CVE-2026-79611
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Xen Hypervisor VMEntry Failure Due to Debug State

Publication date: 2026-10-13

Last updated on: 2026-10-13

Assigner: Xen Project

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.

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Meta Information

Published
2026-10-13
Last Modified
2026-10-13
Generated
2026-10-19
AI Q&A
2026-10-13
EPSS Evaluated
2026-10-18
NVD
CVE-2026-79611
EUVD
EUVD-2026-98828

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
xen xen From 4.17.x (inc) to 4.21.x (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-UNKNOWN

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Executive Summary

This vulnerability occurs in Xen's VMX (Intel, VIA, or Zhaoxin CPUs) due to a specific state combination involving branch tracing and blocked conditions during VMEntry. This causes a fatal crash of the guest system, resulting in a denial of service.

Detection Guidance

Detection requires checking Xen version and configuration. Use 'xl info' to verify Xen version and CPU type. If using Intel/VIA/Zhaoxin CPUs with HVM/PVH guests, the system may be vulnerable. No direct commands detect active exploitation, but monitoring guest crashes or VMEntry failures may indicate issues.

Impact Analysis

It allows userspace code to trigger a Denial of Service, crashing HVM/PVH guest systems. Users relying on these guest types may experience unexpected downtime or service disruption.

Compliance Impact

This vulnerability primarily causes Denial of Service (DoS) conditions in guest operating systems, which may impact availability of systems processing personal or sensitive data. Compliance with GDPR or HIPAA could be affected if such disruptions lead to unauthorized access, data breaches, or failure to maintain system availability required by these regulations.

Mitigation Strategies

Mitigation options include switching to PV guests, using AMD/Hygon/Arm hardware, or applying available patches for Xen 4.21.x or 4.20.x to 4.17.x branches. If patches are unavailable, avoid HVM/PVH guests on affected CPUs.

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