[CVE-2026-53183][MODERATE REGULAR] mptcp: allow subflow rcv wnd to shrink

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

CVE: CVE-2026-53183
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mptcp: allow subflow rcv wnd to shrink
Commit: bf364b0f10b27679140699821f88af7f01e2a6e3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf364b0f10b27679140699821f88af7f01e2a6e3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53183
Analysis date: Thu, 25 Jun 2026 05:22:56 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended, but not Strong Important candidate
Manual review required: YES

Summary:
An MPTCP receive-window accounting bug may let a remote peer exceed the receiver rcvbuf limit under out-of-order or backlog conditions, causing network-triggered resource pressure or denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-53183 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53183

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-2026-53183	MODERATE	CHECK	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-400;CWE-770;CWE-682;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'An MPTCP receive-window accounting bug can allow a TCP subflow receive window to be artificially inflated when incoming data is acknowledged at the TCP level but is out-of-order in the MPTCP sequence space or is held in backlog. A remote peer can cause traffic accepted by the receiver to exceed the configured receive buffer limit, creating memory pressure or degraded availability on systems where an MPTCP endpoint is reachable. For the CVSS the PR:N is used because the attacker does not need local code execution or an account on the victim, only network reachability to an MPTCP capable service. The issue is network reachable but depends on MPTCP being enabled and on connection patterns that create the affected out-of-order or backlog state. Impact is denial of service through receive buffer overrun and resource pressure, with no concrete evidence of confidentiality loss, integrity loss, UAF, or arbitrary memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended, but not Strong Important candidate (with actual score 4)	YES	REMOTE SIMPLEFIX NETWORK SKB  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended, but not Strong Important candidate**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  MPTCP is a network protocol path, and a peer connected to an MPTCP-capable endpoint can influence receive ordering and backlog behavior.

* Reliable kernel crash / strong DoS: +1
  The commit states that incoming traffic can exceed the receiver rcvbuf size. This is a resource-accounting DoS concern, but not a demonstrated immediate panic or memory corruption.

* Hard or special triggering conditions: -1
  The issue depends on MPTCP being used and on data being acknowledged at TCP level while out-of-order in MPTCP sequence space or held in backlog.

* Availability impact realistic: +1 in paranoid score only
  In the stronger interpretation, sustained remote traffic could create meaningful memory pressure or service degradation beyond a minor local failure.

No memory corruption signals are applied:

* No UAF.
* No OOB write.
* No double-free.
* No page-cache corruption.
* No privilege escalation primitive.
* No confidentiality or integrity primitive.

## 3. Reachability analysis

A remote peer can potentially trigger the bug if it can connect to an MPTCP-enabled service or otherwise participate in an MPTCP connection with the victim. No local account or code execution on the victim is required, so PR:N is appropriate for CVSS-style reasoning.

The path is not a generic TCP path for all deployments. It requires MPTCP support and an MPTCP-capable endpoint, which reduces practical exposure compared with ordinary TCP receive-path bugs. Containers and namespaces do not significantly change the main attacker model unless they expose or depend on MPTCP services.

Realistic exploitation is a network DoS/resource-pressure scenario, not a memory-corruption or LPE scenario.

## 4. Severity interpretation

This behaves like an actionable resource-exhaustion Moderate, not an Important-class kernel memory corruption issue.

The theoretical worst case is degraded availability from receive-buffer accounting failure and memory pressure. The realistic evidence from the patch supports DoS only. There is no concrete primitive for attacker-controlled memory corruption, privilege escalation, data disclosure, or integrity violation.

The score should not be auto-closed as trivial because the affected path is network reachable with PR:N when MPTCP is exposed. But it also should not be escalated to Important without additional evidence of system-wide reliable exhaustion, panic, or corruption.

## 5. One-sentence report phrase

An MPTCP receive-window accounting bug may let a remote peer exceed the receiver rcvbuf limit under out-of-order or backlog conditions, causing network-triggered resource pressure or denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Because the issue is PR:N and network reachable when MPTCP is exposed, it should receive manual review. However, the patch supports a DoS/resource-accounting classification only, so manual review is recommended rather than required for Important-class escalation.

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

Patch: mptcp: allow subflow rcv wnd to shrink
Commit: bf364b0f10b27679140699821f88af7f01e2a6e3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf364b0f10b27679140699821f88af7f01e2a6e3

Commit description:

In MPTCP connection, the `window` field in the TCP header refers to the
MPTCP-level rcv_nxt and it's right edge should not move backward. Such
constraint is enforced at DSS option generation time.

At the same time, the TCP stack ensures independently that the TCP-level
rcv wnd right's edge does not move backward. That in turn causes artificial
inflating of the MPTCP rcv window when the incoming data is acked at the
TCP level and is OoO in the MPTCP sequence space (or lands in the backlog).

As a consequence, the incoming traffic can exceed the receiver rcvbuf size
even when the sender is not misbehaving.

Prevent such scenario forcibly allowing the TCP subflow to shrink the
TCP-level rcv wnd regardless of the current netns setting.

Fixes: f3589be ("mptcp: never shrink offered window")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260602-net-mptcp-misc-fixes-7-1-rc7-v2-4-856831229976@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

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=bf364b0f10b27679140699821f88af7f01e2a6e3

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

The original Linux kernel CVE announcement for CVE-2026-53183 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53183

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: 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).