CVE-2026-23553 — x86: incomplete IBPB for vCPU isolation

Description (CNA)

In the context switch logic Xen attempts to skip an IBPB in the case of a vCPU returning to a CPU on which it was the previous vCPU to run. While safe for Xen's isolation between vCPUs, this prevents the guest kernel correctly isolating between tasks. Consider: 1) vCPU runs on CPU A, running task 1. 2) vCPU moves to CPU B, idle gets scheduled on A. Xen skips IBPB. 3) On CPU B, guest kernel switches from task 1 to 2, issuing IBPB. 4) vCPU moves back to CPU A. Xen skips IBPB again. Now, task 2 is running on CPU A with task 1's training still in the BTB.

Affected (CNA)

Vendor / product	Versions
Xen / Xen	unknown consult Xen advisory XSA-479

CWE

No CWE identifiers supplied.

References

https://xenbits.xenproject.org/xsa/advisory-479.html

ADP enrichment

CVE

No English description supplied.

http://www.openwall.com/lists/oss-security/2026/01/27/3
http://xenbits.xen.org/xsa/advisory-479.html

CISA-ADP

No English description supplied.

No references supplied.