# CVE-2026-80793: ipv4: reject undersized MTUs in ip_do_fragment()

> A remote attacker can cause a kernel softlockup and denial of service by sending packets that trigger infinite fragmentation loops due to undersized MTUs.

- **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:** 19 Oct 2026
- **Also known as:** AZL-99552, BELL-CVE-2026-80793, DEBIAN-CVE-2026-80793, GHSA-vc8f-29mh-24xv, UBUNTU-CVE-2026-80793

## What is CVE-2026-80793?

CVE-2026-80793 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:

ipv4: reject undersized MTUs in ip_do_fragment()

ip_do_fragment() subtracts the IPv4 header length from the effective
MTU and passes the resulting payload MTU to ip_frag_next().

If the effective MTU is smaller than hlen + 8, ip_frag_next() rounds
the fragment payload length down to zero. The fragmentation state then
never makes forward progress: state->left, state->ptr and state->offset
stay unchanged while ip_do_fragment() keeps allocating and transmitting
header-only fragments until the softlockup detector fires.

This is reproducible with a route installed using "mtu lock 20", but it
is also reproducible without route MTU lock, for example by forwarding a
packet to a device whose MTU is 20.

Fix it in ip_do_fragment() by rejecting mtu < hlen + 8 with -EMSGSIZE,
matching the existing IPv6 fragmentation check.

## Which versions are affected by CVE-2026-80793?


## How do I fix CVE-2026-80793?

Apply the vendor's patch.

## Is CVE-2026-80793 being exploited?

There is no known exploitation of CVE-2026-80793 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80793 and fix it if it is affected.

What it is: CVE-2026-80793 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-2026-80793 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-2026-80793/ (Markdown: https://insights.sbomify.com/CVE-2026-80793/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80793
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80793
- OSV: https://osv.dev/vulnerability/CVE-2026-80793

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80793/
Summaries are generated and may contain mistakes; the official description above is the record.