# CVE-2026-63836: batman-adv: tp_meter: avoid divide-by-zero for dec_cwnd

> An integer overflow in the batman-adv tp_meter module causes a divide-by-zero error, leading to a kernel crash and 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:** 19 Jul 2026
- **Updated:** 17 Aug 2026
- **Also known as:** BELL-CVE-2026-63836, DEBIAN-CVE-2026-63836, GHSA-f3pj-q27g-923q, ROOT-OS-DEBIAN-11-CVE-2026-63836, ROOT-OS-UBUNTU-2204-CVE-2026-63836, UBUNTU-CVE-2026-63836

## What is CVE-2026-63836?

CVE-2026-63836 is a vulnerability in Linux, published on 19 Jul 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: tp_meter: avoid divide-by-zero for dec_cwnd

The cwnd is always MSS <= cwnd <= 0x20000000. But the calculation in
batadv_tp_update_cwnd() assumes unsigned 32 bit arithmetics.

    ((mss * 8) ** 2) / (cwnd * 8)

In case cwnd is actually 0x20000000, it will be shifted by 3 bit to the
left end up at 0x100000000 or U32_MAX + 1. It will therefore wrap around
and be 0 - resulting in:

    ((mss * 8) ** 2) / 0

This is of course invalid and cannot be calculated. The calculation should
must be simplified to avoid this overflow:

   (mss ** 2) * 8 / cwnd

It will keep the precision enhancement from the scaling (by 8) but avoid
the overflow in the divisor.

In theory, there could still be an overflow in the dividend. It is at the
moment fixed to BATADV_TP_PLEN in batadv_tp_recv_ack() - so it is not an
imminent problem. But allowing it to use the whole u32 bit range, would
mean that it can still use up to 67 bits. To keep this calculation safe for
32 bit arithmetic, mss must never use more than floor((32 - 3) / 2) bits -
or in other words: must never be larger than 16383.

## Which versions are affected by CVE-2026-63836?


## How do I fix CVE-2026-63836?

Apply the vendor's patch.

## Is CVE-2026-63836 being exploited?

There is no known exploitation of CVE-2026-63836 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63836 and fix it if it is affected.

What it is: CVE-2026-63836 is a vulnerability in Linux, published on 19 Jul 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-2026-63836 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-2026-63836/ (Markdown: https://insights.sbomify.com/CVE-2026-63836/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63836
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63836
- OSV: https://osv.dev/vulnerability/CVE-2026-63836

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63836/
Summaries are generated and may contain mistakes; the official description above is the record.