CVE-2026-89554 (CVSS 8.2)

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.

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CVSS v3.1 Score

8.2

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

EPSS — Exploit Prediction

0.0061

Probability of exploitation

0.47%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

References

Other References

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

Frequently Asked Questions

What is CVE-2026-89554? +

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. It has a CVSS v3.1 base score of 8.2 (HIGH).

How severe is CVE-2026-89554? +

CVE-2026-89554 has a CVSS v3.1 score of 8.2 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0061, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-89554? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

CVE-2026-89554