[CVE-2026-53220][MODERATE 7.0] netfilter: revalidate bridge ports [ Upstream

[CVE-2026-53220][MODERATE 7.0] netfilter: revalidate bridge ports [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53220
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: revalidate bridge ports [ Upstream
Commit: 43330a1e8aace6b5a8de9aba127e9e394ab49b0f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=43330a1e8aace6b5a8de9aba127e9e394ab49b0f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53220
Analysis date: Thu, 25 Jun 2026 06:09:00 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A bridge netfilter race during NFQUEUE reinjection can dereference a stale or invalid bridge port after device reconfiguration, causing a kernel panic and requiring manual review because delegated CAP_NET_ADMIN environments may make the host DoS path practical.

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

The original Linux kernel CVE announcement for CVE-2026-53220 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53220

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-2026-53220	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:L/I:L/A:H';**CWE-476;CWE-362;CWE-843;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'Bridge netfilter code can dereference an invalid bridge port pointer when a packet is queued through NFQUEUE and the input or output device is removed from the bridge before reinjection. The device reference keeps the net_device alive, but it does not guarantee that rx_handler_data still points to a net_bridge_port, so the reinjected path may hit a NULL pointer dereference or interpret another virtual device context as a bridge port. For the CVSS the PR:L is used for the paranoid score because delegated CAP_NET_ADMIN, container root, reduced capability root, or a service account can manage bridge and netfilter state in some deployments without full host root. The issue is not directly reachable by packet traffic alone because reliable triggering also requires bridge or netfilter control plane changes. Impact is at least denial of service via kernel crash, with limited confidentiality or integrity impact kept only for the paranoid score due to the invalid pointer and type confusion aspect.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	LOCK DANGER NETFILTER SKB LOG HARDWARE PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Weak/indirect corruption candidate: +1. The old code dereferences `br_port_get_rcu()` and assumes `rx_handler_data` still represents a `net_bridge_port`.
* Real lifetime / RCU context issue: +1. Queued packets leave the original RCU-protected hook context, while the device can be removed from the bridge before reinjection.
* Reliable kernel crash / strong DoS: +1. The commit explicitly describes a NULL dereference causing kernel panic.
* Common packet-processing path: +1. This is netfilter bridge/NFQUEUE packet reinjection logic.
* Availability impact realistic: +1. The impact is a host kernel panic, not merely a warning.
* Requires CAP_NET_ADMIN in typical deployments: -2. Reliable triggering usually requires managing bridge membership, NFQUEUE rules, or related network configuration.

Conservative total: 3.

Paranoid additional interpretation:

* Weak LPE concern: +1. The patch notes that `rx_handler_data` may point to data that is not a `net_bridge_port`, which is an invalid struct interpretation / weak type confusion concern, but no write primitive is shown.
* Cross-boundary isolation concern: +1. In delegated network namespaces or container-like setups, a reduced-privilege actor with network administration rights may be able to affect host kernel availability.
* Reduced privilege / delegated capability penalty: -1. This is not full host-root in all practical deployments, but it is also not a pure unprivileged trigger.

Paranoid total: 5.

## 3. Reachability analysis

The bug is not remotely reachable by packet traffic alone. A packet must be queued and later reinjected through NFQUEUE while the relevant bridge port state changes underneath it.

Typical reliable triggering requires CAP_NET_ADMIN or equivalent control over bridge, ebtables/nftables, NFQUEUE, or virtual network device configuration. In container or namespace deployments, this may be delegated to container root or a reduced-capability service account, so treating it only as full host-root is too dismissive.

The affected area is a real packet-processing path, but the vulnerable scenario requires bridge netfilter plus NFQUEUE/reinject behavior and concurrent bridge-port reconfiguration.

## 4. Severity interpretation

This behaves above an ordinary low-risk NULL dereference because the patch context explicitly mentions stale bridge-port assumptions and possible invalid interpretation of `rx_handler_data`. However, the evidence still mainly supports DoS: kernel panic from NULL dereference or invalid bridge-port dereference.

Theoretical memory corruption concern exists due to invalid struct interpretation, but the patch does not show attacker-controlled reclaim, arbitrary write, callback control, or a strong LPE primitive. Therefore this is best treated as Actionable Moderate for manual review, not a strong Important candidate by default.

## 5. One-sentence report phrase

A bridge netfilter race during NFQUEUE reinjection can dereference a stale or invalid bridge port after device reconfiguration, causing a kernel panic and requiring manual review because delegated CAP_NET_ADMIN environments may make the host DoS path practical.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a netfilter bridge race in a packet-processing path with confirmed kernel panic and a weak type-confusion concern around `rx_handler_data`, and namespace/delegated CAP_NET_ADMIN deployments materially affect practical exposure.

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

Patch: netfilter: revalidate bridge ports [ Upstream
Commit: 43330a1e8aace6b5a8de9aba127e9e394ab49b0f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=43330a1e8aace6b5a8de9aba127e9e394ab49b0f

Changed files:
  net/bridge/netfilter/ebt_dnat.c
  net/bridge/netfilter/ebt_redirect.c
  net/netfilter/nfnetlink_log.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=43330a1e8aace6b5a8de9aba127e9e394ab49b0f

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

The original Linux kernel CVE announcement for CVE-2026-53220 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53220

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 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: NO
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).