# CVE-2026-80839: batman-adv: reject unrepresentable multicast TVLV offsets

> An attacker can trigger memory corruption in the Linux kernel batman-adv module by sending crafted multicast TVLV packets, potentially leading to code execution or system crash.

- **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:** 4 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-80839, DEBIAN-CVE-2026-80839, GHSA-676r-cqgf-36vf, UBUNTU-CVE-2026-80839

## What is CVE-2026-80839?

CVE-2026-80839 is a memory corruption vulnerability in Linux, published on 4 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: reject unrepresentable multicast TVLV offsets

The network and transport header fields in struct sk_buff are 16-bit
offsets from skb->head, and U16_MAX is reserved as the unset transport
header value. batadv_tvlv_call_handler() sets both fields from a received
multicast TVLV without checking whether the TVLV end is representable.

If the end offset exceeds the field's range, skb_set_transport_header()
truncates it so that the transport header precedes the network header.
The negative difference is then returned by skb_network_header_len() as
a large u32. batadv_mcast_forw_packet() consequently accepts an oversized
multicast tracker and accesses memory beyond the skb data.

Add skb_set_transport_header_careful(), an offset-aware counterpart to
skb_reset_transport_header_careful(), which validates the final
head-relative offset before assigning it. Use the new helper in
batadv_tvlv_call_handler() and reject unrepresentable TVLVs before
setting the network header.

## Which versions are affected by CVE-2026-80839?


## How do I fix CVE-2026-80839?

Apply the vendor's patch.

## Is CVE-2026-80839 being exploited?

There is no known exploitation of CVE-2026-80839 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80839 and fix it if it is affected.

What it is: CVE-2026-80839 is a memory corruption vulnerability in Linux, published on 4 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-2026-80839 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-2026-80839/ (Markdown: https://insights.sbomify.com/CVE-2026-80839/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80839
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80839
- OSV: https://osv.dev/vulnerability/CVE-2026-80839

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80839/
Summaries are generated and may contain mistakes; the official description above is the record.