[CVE-2026-89554][MODERATE REGULAR] mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-89554
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction
Commit: 64f2c5dd49b956a542c8c02b8dad5d262a462bba
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=64f2c5dd49b956a542c8c02b8dad5d262a462bba
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89554
Analysis date: Sat, 12 Sep 2026 06:47:40 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
An uninitialized `local_id` in MPTCP syncookie MP_JOIN reconstruction can let a remote peer influence stale request-sock data used as a subflow address ID, corrupting MPTCP path-manager bookkeeping without a demonstrated kernel memory-corruption or privilege-escalation primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89554 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89554

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89554	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89554 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';*CWE-457;CWE-665;CWE-908;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.6';DESCR 'An uninitialized local_id in the MPTCP syncookie MP_JOIN reconstruction path can cause the joined subflow to inherit a stale address ID from a previously freed request socket. This happens because mptcp_token_join_cookie_init_state restores other MP_JOIN fields from the saved cookie entry but does not restore local_id before subflow_ulp_clone uses it for path manager bookkeeping. A remote peer can influence the stale byte by sending concurrent MP_JOIN SYN packets when SYN cookies are in use, which can corrupt id based subflow tracking for the affected connection. For the CVSS the PR:N is used because no local account is required when the MPTCP listener is reachable over the network. The attack complexity is high because it depends on the SYN cookie reconstruction path and request socket reuse timing. Impact is primarily limited integrity and availability degradation for MPTCP subflow management, with a conservative low confidentiality component in the paranoid score due to stale slab data use.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE DANGER UAF NETWORK LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied.

## 2. Signal breakdown

Positive signals:

* +2 Remote reachable / directly network-triggerable: attacker-controlled MPTCP MP_JOIN traffic can reach the vulnerable syncookie reconstruction path when the MPTCP listener is exposed and SYN cookies are in use.
* +1 Weak / indirect corruption candidate: `local_id` is left uninitialized and later consumed as the joined subflow address ID, corrupting path-manager ID-based bookkeeping for the connection.

Negative signals:

* No privilege penalty: the supplied evidence supports network triggering without a local account on the target.
* No race/timing penalty: although the issue depends on SYN cookie reconstruction and request-sock reuse, the input states an off-path peer can influence the stale byte with concurrent MP_JOIN SYNs, and repeated network attempts appear plausible.

Important non-awarded signals:

* RealUAF not awarded: this is use of an uninitialized field containing stale slab data, not a stale reference to a freed object being dereferenced after lifetime end.
* Strong corruption not awarded: no controlled reclaim, arbitrary write, OOB write, callback control, refcount takeover, or controlled object replacement is established.
* Confidentiality not awarded: stale byte use is described as influencing `local_id`, but no disclosure path to the attacker is established.
* Integrity impact is not scored as a strong security-boundary primitive: the demonstrated corruption is connection/path-manager bookkeeping, not protected kernel state modification with broader security effect.
* Reliable kernel crash / strong DoS not awarded: the supplied consequence is corrupted MPTCP subflow bookkeeping, not a demonstrated kernel panic or host-wide fatal fault.
* Extra Availability not awarded: connection-level disruption is plausible, but no distinct host-wide or persistent availability consequence is established.
* ImportantCandidate not set: the primitive is not a real UAF or strong unresolved kernel corruption primitive.

## 3. Reachability analysis

The trigger is remote network traffic against an MPTCP-capable listener. The vulnerable path is specific to MP_JOIN request reconstruction under SYN cookies. The attacker does not need a local account or host-admin-equivalent privilege on the target, so `HostAdminRequired=NO`.

The complete trigger depends on the target having MPTCP enabled and reachable, and on the SYN cookie path being used. This is a deployment and runtime condition, but not a privilege requirement for the attacker once the service is exposed. The peer can influence the stale byte by sending concurrent MP_JOIN SYNs, so the condition is not treated as a one-shot externally uncontrollable race.

Namespace-scoped privilege is not the relevant access model here. The issue is reached through network traffic, not through a local namespace capability chain.

## 4. Primitive and severity interpretation

The exact primitive is an uninitialized read of `subflow_req->local_id` during syncookie MP_JOIN reconstruction. The field is then consumed by `subflow_ulp_clone()` and stored as the joined subflow address ID. Because the request-sock slab is `SLAB_TYPESAFE_BY_RCU` and not zeroed on allocation, the byte may contain stale data from a previously freed request socket.

This is not classified as a real UAF because the vulnerable path does not use an old dangling reference to the freed request socket. It uses a newly allocated request object with an uninitialized field that contains stale storage contents.

The demonstrated consequence is corruption of MPTCP path-manager ID-based subflow bookkeeping for the affected connection. The practical impact is likely connection-level integrity or availability degradation rather than host compromise. A stronger consequence such as privilege escalation, arbitrary kernel read/write, or host-wide crash is not supported by the supplied patch and context.

Primitive class is `CLASS_C`: invalid state / stale uninitialized value consumption with protocol bookkeeping corruption, but without an independently useful memory-corruption or lifetime primitive.

`ImportantCandidate=NO` because the bug does not establish Class A behavior and does not establish a Class B UAF/lifetime primitive requiring Important-vs-Moderate exploitation review.

`AutoDowngradeAllowed=YES` because no real UAF, repeated lifetime mutation, authorization bypass, shared-page ownership violation, controlled corruption, or demonstrated privilege escalation remains unresolved.

## 5. One-sentence report phrase

An uninitialized `local_id` in MPTCP syncookie MP_JOIN reconstruction can let a remote peer influence stale request-sock data used as a subflow address ID, corrupting MPTCP path-manager bookkeeping without a demonstrated kernel memory-corruption or privilege-escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network reachable and attacker-influenced, but the established primitive is an uninitialized stale byte used for MPTCP bookkeeping rather than a real UAF, arbitrary write, authorization bypass, or demonstrated host-wide crash.

## 7. Call-site confidence

Call-site confidence: medium

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction
Commit: 64f2c5dd49b956a542c8c02b8dad5d262a462bba
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=64f2c5dd49b956a542c8c02b8dad5d262a462bba

Commit description:

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.

Fixes: 9466a1c ("mptcp: enable JOIN requests even if cookies are in use")
Cc: stable@vger.kernel.org
Signed-off-by: Harshit Varu <harshitvaru666@gmail.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260815115205.197151-1-harshitvaru666@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/mptcp/syncookies.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=64f2c5dd49b956a542c8c02b8dad5d262a462bba

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89554 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89554

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.6

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: unknown
  Paranoid ActionableScore: 3

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).