[CVE-2026-63867][MODERATE REGULAR] mptcp: close TOCTOU race while computing rcv_wnd [ Upstream

[CVE-2026-63867][MODERATE REGULAR] mptcp: close TOCTOU race while computing rcv_wnd [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63867
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mptcp: close TOCTOU race while computing rcv_wnd [ Upstream
Commit: 301a33fd590c408a05c5df800e0cc1e6a8a2f8f8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=301a33fd590c408a05c5df800e0cc1e6a8a2f8f8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63867
Analysis date: Sun, 19 Jul 2026 11:20:48 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A TOCTOU race in the MPTCP output path can build a packet with inconsistent DSS ACK and receive-window values, allowing a remote MPTCP peer under timing-dependent conditions to disrupt the connection without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63867 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63867

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-2026-63867	MODERATE	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:N/I:N/A:H';CWE-367;CWE-362;CWE-703;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'A TOCTOU race in the MPTCP output path can use different lockless reads of the MPTCP ack_seq value while building one packet. This may place one data_ack value in the DSS option while computing the advertised receive window from another value, creating an inconsistent packet that can confuse the peer and disrupt an MPTCP connection. For the CVSS the PR:N is used because a remote peer does not need local privileges on the target if it can establish or participate in an MPTCP connection. The issue is network reachable through MPTCP traffic, but reliable triggering depends on timing and concurrent state changes, so AC:H is used. Impact is denial of service only. No UAF, OOB access, information leak, or privilege escalation primitive is indicated by the code change.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)	YES	REMOTE RACE NETWORK SKB PACKET  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

## 1. ActionableScore

ActionableScore=3
ActionableScoreLower=2

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  MPTCP is a network protocol path, and a peer participating in an MPTCP connection can influence ACK progression and timing.
* Hard or unreliable race / special timing required: -1
  The bug is a TOCTOU race involving multiple lockless reads of `msk->ack_seq` while constructing one outgoing packet.
* Availability impact realistic: +1
  The described consequence is peer confusion and possible MPTCP connection disruption, not host memory corruption or privilege escalation.

Conservative total: 2

Paranoid additional interpretation:

* Network protocol state disruption deserves manual triage: +1
  Even without memory corruption, the affected code is in packet construction for MPTCP DSS ACK and receive-window tracking. A malicious peer may be able to stress timing-sensitive ACK/window transitions.

Paranoid total: 3

Not counted:

* No generic memory corruption. The patch does not show UAF, OOB access, double-free, type confusion, refcount misuse, or arbitrary write.
* No privilege escalation plausible signal. There is no concrete primitive leading beyond protocol inconsistency.
* No confidentiality or integrity impact. The bug affects consistency of DSS ACK and receive-window values, not data disclosure or unauthorized modification.
* No reliable kernel crash / strong DoS. The commit says the peer may be confused, not that the host crashes or panics.

## 3. Reachability analysis

A remote MPTCP peer can plausibly influence this path by participating in an MPTCP connection and causing ACK sequence changes while the local host builds outgoing packets. No local privileges on the target are required for the network-side interpretation.

The path is only relevant when MPTCP is enabled and used by applications or services. This is not as broadly exposed as ordinary TCP packet processing, but it is still a real network protocol path and should not be treated as purely local.

Namespaces and containers do not materially change the main network-peer model. A local container or namespace user may create MPTCP sockets only if policy allows it, but the more important exposure is a remote peer of an existing MPTCP connection.

## 4. Severity interpretation

This behaves like an ordinary Moderate to borderline manual-review issue, not an Important-class kernel vulnerability.

Theoretical risk is limited to inconsistent protocol state in one packet: one `ack_seq` value may be used for the DSS ACK while another value is used for the advertised receive window. Realistic impact is MPTCP connection disruption or degraded behavior.

There is no evidence of memory corruption, kernel crash, information disclosure, privilege escalation, page-cache corruption, or cross-boundary policy bypass. The issue is network reachable, which justifies manual review, but the demonstrated impact remains DoS-like and connection-scoped.

## 5. One-sentence report phrase

A TOCTOU race in the MPTCP output path can build a packet with inconsistent DSS ACK and receive-window values, allowing a remote MPTCP peer under timing-dependent conditions to disrupt the connection without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network reachable and race-based in a protocol output path, but the patch supports only protocol-level DoS or connection disruption. It does not show a memory corruption or privilege-escalation primitive, so Important-class handling is not currently justified.

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

Patch: mptcp: close TOCTOU race while computing rcv_wnd [ Upstream
Commit: 301a33fd590c408a05c5df800e0cc1e6a8a2f8f8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=301a33fd590c408a05c5df800e0cc1e6a8a2f8f8

Changed files:
  net/mptcp/options.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=301a33fd590c408a05c5df800e0cc1e6a8a2f8f8

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

The original Linux kernel CVE announcement for CVE-2026-63867 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63867

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:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 2
  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).