[CVE-2026-80587][MODERATE REGULAR] mptcp: avoid combining some incoming suboptions

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

CVE: CVE-2026-80587
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mptcp: avoid combining some incoming suboptions
Commit: dc1d8d3eb345c616fbe922a010fa391c72c54d52
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dc1d8d3eb345c616fbe922a010fa391c72c54d52
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80587
Analysis date: Wed, 26 Aug 2026 11:55:29 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline network-reachable Moderate, manual review recommended, not an Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-80587 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80587

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-80587	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';*CWE-20;CWE-345;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'MPTCP incoming option parsing could accept mutually exclusive or conflicting suboptions in the same TCP option block, while later MPTCP code expects only a limited set of combinations. A malicious remote MPTCP peer can craft packets with inconsistent suboption combinations and cause incorrect protocol state handling or connection level denial of service. For the CVSS the PR:N is used because the peer only needs network reachability to an MPTCP endpoint and does not need a local account on the target. The issue is network reachable when MPTCP is enabled and exposed to untrusted peers, but the known impact is limited to protocol state confusion and availability or limited integrity of the affected MPTCP connection. No direct UAF, OOB write, arbitrary write, or privilege escalation primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline network-reachable Moderate, manual review recommended, not an Important candidate (with actual score 4)	YES	REMOTE NETWORK LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

1. ActionableScore
* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline network-reachable Moderate, manual review recommended, not an Important candidate**::

2. Signal breakdown
* Remote reachable / network-triggerable: +2. A malicious MPTCP peer can send crafted TCP options containing conflicting MPTCP suboptions.
* Availability impact realistic: +1. The likely impact is incorrect MPTCP state handling or connection-level denial of service. No reliable kernel crash is shown.
* Integrity impact plausible, paranoid only: +1. Conflicting control suboptions may affect protocol state semantics for the affected MPTCP connection, but this is limited and not evidence of kernel memory corruption.
* No generic memory corruption: +0. The patch adds validation and early breaks for invalid option combinations. It does not show UAF, OOB write, double free, type confusion, arbitrary write, or stale lifetime use.
* No privilege escalation plausible: +0. There is no supported LPE primitive.
* No strong DoS: +0. The commit does not demonstrate a panic, BUG, soft lockup, or host-wide crash.
* No rare subsystem penalty applied: 0. MPTCP is configuration and deployment dependent, but the affected path is normal network protocol parsing when MPTCP is enabled.

3. Reachability analysis
A remote peer can trigger the parsing path by establishing or interacting with an MPTCP connection and sending malformed combinations of MPTCP TCP options. No local account is required on the target, so PR:N is appropriate for CVSS-style reachability when the MPTCP endpoint is exposed. Containers and namespaces are not central to the trigger because the input arrives through the network stack. Realistic exposure depends on whether MPTCP is enabled and used by the service or host. Public Internet exposure is less common than ordinary TCP, but internal or controlled network exposure is plausible.

4. Severity interpretation
This behaves like an actionable or borderline Moderate networking parser issue, not an Important-class kernel memory corruption issue. The theoretical concern is protocol state confusion caused by mutually exclusive suboptions being accepted together. The realistic evidence supports connection disruption or limited protocol-state integrity impact. The patch context does not support privilege escalation, information disclosure, arbitrary write, or host-wide memory corruption.

5. One-sentence report phrase
MPTCP incoming option parsing allowed mutually exclusive suboptions to be accepted in one TCP option block, allowing a malicious remote peer to cause inconsistent MPTCP state handling and possible connection-level denial of service when MPTCP is enabled.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED. The issue is network reachable protocol parsing logic, but current evidence supports only limited availability or protocol-state impact and does not show a concrete memory corruption or privilege escalation primitive.

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

Patch: mptcp: avoid combining some incoming suboptions
Commit: dc1d8d3eb345c616fbe922a010fa391c72c54d52
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dc1d8d3eb345c616fbe922a010fa391c72c54d52

Commit description:

Some MPTCP suboptions are mutually exclusive according to the RFC8684,
but also because in different places, the code doesn't expect some
combinations to be present. That's specially true for suboptions that
would be present twice, but with different attributes.

The new restrictions are the same as the ones applied on the output
side, with mptcp_write_options. The same rules can be reused with a
small fix: an MP_FASTCLOSE can be used with a DSS when the sender picks
this option [1], which is not the case on Linux. Here are the rules:

  Which options can be used together?

  X: mutually exclusive
  O: often used together
  C: can be used together in some cases
  P: could be used together but we prefer not to (optimisations)

  | Opt: | MPC  | MPJ  | DSS  | ADD  |  RM  | PRIO | FAIL |  FC  |
  |------|------|------|------|------|------|------|------|------|
  | MPC  |------|------|------|------|------|------|------|------|
  | MPJ  |  X   |------|------|------|------|------|------|------|
  | DSS  |  X   |  X   |------|------|------|------|------|------|
  | ADD  |  X   |  X   |  P   |------|------|------|------|------|
  | RM   |  C   |  C   |  C   |  P   |------|------|------|------|
  | PRIO |  X   |  C   |  C   |  C   |  C   |------|------|------|
  | FAIL |  X   |  X   |  C   |  X   |  X   |  X   |------|------|
  | FC   |  X   |  X   |  P   |  X   |  X   |  X   |  X   |------|
  | RST  |  X   |  X   |  X   |  X   |  X   |  X   |  O   |  O   |
  |------|------|------|------|------|------|------|------|------|

The only difference is with the 'P': another stack could send and
ADD_ADDR with other suboptions (DSS, RM_ADDR), and this should be
allowed.

A few points of attention:

 - In theory, an MP_CAPABLE could be used with a RM_ADDR, but there is
   no reason to add it with a SYN. Note that even with a 4th ACK, it
   doesn't seem to be useful, except when IDs are known in advance via
   another channel. Better not to break that.

 - Now, combining both an MP_CAPABLE and an MP_JOIN will no longer
   result to a reject of the two options, but only the second suboption
   is ignored. That seems OK to do that for this unexpected error. At
   least now all inconsistent combinations are handled the same way.
   This could change later in next. This also means the explicit checks
   for having both MPC + MPJ in subflow.c will now be unreachable.
   That's fine, they will be removed in a follow-up patch.

 - In case of conflicting combinations, the extra suboption(s) is/are
   ignored: having such combinations either means the remote peer is
   buggy, or is evil. The simplest action is then taken in this case:
   stop processing the current suboption.

 - In mp_opt->suboptions, there is also a bit reserved to the checksum,
   which can be used in an MP_CAPABLE and a DSS. Each time a DSS option
   can be used in parallel with another option, the checksum can be set,
   so the verification is combined into a new OPTIONS_MPTCP_DSS macro.

 - An MP_CAPABLE ACK can carry a Data-Level Length, and an optional
   Checksum: they are the same as the ones found in a DSS, because a DSS
   cannot be used in parallel to an MP_CAPABLE. Similarly, even if there
   is room, a DSS cannot be used with an MP_JOIN.

Fixes: eda7acd ("mptcp: Handle MPTCP TCP options")
Cc: stable@vger.kernel.org
Link: https://www.rfc-editor.org/rfc/rfc8684.html#section-3.5-5.1 [1]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260803-net-mptcp-misc-fixes-7-2-rc6-v2-2-b8f496d71664@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/mptcp/options.c
  net/mptcp/protocol.h

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=dc1d8d3eb345c616fbe922a010fa391c72c54d52

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

The original Linux kernel CVE announcement for CVE-2026-80587 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80587

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:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 5.3

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: 3
  Paranoid ActionableScore: 4

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).