---
id: CVE-2026-89554
title: >-
  kernel: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction
  (CVE-2026-89554)
summary: >-
  A flaw was found in the Linux kernel's Multipath TCP (MPTCP) implementation.
  When reconstructing a Multipath TCP (MPTCP) join request under SYN cookies,
  the `local_id` field is not properly initialized. An off-path attacker can
  influence t…
severity: medium
cvss: 5.5
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'
cvssSource: vendor
cwe: CWE-824
vendor: Red Hat
product: Red Hat Enterprise Linux 9
affected:
  - enterprise_linux 10
  - enterprise_linux 8
  - enterprise_linux 9
  - openshift_container_platform 4
published: '2026-09-11'
updated: '2026-09-15'
sourceUpdated: '2026-09-15T06:19:13+00:00'
source: CSAF
sourceUrl: 'https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89554.json'
references:
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89554.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-89554'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2532504'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-89554'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-89554'
  - url: >-
      https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89554.mbox
  - url: 'https://git.kernel.org/stable/c/4534e0eead692bb069ad1e6a5a245fc2e2078300'
  - url: 'https://git.kernel.org/stable/c/63cacb05e51a1c6e1349d2e593d00fd6ef43def4'
  - url: 'https://git.kernel.org/stable/c/954b5ea836eb118d188975ced803448537ab8479'
  - url: 'https://git.kernel.org/stable/c/9df36a4846375a9b75bf42d77bfed216b0f366f9'
  - url: 'https://git.kernel.org/stable/c/64f2c5dd49b956a542c8c02b8dad5d262a462bba'
  - url: 'https://git.kernel.org/stable/c/51887ccd88791ddaa8755a7c614fda031ecf7982'
  - url: 'https://git.kernel.org/stable/c/bf19d166337c6488b39cb03eeeffb30a941d6326'
  - url: 'https://git.kernel.org/stable/c/b878dfdd12d7a5b8722a78d35e313506140ca3d9'
tags:
  - csaf
  - vex
  - red-hat
  - cve.org
  - score-dispute
epss: 0.00606
epssPercentile: 0.47387
scores:
  vendor: 5.5
  cna: 8.2
ingestedAt: '2026-09-14T15:23:07.451Z'
---

## Overview

A flaw was found in the Linux kernel's Multipath TCP (MPTCP) implementation. When reconstructing a Multipath TCP (MPTCP) join request under SYN cookies, the `local_id` field is not properly initialized. An off-path attacker can influence this uninitialized field by sending concurrent MP_JOIN SYN requests, leading to corruption of the path manager's subflow bookkeeping. This can result in a denial of service for affected MPTCP connections.

## Vendor advisories

- **Red Hat VEX** · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · updated 2026-09-15 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89554.json)

**kernel: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction** — rated Moderate by Red Hat. Released 2026-09-11, updated 2026-09-15.

Affected:

- Red Hat Enterprise Linux 10
- Red Hat Enterprise Linux 8
- Red Hat Enterprise Linux 9
- Red Hat OpenShift Container Platform 4

No fix planned:

- Red Hat Enterprise Linux 10
- Red Hat Enterprise Linux 8
- Red Hat Enterprise Linux 9
- Red Hat OpenShift Container Platform 4

Not affected:

- Red Hat Enterprise Linux 6
- Red Hat Enterprise Linux 7

## Remediation

Fix deferred