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

Q: Is CVE-2026-63836 being exploited?

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

Not scored Published 19 Jul 2026 Updated 17 Aug 2026

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.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

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?