[CVE-2026-97568][MODERATE REGULAR] mptcp: syncookies: remember the request backup flag

[CVE-2026-97568][MODERATE REGULAR] mptcp: syncookies: remember the request backup flag

[AL-KERNEL]

CVE: CVE-2026-97568
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mptcp: syncookies: remember the request backup flag
Commit: 61178d0b85b4b1b53e78b06e89d018d22d102f88
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=61178d0b85b4b1b53e78b06e89d018d22d102f88
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97568
Analysis date: Fri, 25 Sep 2026 08:28:49 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-97568 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97568

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: NO

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
======================================================================

CVE-2026-97568	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97568 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';*CWE-457;CWE-665;CWE-682;BEST CVSS score: '3.7';DESCR 'An MPTCP syncookie state restoration bug can use an uninitialized request backup flag because join_entry stores backup but does not preserve request_bkup. A remote peer may trigger the affected MP_JOIN handling path when MPTCP syncookies are in use, which can incorrectly mark a subflow as backup and degrade path management or availability for an affected MPTCP connection. For the CVSS the PR:N value is used because a remote network peer does not require local access to the target system. Triggering requires specific MPTCP syncookie and MP_JOIN conditions, so AC:H is appropriate. No memory corruption, information disclosure, integrity impact, or privilege escalation primitive is indicated by the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	REMOTE DANGER SIMPLEFIX NETWORK LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied.

2. Signal breakdown

* +2 Remote reachable protocol path. A remote MPTCP peer can reach MP_JOIN processing without local execution on the target, provided the target uses MPTCP and the syncookie path is activated.
* +0 No reliable kernel crash or host wide denial of service is established. The patch supports incorrect restoration of a one bit subflow policy field.
* +0 No memory corruption primitive is established. The uninitialized value is a logical state bit and is copied into request_bkup.
* +0 No UAF, stale lifetime reference, refcount error, double release, OOB access, arbitrary write, or controlled reclaim is established.
* +0 No confidentiality impact is established. The patch does not expose a path that returns uninitialized kernel data to userspace or the network.
* +0 No integrity impact is established. Incorrect backup policy selection does not establish unauthorized modification of kernel state or another principals data.
* +0 No privilege escalation or authorization bypass is established.
* +0 No strong availability point is awarded. The likely result is degraded path selection or availability for an affected MPTCP connection, not a demonstrated system wide DoS.
* +0 No hard race penalty is applied. This is state preservation failure during syncookie state reconstruction, not a timing race.
* +0 No niche subsystem penalty is applied. MPTCP configuration is a prerequisite, but the supplied evidence does not establish an attacker inaccessible or unusually rare execution condition.

3. Reachability analysis

The evaluated attacker is a remote MPTCP peer. The affected path requires an MPTCP capable target and activation of the MPTCP syncookie handling path. The remote peer does not need host administrator privilege to send network traffic that reaches the listener.

HostAdminRequired is NO because the complete attacker action is sending protocol traffic to an already exposed MPTCP service. However, the target must already be configured to use MPTCP and must enter the relevant syncookie flow. Those are deployment prerequisites, not privileges required by the remote attacker.

NamespaceScopedPrivilege is UNKNOWN. No namespace capability check or namespace scoped control plane operation is relevant to the supplied trigger path. The patch does not establish whether an unprivileged local namespace user can create a complete equivalent test environment.

No special hardware, boot parameter, BPF setup, device setup, or host managed object lifecycle is indicated. No attacker controlled race, object reuse, memory placement, or repeated lifetime transition is present.

4. Primitive and severity interpretation

The exact defect is an uninitialized logical state bit. join_entry preserved backup but did not preserve request_bkup before restoring state into mptcp_subflow_request_sock. The restored request_bkup field could therefore contain an unintended value.

This is not an active structured object lifetime violation and not fixed stale storage. There is no object free, stale dereference, second release, refcount mutation, OOB operation, or ownership transfer failure. RealUAF is NO.

The demonstrated consequence is incorrect MPTCP subflow backup state. This can affect subflow role selection and may degrade connectivity or failover behavior for the affected MPTCP connection. A host wide kernel crash, information disclosure, integrity violation, privilege escalation, or cross boundary effect is not supported by the patch.

PrimitiveClass is NONE. ImportantCandidate is NO because the patch establishes a protocol state logic defect only and does not leave unresolved memory corruption, authorization, ownership, or lifetime exploitation leverage. AutoDowngradeAllowed is YES because no concrete Important preserving primitive would be discarded by ordinary low priority handling.

Likely operational severity is low. Systems that use MPTCP backup subflows together with syncookies may want to patch during normal maintenance, especially where path failover correctness is operationally important.

5. One-sentence report phrase

An MPTCP syncookie state restoration bug can use an uninitialized request backup flag and may incorrectly classify an MPTCP subflow as a backup path, causing connection specific path management or availability degradation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is network reachable in deployments that expose MPTCP services and use the affected syncookie path, but the patch supports only a low impact connection state logic failure and does not establish memory corruption or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: mptcp: syncookies: remember the request backup flag
Commit: 61178d0b85b4b1b53e78b06e89d018d22d102f88
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=61178d0b85b4b1b53e78b06e89d018d22d102f88

Commit description:

Instead of using an uninitialised bit when copying the info in
subflow_ulp_clone().

To fix this, no need to extend the join_entry structure: backup is
coming from struct mptcp_subflow_request_sock, only one bit. Do the same
here by using one bit for both.

Fixes: efd340b ("mptcp: distinguish rcv vs sent backup flag in requests")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260908-net-mptcp-misc-fixes-7-3-rc1-v2-3-df1de70348b6@kernel.org
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=61178d0b85b4b1b53e78b06e89d018d22d102f88

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

The original Linux kernel CVE announcement for CVE-2026-97568 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97568

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 3.7

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: 2

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).