# CVE-2026-31788: xen/privcmd: restrict usage in unprivileged domU

> A local attacker with root access in an unprivileged Xen guest can bypass Secure Boot restrictions by modifying kernel memory via the privcmd driver.

- **Severity:** High 8.2 (`CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Mar 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-31788, DEBIAN-CVE-2026-31788, GHSA-q397-v647-xvh6, ROOT-OS-DEBIAN-11-CVE-2026-31788, ROOT-OS-DEBIAN-12-CVE-2026-31788, ROOT-OS-DEBIAN-13-CVE-2026-31788, ROOT-OS-UBUNTU-2204-CVE-2026-31788, ROOT-OS-UBUNTU-2404-CVE-2026-31788, UBUNTU-CVE-2026-31788

## What is CVE-2026-31788?

CVE-2026-31788 is a high-severity vulnerability in Linux kernel, published on 25 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

xen/privcmd: restrict usage in unprivileged domU

The Xen privcmd driver allows to issue arbitrary hypercalls from
user space processes. This is normally no problem, as access is
usually limited to root and the hypervisor will deny any hypercalls
affecting other domains.

In case the guest is booted using secure boot, however, the privcmd
driver would be enabling a root user process to modify e.g. kernel
memory contents, thus breaking the secure boot feature.

The only known case where an unprivileged domU is really needing to
use the privcmd driver is the case when it is acting as the device
model for another guest. In this case all hypercalls issued via the
privcmd driver will target that other guest.

Fortunately the privcmd driver can already be locked down to allow
only hypercalls targeting a specific domain, but this mode can be
activated from user land only today.

The target domain can be obtained from Xenstore, so when not running
in dom0 restrict the privcmd driver to that target domain from the
beginning, resolving the potential problem of breaking secure boot.

This is XSA-482

---
V2:
- defer reading from Xenstore if Xenstore isn't ready yet (Jan Beulich)
- wait in open() if target domain isn't known yet
- issue message in case no target domain found (Jan Beulich)

Weakness: CWE-862 (Missing Authorization)

## Which versions are affected by CVE-2026-31788?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31788?

Apply the vendor's patch.

## Is CVE-2026-31788 being exploited?

There is no known exploitation of CVE-2026-31788 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31788 and fix it if it is affected.

What it is: CVE-2026-31788 is a high-severity vulnerability in Linux kernel, published on 25 Mar 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-2026-31788 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-31788/ (Markdown: https://insights.sbomify.com/CVE-2026-31788/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31788
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31788
- OSV: https://osv.dev/vulnerability/CVE-2026-31788

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31788/
Summaries are generated and may contain mistakes; the official description above is the record.