[CVE-2026-80585][MODERATE 7.0] mptcp: fastopen: only mark MPTFO subflows with SYN data

[CVE-2026-80585][MODERATE 7.0] mptcp: fastopen: only mark MPTFO subflows with SYN data

[AL-KERNEL]

CVE: CVE-2026-80585
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mptcp: fastopen: only mark MPTFO subflows with SYN data
Commit: f75f174edc865738522e514d042cf5627f084859
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f75f174edc865738522e514d042cf5627f084859
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80585
Analysis date: Wed, 26 Aug 2026 14:07:33 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Strong Important only if follow-up confirms reliable kernel crash beyond WARN or a concrete state-confusion failure path.
Manual review required: YES

Summary:
A remote client can send a data-less TCP Fast Open SYN to an exposed MPTCP/TFO listener, causing stale MPTFO state after a WARN and potentially triggering later connection state confusion or kernel DoS.

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

The original Linux kernel CVE announcement for CVE-2026-80585 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80585

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 7.0
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-2026-80585	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:H';CWE-754;CWE-703;CWE-696;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '7.5';DESCR 'MPTCP Fast Open handling can mark a subflow as MPTFO before confirming that SYN data was queued on the child socket. A remote client with a valid TCP Fast Open cookie can send a data-less TFO SYN, causing a WARN and leaving stale MPTFO state if the warning is non fatal. That stale flag can later confuse check_fully_established and may affect connection state handling or availability. For the CVSS the PR:N is used because the attacker does not need local access or authenticated privileges on the target, only network reachability to an MPTCP/TFO listener and a valid TFO cookie. The issue is network reachable when passive TCP Fast Open and MPTCP are enabled on an exposed service. Impact is primarily denial of service through WARN/panic configurations or state confusion, with no concrete evidence of information disclosure or arbitrary write from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE SIMPLEFIX NETWORK SKB LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Strong Important only if follow-up confirms reliable kernel crash beyond WARN or a concrete state-confusion failure path.

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  A remote client can send a data-less TFO SYN to an exposed MPTCP/TFO listener.
* Availability impact realistic: +1
  The commit states that the bad state hits a WARN and can leave stale MPTFO state that later affects `check_fully_established()`.
* Common networking protocol path: +1
  This is in the TCP/MPTCP Fast Open connection setup path, reachable through packet-driven protocol behavior.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1
  Defensible only for paranoid scoring because WARN can become fatal with `panic_on_warn`, and the stale MPTFO state is explicitly said to later trigger state confusion.

Not awarded:

* No generic memory corruption: +0
  The patch shows stale state and WARN/state confusion, not UAF, OOB write, double-free, refcount takeover, or arbitrary write.
* No LPE signal: +0
  No plausible privilege-escalation primitive is shown.
* No C/I impact: +0
  The evidence supports availability impact only.

## 3. Reachability analysis

The likely trigger is a remote client reaching a service that uses MPTCP with passive TCP Fast Open enabled. The attacker needs network reachability and a valid TFO cookie, but no local account or authenticated privilege on the target. The path is not as broadly exposed as ordinary TCP because it depends on MPTCP and server-side TFO configuration, but it is still a network-facing kernel protocol path rather than a local control-plane interface. Namespaces do not materially reduce the remote-trigger concern, though containerized services may affect whether MPTCP/TFO is actually exposed.

Call-site confidence: medium. The changed function and commit message describe the immediate stale flag and downstream `check_fully_established()` confusion, but the full downstream failure mode is not included in the supplied diff.

## 4. Severity interpretation

This behaves more like an actionable Moderate network DoS than an ordinary low-risk warning cleanup. The realistic impact is availability loss through WARN/panic configurations or MPTCP state-machine confusion. Theoretical impact beyond DoS is not supported by the patch because there is no demonstrated memory corruption, attacker-controlled reuse, write primitive, or information disclosure.

## 5. One-sentence report phrase

A remote client can send a data-less TCP Fast Open SYN to an exposed MPTCP/TFO listener, causing stale MPTFO state after a WARN and potentially triggering later connection state confusion or kernel DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the issue is network-triggerable in a kernel networking path and the commit explicitly mentions stale MPTFO state leading to later state confusion, even though current evidence supports DoS rather than memory corruption or privilege escalation.

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

Patch: mptcp: fastopen: only mark MPTFO subflows with SYN data
Commit: f75f174edc865738522e514d042cf5627f084859
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f75f174edc865738522e514d042cf5627f084859

Commit description:

Passive TCP Fast Open accepts a valid-cookie SYN even when it carries
no data. In that case the child socket's receive queue is intentionally
left empty.

mptcp_fastopen_subflow_synack_set_params() set is_mptfo before checking
for queued SYN data. That made data-less TFO SYNs hit a WARN and, if
the warning was non-fatal, left stale MPTFO state behind. The stale
flag could later trigger a state-confusion bug in
check_fully_established().

Only mark the subflow as MPTFO after confirming that an SKB was queued.
Return quietly when the receive queue is empty.

Note that mptcp_subflow_context's is_mptfo field is now not just about
subflows where the TFO was present, but about MPTFO subflow that
consumed SYN data. Only having a valid cookie but not carrying data is
not really "doing TFO".

Fixes: 36b122b ("mptcp: add subflow_v(4,6)_send_synack()")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Wyatt Feng <bronzed_45_vested@icloud.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260803-net-mptcp-misc-fixes-7-2-rc6-v2-7-b8f496d71664@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/mptcp/fastopen.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=f75f174edc865738522e514d042cf5627f084859

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

The original Linux kernel CVE announcement for CVE-2026-80585 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80585

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:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).