# CVE-2026-31657: batman-adv: hold claim backbone gateways by reference

> A use-after-free vulnerability in the batman-adv kernel module allows attackers to cause a denial of service or potentially execute code via improper reference handling.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Apr 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-83810, BELL-CVE-2026-31657, DEBIAN-CVE-2026-31657, GHSA-w24x-3wrx-8q34, ROOT-OS-DEBIAN-11-CVE-2026-31657, ROOT-OS-DEBIAN-12-CVE-2026-31657, ROOT-OS-UBUNTU-2204-CVE-2026-31657, ROOT-OS-UBUNTU-2404-CVE-2026-31657, UBUNTU-CVE-2026-31657

## What is CVE-2026-31657?

CVE-2026-31657 is a critical use after free vulnerability in Linux kernel, published on 24 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

batman-adv: hold claim backbone gateways by reference

batadv_bla_add_claim() can replace claim->backbone_gw and drop the old
gateway's last reference while readers still follow the pointer.

The netlink claim dump path dereferences claim->backbone_gw->orig and
takes claim->backbone_gw->crc_lock without pinning the underlying
backbone gateway. batadv_bla_check_claim() still has the same naked
pointer access pattern.

Reuse batadv_bla_claim_get_backbone_gw() in both readers so they operate
on a stable gateway reference until the read-side work is complete.
This keeps the dump and claim-check paths aligned with the lifetime
rules introduced for the other BLA claim readers.

Weakness: CWE-476 (NULL Pointer Dereference), CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-31657?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31657?

Apply the vendor's patch.

## Is CVE-2026-31657 being exploited?

There is no known exploitation of CVE-2026-31657 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-2026-31657 and fix it if it is affected.

What it is: CVE-2026-31657 is a critical use after free vulnerability in Linux kernel, published on 24 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-31657 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-2026-31657/ (Markdown: https://insights.sbomify.com/CVE-2026-31657/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31657
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31657
- OSV: https://osv.dev/vulnerability/CVE-2026-31657

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31657/
Summaries are generated and may contain mistakes; the official description above is the record.