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

Low 2.9 Published 28 Jan 2026 Updated 28 Jan 2026

Xen hypervisor fails to isolate guest tasks correctly by skipping IBPB, allowing speculative execution side-channel attacks between guest processes.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2026-23553?

CVE-2026-23553 is a low-severity vulnerability in Xen Xen, published on 28 Jan 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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.

Weakness: CWE-693, CWE-665

Which versions are affected by CVE-2026-23553?

It affects 1 product.

Products include Xen Xen.

How do I fix CVE-2026-23553?

Apply the vendor's patch.

Is CVE-2026-23553 being exploited?

There is no known exploitation of CVE-2026-23553 in the wild and no public exploit code.

All Xen Xen vulnerabilities

Sources

Also known as ALPINE-CVE-2026-23553, DEBIAN-CVE-2026-23553, GHSA-pxqc-5jg3-xfqm, UBUNTU-CVE-2026-23553.