# CVE-2026-89554: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction

> An off-path attacker can influence uninitialized memory in Linux kernel MPTCP SYN cookie handling, potentially leaking stale data from freed sockets.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100725, BELL-CVE-2026-89554, DEBIAN-CVE-2026-89554, GHSA-9vj3-qjq5-h6jg, UBUNTU-CVE-2026-89554

## What is CVE-2026-89554?

CVE-2026-89554 is a high-severity vulnerability in Linux, published on 11 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:

mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction

mptcp_token_join_cookie_init_state() restores remote_nonce, local_nonce,
backup, join_id, token and msk from the saved cookie entry when rebuilding
the request socket for a MP_JOIN 4th-ACK handled under SYN cookies, but it
does not restore local_id, even though the SYN path saved it.
subflow_ulp_clone() then reads that uninitialized field and stores it as
the joined subflow's address-ID. Because the request-sock slab is
SLAB_TYPESAFE_BY_RCU and not zeroed on allocation, the value is the stale
byte of a previously freed request socket, which an off-path peer can
influence by sending concurrent MP_JOIN SYNs. This corrupts the path
manager's id-based subflow bookkeeping for the connection.

Restore subflow_req->local_id from the cookie entry, as done for the other
fields.

## Which versions are affected by CVE-2026-89554?


## How do I fix CVE-2026-89554?

Apply the vendor's patch.

## Is CVE-2026-89554 being exploited?

There is no known exploitation of CVE-2026-89554 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89554 and fix it if it is affected.

What it is: CVE-2026-89554 is a high-severity vulnerability in Linux, published on 11 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-2026-89554 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-2026-89554/ (Markdown: https://insights.sbomify.com/CVE-2026-89554/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89554
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89554
- OSV: https://osv.dev/vulnerability/CVE-2026-89554

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89554/
Summaries are generated and may contain mistakes; the official description above is the record.