# CVE-2026-98438: mptcp: options: fix uninit-value in mptcp_write_data_fin

> An attacker can read uninitialized kernel memory via crafted MPTCP packets, potentially exposing sensitive data.

- **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:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-98438, GHSA-232g-622w-prrg, UBUNTU-CVE-2026-98438

## What is CVE-2026-98438?

CVE-2026-98438 is a vulnerability in Linux, published on 18 Oct 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:

mptcp: options: fix uninit-value in mptcp_write_data_fin

When sending a DATA_FIN without data, and because the DATA_FIN occupies
1 octet of the connection-level sequence space [1], it is then required
to add a DSS mapping with specific values.

If the checksum has been negotiated, it also needs to be computed, and
included in the outgoing packet, and thus the initial csum data needs to
be reset to 0 as well. This is no longer the case since commit
cfcceb7a39fc ("tcp: shrink per-packet memset in __tcp_transmit_skb()"),
because the whole ext_copy structure is no longer zeroed by default.

This seems to be the only case where use_map is changed and set
afterwards, so initialising the csum field only in this case, along with
other fields for this specific case.

## Which versions are affected by CVE-2026-98438?


## How do I fix CVE-2026-98438?

Apply the vendor's patch.

## Is CVE-2026-98438 being exploited?

There is no known exploitation of CVE-2026-98438 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98438 and fix it if it is affected.

What it is: CVE-2026-98438 is a vulnerability in Linux, published on 18 Oct 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-2026-98438 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-2026-98438/ (Markdown: https://insights.sbomify.com/CVE-2026-98438/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98438
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98438
- OSV: https://osv.dev/vulnerability/CVE-2026-98438

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98438/
Summaries are generated and may contain mistakes; the official description above is the record.