# CVE-2026-80847: tcp: clamp route advmss to TCP_MIN_MSS

> A remote attacker can cause a denial of service in the Linux kernel by triggering a divide-by-zero error via crafted TCP route metrics.

- **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:** 21 Oct 2026
- **Also known as:** AZL-99588, BELL-CVE-2026-80847, DEBIAN-CVE-2026-80847, GHSA-m3f3-9j4q-c4f2, UBUNTU-CVE-2026-80847

## What is CVE-2026-80847?

CVE-2026-80847 is a denial of service 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:

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.

## Which versions are affected by CVE-2026-80847?


## How do I fix CVE-2026-80847?

Apply the vendor's patch.

## Is CVE-2026-80847 being exploited?

There is no known exploitation of CVE-2026-80847 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80847 and fix it if it is affected.

What it is: CVE-2026-80847 is a denial of service 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-2026-80847 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-2026-80847/ (Markdown: https://insights.sbomify.com/CVE-2026-80847/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80847
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80847
- OSV: https://osv.dev/vulnerability/CVE-2026-80847

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80847/
Summaries are generated and may contain mistakes; the official description above is the record.