CVE-2026-89554
HighCVSS 8.2Exploitation Probability (EPSS)
Low risk45th percentile - higher than 45% of all known CVEs
Summary
In the Linux kernel, mptcp_token_join_cookie_init_state() fails to restore the local_id field from the saved cookie entry when rebuilding the request socket for an MP_JOIN handled under SYN cookies. The uninitialized field is then read by subflow_ulp_clone() and stored as the joined subflow's address-ID, which an off-path peer can influence by sending concurrent MP_JOIN SYNs.
Risk Assessment
This uninitialized memory/use-after-free vulnerability can corrupt the path manager's id-based subflow bookkeeping for MPTCP connections, potentially causing connection errors or further security impacts. A remote attacker can influence the value by sending concurrent MP_JOIN SYN packets.
Recommendation
Update the Linux kernel to a version containing the fix that restores subflow_req->local_id from the cookie entry. Verify that systems using MPTCP have the latest kernel installed.
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Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

