Vulnerability · Published Jan 28, 2026 · Assigned by XEN
CVE-2026-23553x86: incomplete IBPB for vCPU isolation
Low 2.9
In plain English
A low-severity security flaw in Xen from Xen. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 4% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine:v3.20 | xen | 4.18.5-r4 | ALPINE-CVE-2026-23553 |
| Alpine:v3.21 | xen | 4.19.4-r1 | ALPINE-CVE-2026-23553 |
| Alpine:v3.22 | xen | 4.20.2-r1 | ALPINE-CVE-2026-23553 |
| Alpine:v3.23 | xen | 4.20.2-r1 | ALPINE-CVE-2026-23553 |
| Alpine:v3.24 | xen | 4.21.0-r2 | ALPINE-CVE-2026-23553 |
| Debian:12 | xen | 4.17.7-0+deb12u1 | DEBIAN-CVE-2026-23553 |
| Debian:13 | xen | 4.20.2+37-g61ff35323e-0+deb13u1 | DEBIAN-CVE-2026-23553 |
| Debian:14 | xen | 4.20.2+37-g61ff35323e-1 | DEBIAN-CVE-2026-23553 |
| Ubuntu:22.04:LTS | xen | no fix listed | UBUNTU-CVE-2026-23553 |
| Ubuntu:24.04:LTS | xen | no fix listed | UBUNTU-CVE-2026-23553 |
| Ubuntu:25.10 | xen | no fix listed | UBUNTU-CVE-2026-23553 |
| Ubuntu:26.04:LTS | xen | no fix listed | UBUNTU-CVE-2026-23553 |
| Ubuntu:26.10 | xen | no fix listed | UBUNTU-CVE-2026-23553 |
Affected products (as reported)
Weakness: CWE-665, CWE-693 · Vector: CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N