CVE-2026-79611 VMX: VMentry failure with branch tracing and blocked states
A guest VM can cause a fatal VMEntry failure and crash itself by triggering a specific state combination involving branch tracing and blocked states.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-79611?
CVE-2026-79611 is a medium-severity denial of service vulnerability in Xen, published on 13 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
For XSA-308 logic was introduced, as VMX VMEntry checks do not like the exact combination of state which occurs when #DB in intercepted, Single Stepping is active, and blocked by STI/MovSS is active, despite this being a legitimate state to be in. The resulting VMEntry failure is fatal to the guest.
Sadly there is another special sub-case of the above, with branch tracing enabled. In that case the prior adjustment would still cause a VMentry failure, as before fatal to the guest.
Weakness: CWE-843
Which versions are affected by CVE-2026-79611?
How do I fix CVE-2026-79611?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-79611 being exploited?
There is no known exploitation of CVE-2026-79611 in the wild and no public exploit code.
Sources
Also known as ALPINE-CVE-2026-79611, DEBIAN-CVE-2026-79611, GHSA-28w8-qvww-f539, UBUNTU-CVE-2026-79611.