[CVE-2026-52912][MODERATE 7.0] netfilter: nf_queue: hold bridge skb->dev while queued [ Upstream

[CVE-2026-52912][MODERATE 7.0] netfilter: nf_queue: hold bridge skb->dev while queued [ Upstream

[AL-KERNEL]

CVE: CVE-2026-52912
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nf_queue: hold bridge skb->dev while queued [ Upstream
Commit: 950d809f154dca04e5fbe5d3c8b9c5e44769cd57
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=950d809f154dca04e5fbe5d3c8b9c5e44769cd57
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52912
Analysis date: Wed, 24 Jun 2026 04:52:29 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A netfilter NFQUEUE bridge packet can retain skb->dev pointing to a freed bridge master until reinjection, causing a net_device use-after-free and a likely kernel crash, with manual review recommended because namespace delegated CAP_NET_ADMIN may make the trigger reachable to reduced-privilege local users.

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

The original Linux kernel CVE announcement for CVE-2026-52912 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52912

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-52912	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the netfilter NFQUEUE bridge path because a queued bridge LOCAL_IN skb can keep skb dev pointing to a bridge master device after that device is unregistered and freed. The packet can later be reinjected and br_netif_receive_skb can re-enter the receive path with the stale device pointer, which creates a kernel memory corruption condition and can crash the host. For the CVSS the PR:L is used for the paranoid score because a local user or service with effective network administration inside a namespace may be able to create the bridge and NFQUEUE conditions and trigger device teardown, while plain remote packet traffic alone is not sufficient for reliable triggering. The issue is not directly Internet reachable, but traffic from a local L2 or controlled network can participate when the vulnerable bridge NFQUEUE configuration exists. Impact is at least denial of service and in the worst case may allow confidentiality or integrity impact due to use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE DANGER NETFILTER UAF SKB HARDWARE KPANIC PACKET 	NO	NO	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

* Local unprivileged trigger: +1
  In normal host deployments this usually needs CAP_NET_ADMIN to configure bridge and NFQUEUE state, but user namespaces or delegated network administration can make the practical attacker closer to PR:L.

* Memory corruption, weak primitive: +1
  The patch and commit describe a real use-after-free of skb->dev pointing to a freed bridge master net_device. No controlled reclaim, overwrite, callback dispatch, or type confusion primitive is shown.

* Real lifetime corruption: +1
  NFQUEUE failed to hold a reference to skb->dev while the packet was queued, allowing the net_device lifetime to end before reinjection.

* Reliable kernel crash / strong DoS: +1
  Reinjection can re-enter br_netif_receive_skb() with a stale net_device pointer, which is a credible kernel crash path.

* Common networking / packet-processing path: +1 in paranoid score
  This is in netfilter NFQUEUE bridge packet handling. It is not ordinary Internet remote exposure, but it is still a network packet path and deserves manual triage.

* Requires CAP_NET_ADMIN in typical deployments: conservative penalty applied indirectly
  Conservative score is kept at 4 because setup and teardown usually require bridge and netfilter administration.

* Race-UAF / stale lifetime cap applied
  Because the patch shows stale pointer lifetime corruption but not attacker-controlled reclaim, write-after-free, callback corruption, or refcount takeover, the conservative score is capped at 4 and paranoid score at 5.

## 3. Reachability analysis

The bug is triggered through bridge netfilter NFQUEUE handling. An attacker needs a vulnerable bridge LOCAL_IN NFQUEUE configuration, queued packets, and bridge master teardown or unregister before verdict reinjection.

Plain remote Internet traffic alone is not sufficient for reliable triggering. Local L2 or controlled network traffic can participate once the vulnerable NFQUEUE bridge setup exists. In typical host deployments, creating the bridge, NFQUEUE rules, and teardown requires CAP_NET_ADMIN. In containerized or user-namespace-enabled environments, a reduced-privilege user may obtain effective network administration inside a namespace, so PR:L is a defensible paranoid interpretation.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a real UAF in a network-related kernel object lifetime path. However, the patch does not demonstrate a strong exploitation primitive such as controlled reclaim, writable stale object reuse, type confusion, callback dispatch, or arbitrary write.

Realistic impact is kernel crash / DoS. Theoretical confidentiality or integrity impact cannot be fully excluded because this is a net_device UAF, but the available patch evidence supports Actionable Moderate rather than a clear Important classification.

## 5. One-sentence report phrase

A netfilter NFQUEUE bridge packet can retain skb->dev pointing to a freed bridge master until reinjection, causing a net_device use-after-free and a likely kernel crash, with manual review recommended because namespace delegated CAP_NET_ADMIN may make the trigger reachable to reduced-privilege local users.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real use-after-free in a networking packet path with namespace-dependent privilege assumptions. It should not be auto-closed even though the currently demonstrated impact is mainly DoS.

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

Patch: netfilter: nf_queue: hold bridge skb->dev while queued [ Upstream
Commit: 950d809f154dca04e5fbe5d3c8b9c5e44769cd57
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=950d809f154dca04e5fbe5d3c8b9c5e44769cd57

Changed files:
  include/net/netfilter/nf_queue.h
  net/netfilter/nf_queue.c
  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=950d809f154dca04e5fbe5d3c8b9c5e44769cd57

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

The original Linux kernel CVE announcement for CVE-2026-52912 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52912

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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).