# CVE-2026-89950: batman-adv: mcast: linearize skbuff for packet generation

> A missing linearization check in the batman-adv multicast forwarding code can trigger a fatal kernel bug, causing a denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89950, DEBIAN-CVE-2026-89950, GHSA-3h9h-h4qr-r799, UBUNTU-CVE-2026-89950

## What is CVE-2026-89950?

CVE-2026-89950 is a vulnerability in Linux, published on 16 Sep 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: mcast: linearize skbuff for packet generation

batadv_mcast_forw_packet() and batadv_mcast_forw_scrape() is not only
called (indirectly) by the unsharing+linearizing batadv_recv_mcast_packet()
handler. When it is called (indirectly) by batadv_mcast_forw_mcsend() then
it will be unshared but not linearized. The SKB_LINEAR_ASSERT() can
therefore cause a fatal BUG().

The linearization should happen during the expansion of the head because
the scrape function can be hit already during the initial
batadv_mcast_forw_mode() selection code:

* batadv_interface_tx
* batadv_mcast_forw_mode
* batadv_mcast_forw_mode_by_count()
* batadv_mcast_forw_push()
  -> calls batadv_mcast_forw_expand_head() before everything else
* batadv_mcast_forw_push_tvlvs()
* batadv_mcast_forw_push_dests()
* batadv_mcast_forw_push_adjust_padding()
* batadv_mcast_forw_scrape()

## Which versions are affected by CVE-2026-89950?


## How do I fix CVE-2026-89950?

Apply the vendor's patch.

## Is CVE-2026-89950 being exploited?

There is no known exploitation of CVE-2026-89950 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89950 and fix it if it is affected.

What it is: CVE-2026-89950 is a vulnerability in Linux, published on 16 Sep 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-2026-89950 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-2026-89950/ (Markdown: https://insights.sbomify.com/CVE-2026-89950/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89950
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89950
- OSV: https://osv.dev/vulnerability/CVE-2026-89950

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89950/
Summaries are generated and may contain mistakes; the official description above is the record.