# CVE-2026-89951: batman-adv: fix stale receive device on merged fragments

> A use-after-free vulnerability in the Linux kernel's batman-adv module allows attackers to cause a denial of service via stale device references in merged fragments.

- **Severity:** High 8.8 (`CVSS:3.1/AV:A/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:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-101675, BELL-CVE-2026-89951, DEBIAN-CVE-2026-89951, GHSA-27vc-6rpf-73gh, UBUNTU-CVE-2026-89951

## What is CVE-2026-89951?

CVE-2026-89951 is a high-severity use after free vulnerability in Linux, published on 16 Sep 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:

batman-adv: fix stale receive device on merged fragments

Fragment reassembly reuses the skb from the highest-numbered buffered
fragment as the merged packet. When that fragment was received on a hard
interface which is deleted before the chain completes, the merged skb can
re-enter the receive path with a stale skb->dev and skb_iif.

batadv_batman_skb_recv() passes such merged packets through the normal
receive handlers again. DAT and bridge loop avoidance both derive the ARP
header length from skb->dev, so they can dereference the freed net_device
before the packet reaches the local mesh interface.

Refresh the receive device metadata from the current receive device before
running the packet handlers. This keeps internally reinjected merged
fragments consistent with the normal receive path after hard interface
teardown.

## Which versions are affected by CVE-2026-89951?


## How do I fix CVE-2026-89951?

Apply the vendor's patch.

## Is CVE-2026-89951 being exploited?

There is no known exploitation of CVE-2026-89951 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89951 and fix it if it is affected.

What it is: CVE-2026-89951 is a high-severity use after free vulnerability in Linux, published on 16 Sep 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-2026-89951 in the wild and no public exploit code.

Affected product: Linux. 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-2026-89951/ (Markdown: https://insights.sbomify.com/CVE-2026-89951/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89951
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89951
- OSV: https://osv.dev/vulnerability/CVE-2026-89951

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89951/
Summaries are generated and may contain mistakes; the official description above is the record.