[CVE-2026-74696][MODERATE REGULAR] tcp: fix TFO max_qlen accounting across reuseport migration [ Upstream

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

CVE: CVE-2026-74696
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tcp: fix TFO max_qlen accounting across reuseport migration [ Upstream
Commit: e98f0d80b9cccb5f828425d2004f9686e7d1ae24
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e98f0d80b9cccb5f828425d2004f9686e7d1ae24
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74696
Analysis date: Sat, 22 Aug 2026 15:33:22 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / Actionable Moderate consideration
Manual review required: YES

Summary:
TCP Fast Open max_qlen accounting can become inconsistent across SO_REUSEPORT listener migration, allowing a remote client to exceed the configured pending TFO request limit under specific listener migration conditions and causing potential denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-74696 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74696

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-2026-74696	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-682;CWE-770;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'TCP Fast Open max_qlen accounting can become incorrect when pending TFO children are migrated between SO_REUSEPORT listeners. The child request is charged to the old listener when it is created but can be uncharged from the new listener after inet_reqsk_clone repoints rsk_listener, allowing the new listener queue length to drift negative and stop enforcing the configured pending Fast Open limit. A remote client may then be able to send more pending TFO requests than intended if TCP_FASTOPEN and SO_REUSEPORT listener migration are used and a listener is closed while TFO children are still pending. For the CVSS the PR:N is selected because no privileges are required beyond network reachability to the affected TCP service once the vulnerable listener state exists. The issue is network reachable but requires a specific server configuration and migration timing, so AC:H is used. Impact is denial of service through resource consumption or service degradation rather than memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate consideration (with actual score 4)	YES	REMOTE LOCK SIMPLEFIX NETWORK  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate consideration**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  The affected path is TCP Fast Open on a listening TCP service. A remote client can contribute pending TFO requests once the vulnerable listener state exists.

* Availability impact realistic: +1
  The bug can bypass the configured TCP_FASTOPEN max_qlen limit and allow more pending Fast Open requests than intended, causing resource pressure or service degradation.

Paranoid additional signal:

* Common networking core path or packet-processing path: +1
  This is in TCP listener/request socket handling, which is a core networking path, even though the exact trigger depends on TCP_FASTOPEN and SO_REUSEPORT listener migration.

Negative or limiting factors:

* Hard or special timing/configuration required: -1
  The issue requires TCP_FASTOPEN, SO_REUSEPORT listener migration, closing one listener while TFO children are pending, and continued attacker traffic. This is not a simple always-on TCP receive bug.

Not awarded:

* No generic memory corruption signal.
  The patch fixes accounting mismatch, not UAF, OOB write, double free, type confusion, or arbitrary write.

* No privilege escalation signal.
  The demonstrated consequence is resource limit bypass and DoS, not LPE.

* No confidentiality or integrity signal.
  The bug affects queue accounting and availability only.

## 3. Reachability analysis

A remote client can potentially trigger the resource pressure by sending TCP Fast Open requests to an exposed service, but only if the server uses TCP_FASTOPEN and SO_REUSEPORT and a listener migration occurs while pending TFO children exist. The attacker does not need local privileges on the victim, so PR:N is reasonable for CVSS once the vulnerable service state is present. Namespaces and containers mainly affect the server-side setup, not the remote attacker model. This path is network reachable, but the practical trigger is configuration and timing dependent.

## 4. Severity interpretation

This behaves like an actionable Moderate or borderline manual-review issue, not an Important-class memory corruption bug. The realistic impact is remote or adjacent-service DoS through bypassing a configured TCP Fast Open pending-request limit. Theoretical escalation beyond DoS is not supported by the patch because the changed code only restores fastopenq.qlen accounting during request migration. The issue should not be auto-closed solely as low risk because it is network reachable in a core TCP path, but it also should not be treated as strong Important without evidence of memory corruption or a stronger resource-exhaustion primitive.

## 5. One-sentence report phrase

TCP Fast Open max_qlen accounting can become inconsistent across SO_REUSEPORT listener migration, allowing a remote client to exceed the configured pending TFO request limit under specific listener migration conditions and causing potential denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network reachability and TCP core involvement make it worth manual review, but the patch supports a DoS/resource-accounting impact only and does not show memory corruption, privilege escalation, or confidentiality impact.

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

Patch: tcp: fix TFO max_qlen accounting across reuseport migration [ Upstream
Commit: e98f0d80b9cccb5f828425d2004f9686e7d1ae24
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e98f0d80b9cccb5f828425d2004f9686e7d1ae24

Changed files:
  net/ipv4/inet_connection_sock.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=e98f0d80b9cccb5f828425d2004f9686e7d1ae24

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

The original Linux kernel CVE announcement for CVE-2026-74696 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74696

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).