HIGH Introduced in 5.9
mptcp JoinCookie Corruption
CVE-2026-89554
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
KernelScan AI4.8MEDIUM
01Description
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.
02KernelScan AI Analysis
Risk summary
A remote network attacker can exploit a missing field restoration in MPTCP SYN cookie MP_JOIN handling to corrupt the path manager's subflow bookkeeping. The server must be in SYN cookie mode, which occurs under SYN flood conditions. The impact is limited to integrity corruption of internal connection state and potential disruption of the affected MPTCP connection.
Vulnerability analysis
When an MPTCP server falls back to SYN cookie mode (typically under SYN flood conditions) and processes an MP_JOIN request, the code that rebuilds the request socket from saved cookie state fails to restore the local address identifier field. Because the request socket memory is allocated from a slab that does not zero memory on reuse, the field retains a stale value from a previously freed request socket. An off-path network peer can influence this stale value by sending concurrent MP_JOIN SYNs, causing the joined subflow to be registered with an incorrect address ID and corrupting the MPTCP path manager's subflow bookkeeping for that connection. The fix restores the local address identifier from the saved cookie entry, matching how all other fields are already handled. The vulnerability is reachable by any network peer who can send MPTCP traffic to the listening server; no local access or special privileges are required, but the server must be in SYN cookie mode for the bug to trigger.
Lifecycle
03Fix Versions
| Branch | Introduced | Fixed in | Patch commit |
|---|---|---|---|
| 5.10 | 5.9 | 5.10.270 | 4534e0eead69 |
| 5.15 | 5.9 | 5.15.221 | 63cacb05e51a |
| 6.1 | 5.9 | 6.1.188 | 954b5ea836eb |
| 6.6 | 5.9 | 6.6.157 | 9df36a484637 |
| 6.12 | 5.9 | 6.12.109 | 64f2c5dd49b9 |
| 6.18 | 5.9 | 6.18.50 | 51887ccd8879 |
| 7.2 | 5.9 | 7.2.4 | bf19d166337c |
| mainline | 5.9 | 7.3-rc1 | b878dfdd12d7 |
| cip/6.12 | 5.9 | 6.12.111-cip32 | 64f2c5dd49b9 |