CVE-2026-89554 mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction
An off-path attacker can influence uninitialized memory in Linux kernel MPTCP SYN cookie handling, potentially leaking stale data from freed sockets.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89554?
CVE-2026-89554 is a high-severity vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-89554?
How do I fix CVE-2026-89554?
Apply the vendor's patch.
Is CVE-2026-89554 being exploited?
There is no known exploitation of CVE-2026-89554 in the wild and no public exploit code.
Sources
Also known as AZL-100725, BELL-CVE-2026-89554, DEBIAN-CVE-2026-89554, GHSA-9vj3-qjq5-h6jg, UBUNTU-CVE-2026-89554.