Vulnerability/
https://seclists.org/oss-sec/2026/q1/119

https://seclists.org/oss-sec/2026/q1/119: Xen Security Advisory 479 v2 (CVE-2026-23553) - x86: incomplete IBPB for vCPU isolation

Published Jan 27, 2026

Updated

Affected Software

1 affected component

Xen Project Xen>=4.6

Frequently Asked Questions

Which deployments are affected?

Affected systems are x86 Xen deployments with the XSA-254 fixes backported. Upstream Xen 4.6 and newer are vulnerable; Arm systems are not vulnerable.

What must occur for guest isolation to be weakened?

The issue involves a vCPU moving between physical CPUs while the guest switches tasks. Xen can skip IBPB when the vCPU returns to a CPU it previously ran on, leaving branch-target-buffer training from an earlier guest task available while a different task runs.

What is the potential impact inside a guest?

Guest processes may use information leaks to obtain information intended to remain private to other entities in the same guest.

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