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CVE-2026-89554
mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem net/mptcp. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-89554 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/mptcp/syncookies.c
- What this is
- NETWORKING [MPTCP]MAINTAINERS section matched on net/mptcp/ · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -wE 'CONFIG_MPTCP|CONFIG_SYN_COOKIES' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sid, bookworm · linux-6.12y
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switches
- CONFIG_MPTCP, CONFIG_SYN_COOKIES
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-09-11 (37 days ago)source
- CVSS
- 8.2 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.60%, higher than 47.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/mptcp
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.9 | 5.10.270 | 4534e0eead69 |
| 5.9 | 5.15.221 | 63cacb05e51a |
| 5.9 | 6.1.188 | 954b5ea836eb |
| 5.9 | 6.6.157 | 9df36a484637 |
| 5.9 | 6.12.109 | 64f2c5dd49b9 |
| 5.9 | 6.18.50 | 51887ccd8879 |
| 5.9 | 7.2.4 | bf19d166337c |
| 5.9 | 7.3 | b878dfdd12d7 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- The flaw is on the MPTCP MP_JOIN SYN-cookie receive path (tcp_v4_rcv/cookie_v4_check/mptcp_subflow_init_cookie_req/mptcp_token_join_cookie_init_state then subflow_ulp_clone). Any remote peer that can reach an MPTCP listener over routed IP triggers it with MP_JOIN packets when SYN cookies are in use.
- AC:LAttack complexity, low
- An attacker can fill the SYN queue to force cookies (tcp_syncookies defaults to 1), spray concurrent MP_JOIN SYNs to seed the SLAB_TYPESAFE_BY_RCU request-sock cache with chosen local_id values, then complete a cookie MP_JOIN ACK. The attacker drives both the flood and the handshake; success does not depend on uninfluencable victim state.
- PR:NPrivileges required, none
- No local account or capability is required. Any unauthenticated remote client obtains a token by completing MP_CAPABLE to a public MPTCP listener, then drives cookie JOIN; HMAC-valid JOIN from that peer, or slab poisoning of another connection's JOIN, needs no victim credentials.
- UI:NUser interaction, none
- Cookie MP_JOIN reconstruction runs in TCP receive softirq on attacker-sent segments; no local user mount, click, or other action is required beyond ordinary network reachability of an MPTCP service.
- S:UScope, unchanged
- Impact is confined to the host kernel's MPTCP path-manager and connection state and does not cross a VM, IOMMU, container, or other security authority.
- C:NConfidentiality impact, none
- The uninitialized field is a single u8 address-ID leftover from a same-type request-sock slab object. It is not an out-of-bounds or pointer leak and does not disclose kernel memory contents to the attacker.
- I:LIntegrity impact, low
- subflow_ulp_clone stores the stale local_id as the joined subflow's address-ID, corrupting path-manager id-based bookkeeping (endpoint availability, ADD_ADDR/REMOVE_ADDR matching, and ID-0 primary-path identity). The modification is limited to that protocol ID and is not an arbitrary kernel write or control-flow primitive.
- A:HAvailability impact, high
- A poisoned local_id of 0 or a colliding endpoint ID makes later REMOVE_ADDR/RMSUBFLOW close the initial or other still-needed subflows via mptcp_close_ssk, and desynchronized PM state can reject further joins or tear down the MPTCP session. An attacker can repeat this against many connections on an internet-facing MPTCP service.