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

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

- **Severity:** Low 2.9 (`CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 28 Jan 2026
- **Updated:** 28 Jan 2026
- **Also known as:** ALPINE-CVE-2026-23553, DEBIAN-CVE-2026-23553, GHSA-pxqc-5jg3-xfqm, UBUNTU-CVE-2026-23553

## 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 (Protection Mechanism Failure), CWE-665 (Improper Initialization)

## Which versions are affected by CVE-2026-23553?

It affects 1 product.


Products: 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.

## Other vulnerabilities in Xen Xen

- [CVE-2017-10912](https://insights.sbomify.com/CVE-2017-10912/): Privilege escalation in n/a (critical 10.0)
- [CVE-2017-10918](https://insights.sbomify.com/CVE-2017-10918/): Memory corruption in n/a (critical 10.0)
- [CVE-2017-10920](https://insights.sbomify.com/CVE-2017-10920/): Memory corruption in n/a (critical 10.0)
- [CVE-2017-10921](https://insights.sbomify.com/CVE-2017-10921/): Memory corruption in n/a (critical 10.0)
- [CVE-2025-27466](https://insights.sbomify.com/CVE-2025-27466/): Mutiple vulnerabilities in the Viridian interface (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23553 and fix it if it is affected.

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

Affected product: Xen Xen. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-23553/ (Markdown: https://insights.sbomify.com/CVE-2026-23553/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23553
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23553
- OSV: https://osv.dev/vulnerability/CVE-2026-23553

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23553/
Summaries are generated and may contain mistakes; the official description above is the record.