[CVE-2026-45859][MODERATE 7.0] netfilter: nfnetlink_queue: do shared-unconfirmed check before segmentation [ Upstream

[CVE-2026-45859][MODERATE 7.0] netfilter: nfnetlink_queue: do shared-unconfirmed check before segmentation [ Upstream

[AL-KERNEL]

CVE: CVE-2026-45859
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nfnetlink_queue: do shared-unconfirmed check before segmentation [ Upstream
Commit: 79b713ef4261a8ead96af4703f89d0b5f25532e2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=79b713ef4261a8ead96af4703f89d0b5f25532e2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45859
Analysis date: Thu, 28 May 2026 00:26:40 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
nfnetlink_queue could incorrectly drop UDP GSO traffic because the shared unconfirmed conntrack check ran after segmentation, allowing remote matching traffic to cause packet loss in configured NFQUEUE deployments.

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

The original Linux kernel CVE announcement for CVE-2026-45859 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45859

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: NO

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
======================================================================

CVE-2026-45859	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-696;CWE-703;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'nfnetlink_queue could drop UDP GSO traffic incorrectly because the shared unconfirmed conntrack check was performed after skb_gso_segment created extra skb references. A remote peer may trigger packet loss when traffic matches an NFQUEUE rule, conntrack is enabled, the userspace queue handler does not advertise F_GSO, and UDP GRO or GSO aggregation is present. For the CVSS the PR:N is used because the paranoid network scenario only requires the ability to send matching traffic to the protected path, while the vulnerable NFQUEUE policy is already configured by an administrator. The issue is network reachable only in such deployments and the supported impact is denial of service through packet drops. There is no supported memory corruption, information disclosure, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	REMOTE DANGER INIT NETFILTER RACE SKB HARDWARE LINUS TEST PACKET  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. A remote peer can send UDP GSO/GRO traffic that matches an already configured NFQUEUE rule.
* Reliable service DoS / packet drop: +1. The bug can cause packets to be dropped instead of queued when the userspace NFQUEUE handler lacks F_GSO.
* Availability impact realistic: +1. The supported impact is traffic loss or service disruption on affected forwarding/filtering paths.

Paranoid additional signal:

* Common networking / netfilter path: +1. NFQUEUE and conntrack are common firewall and packet-processing components, and the issue affects packet handling in configured deployments.

Not counted:

* No generic memory corruption. The patch addresses ordering of a conntrack ownership check around GSO segmentation.
* No UAF, OOB write, arbitrary write, information leak, or privilege escalation primitive is supported.
* No CAP penalty for the attacker in the network scenario, because the attacker only needs to send matching traffic. Administrative privileges are needed to configure NFQUEUE, but not to trigger the bug after deployment.

## 3. Reachability analysis

The bug is remotely triggerable only when traffic passes through an NFQUEUE rule with conntrack enabled, UDP GSO/GRO aggregation is present, and the userspace queue application does not advertise F_GSO. TCP is largely excluded by the commit message because the unconfirmed TCP packet is the SYN and is not GRO aggregated.

Namespaces and containers mainly affect who can configure NFQUEUE rules, usually requiring CAP_NET_ADMIN. However, once such a policy is active, a remote sender can trigger the packet-loss behavior by sending matching UDP traffic. This is not universally default-enabled, but it is a realistic netfilter deployment pattern.

## 4. Severity interpretation

This behaves like an actionable Moderate issue. It deserves manual triage for firewalls, gateways, or NFQUEUE-based inspection systems because it is network-triggerable in configured deployments.

It does not behave like an Important memory-corruption vulnerability. The realistic evidence supports availability impact through incorrect packet drops, not kernel compromise, data disclosure, or privilege escalation.

## 5. One-sentence report phrase

nfnetlink_queue could incorrectly drop UDP GSO traffic because the shared unconfirmed conntrack check ran after segmentation, allowing remote matching traffic to cause packet loss in configured NFQUEUE deployments.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Network-triggerability in NFQUEUE deployments makes this worth checking for exposed firewalls or packet-inspection gateways, but the impact appears limited to availability and does not show memory corruption or privilege escalation.

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

Patch: netfilter: nfnetlink_queue: do shared-unconfirmed check before segmentation [ Upstream
Commit: 79b713ef4261a8ead96af4703f89d0b5f25532e2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=79b713ef4261a8ead96af4703f89d0b5f25532e2

Changed files:
  include/net/netfilter/nf_queue.h
  net/netfilter/nfnetlink_queue.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=79b713ef4261a8ead96af4703f89d0b5f25532e2

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

The original Linux kernel CVE announcement for CVE-2026-45859 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45859

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: 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).