---
id: CVE-2026-89554
title: 'mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction'
summary: |-
  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 a…
severity: high
cvss: 8.2
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H'
cvssSource: cna
cwe:
  - CWE-824
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    4534e0eead692bb069ad1e6a5a245fc2e2078300
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    63cacb05e51a1c6e1349d2e593d00fd6ef43def4
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    954b5ea836eb118d188975ced803448537ab8479
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    9df36a4846375a9b75bf42d77bfed216b0f366f9
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    64f2c5dd49b956a542c8c02b8dad5d262a462bba
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    51887ccd88791ddaa8755a7c614fda031ecf7982
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    bf19d166337c6488b39cb03eeeffb30a941d6326
  - >-
    Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 <
    b878dfdd12d7a5b8722a78d35e313506140ca3d9
  - Linux 5.9
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-16T19:23:57.686804Z'
published: '2026-09-11'
updated: '2026-10-19'
sourceUpdated: '2026-10-19T08:48:47.068Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-89554'
references:
  - 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'
  - 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
tags:
  - cve.org
  - csaf
  - vex
  - red-hat
  - score-dispute
epss: 0.00598
epssPercentile: 0.48432
scores:
  cna: 8.2
  vendor: 5.5
ingestedAt: '2026-09-14T15:23:07.451Z'
---

## Overview

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.

## Affected

- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < 4534e0eead692bb069ad1e6a5a245fc2e2078300`
- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < 63cacb05e51a1c6e1349d2e593d00fd6ef43def4`
- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < 954b5ea836eb118d188975ced803448537ab8479`
- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < 9df36a4846375a9b75bf42d77bfed216b0f366f9`
- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < 64f2c5dd49b956a542c8c02b8dad5d262a462bba`
- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < 51887ccd88791ddaa8755a7c614fda031ecf7982`
- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < bf19d166337c6488b39cb03eeeffb30a941d6326`
- `Linux >= 9466a1ccebbe54ac57fb8a89c2b4b854826546a8 < b878dfdd12d7a5b8722a78d35e313506140ca3d9`
- `Linux 5.9`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## 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)