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.