CVE-2026-89554

/// Technical 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.

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References