CVE-2026-89554: Linux

High severity, CVSS 8.2. EPSS: 0.6% chance of exploitation in the next 30 days.

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 products

• Linux Linux: from 5.9, before 5.10.270 (fixed in 5.10.270); from 5.11, before 5.15.221 (fixed in 5.15.221); from 5.16, before 6.1.188 (fixed in 6.1.188); from 6.2, before 6.6.157 (fixed in 6.6.157); from 6.7, before 6.12.109 (fixed in 6.12.109); from 6.13, before 6.18.50 (fixed in 6.18.50); …

Published 2026-09-11. Last modified 2026-09-14.