# CVE-2026-31786: Buffer overflow in drivers/xen/sys-hypervisor.c

> A buffer overflow in the Linux kernel Xen driver allows attackers to execute arbitrary code or cause a denial of service via crafted hypervisor version data.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 30 Apr 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-31786, DEBIAN-CVE-2026-31786, GHSA-7p99-gmjx-2f7r, ROOT-OS-DEBIAN-11-CVE-2026-31786, ROOT-OS-UBUNTU-2204-CVE-2026-31786, UBUNTU-CVE-2026-31786

## What is CVE-2026-31786?

CVE-2026-31786 is a high-severity memory corruption vulnerability in Linux kernel, published on 30 Apr 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:

Buffer overflow in drivers/xen/sys-hypervisor.c

The build id returned by HYPERVISOR_xen_version(XENVER_build_id) is
neither NUL terminated nor a string.

The first causes a buffer overflow as sprintf in buildid_show will
read and copy till it finds a NUL.

00000000  f4 91 51 f4 dd 38 9e 9d  65 47 52 eb 10 71 db 50  |..Q..8..eGR..q.P|
00000010  b9 a8 01 42 6f 2e 32                              |...Bo.2|
00000017

So use a memcpy instead of sprintf to have the correct value:

00000000  f4 91 51 f4 dd 00 9e 9d  65 47 52 eb 10 71 db 50  |..Q.....eGR..q.P|
00000010  b9 a8 01 42                                       |...B|
00000014

(the above have a hack to embed a zero inside and check it's
returned correctly).

This is XSA-485 / CVE-2026-31786

Weakness: CWE-787 (Out-of-bounds Write), CWE-120 (Buffer Copy without Checking Size of Input)

## Which versions are affected by CVE-2026-31786?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31786?

Apply the vendor's patch.

## Is CVE-2026-31786 being exploited?

There is no known exploitation of CVE-2026-31786 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-2026-31786 and fix it if it is affected.

What it is: CVE-2026-31786 is a high-severity memory corruption vulnerability in Linux kernel, published on 30 Apr 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-2026-31786 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-2026-31786/ (Markdown: https://insights.sbomify.com/CVE-2026-31786/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31786
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31786
- OSV: https://osv.dev/vulnerability/CVE-2026-31786

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31786/
Summaries are generated and may contain mistakes; the official description above is the record.