# CVE-2026-98393: net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

> A local attacker can trigger a UDP length integer overflow in the Linux kernel by sending large packets with specific MTU settings, causing a warning and potential denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 14 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-108105, BELL-CVE-2026-98393, DEBIAN-CVE-2026-98393, GHSA-m5wc-gf88-8h7m, UBUNTU-CVE-2026-98393

## What is CVE-2026-98393?

CVE-2026-98393 is a vulnerability in Linux, published on 14 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a
possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to
IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an
unusually large MTU.

Steps to reproduce:

1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will
   be set to that MTU in ip_setup_cork.
2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be
   set to device MTU (cork->fragsize) in __ip_append_data, rather than
   to IP_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of
   IPv4 header). Device MTU allows it (it's only one byte bigger than
   IP_MAX_MTU + IPv4 header, and the device MTU is bigger than that).
4. The UDP length in the built packet is 65536, which overflows the
   16-bit length field and triggers the WARN in udp_set_len_short.

Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward
always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward.

The Fixes tag points at the first commit where I could reproduce the
overflow with IPv4 and IP_PMTUDISC_PROBE.

## Which versions are affected by CVE-2026-98393?


## How do I fix CVE-2026-98393?

Apply the vendor's patch.

## Is CVE-2026-98393 being exploited?

There is no known exploitation of CVE-2026-98393 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98393 and fix it if it is affected.

What it is: CVE-2026-98393 is a vulnerability in Linux, published on 14 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-98393 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-2026-98393/ (Markdown: https://insights.sbomify.com/CVE-2026-98393/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98393
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98393
- OSV: https://osv.dev/vulnerability/CVE-2026-98393

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98393/
Summaries are generated and may contain mistakes; the official description above is the record.