero-length Port I/O to KVM/SEV causing kernel underflow, host crash, or potential guest-to-host escape/privilege escalation.
Immediate action required
- affected>= 291bd20d5d88814a73d43b55b9428feab2f28094 and < 3b6035bc6bff20e89752ce4358bc4c9a9d5883f2
- affected>= 291bd20d5d88814a73d43b55b9428feab2f28094 and < 2254972d4d69e279ba4e87bf0968eb08ad0d3c92
- affected>= 291bd20d5d88814a73d43b55b9428feab2f28094 and < c30cde934c7813b4e3069765dac64ce3d31e34f2
- affected>= 291bd20d5d88814a73d43b55b9428feab2f28094 and < 3988bd2723de407ae90fa7a6f6029b4e60238c58
- affected5.11
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.