[CVE-2026-53000][IMPORTANT] netfilter: nat: use kfree_rcu to release ops [ Upstream

[CVE-2026-53000][IMPORTANT] netfilter: nat: use kfree_rcu to release ops [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53000
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: nat: use kfree_rcu to release ops [ Upstream
Commit: 32fdd2e38e7435a368d88f5977a7d6585ebc8b0e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=32fdd2e38e7435a368d88f5977a7d6585ebc8b0e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53000
Analysis date: Thu, 25 Jun 2026 10:00:38 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A netfilter NAT RCU lifetime bug can leave hook ops or private data visible to nfnetlink hook dump readers after immediate free, creating a local UAF candidate with likely DoS impact and limited confidentiality or integrity concern in reduced-privilege CAP_NET_ADMIN deployments.

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

The original Linux kernel CVE announcement for CVE-2026-53000 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53000

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: IMPORTANT
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-2026-53000	IMPORTANT	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-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A use-after-free candidate exists in the netfilter NAT hook lifetime handling because NAT hook ops and private data could be freed immediately while RCU based hook dump readers may still inspect the same objects. The issue is local and control plane based rather than packet triggered over the network. For the CVSS the PR:L is used in the paranoid score because some deployments expose netfilter administration through user namespaces, delegated CAP_NET_ADMIN, container root, or service accounts that are weaker than full host root. Attack complexity is high because reliable triggering depends on a race between hook unregister or partial register error handling and nfnetlink hook inspection. Impact is at least local denial of service via kernel crash or warning. In the worst still-defensible interpretation, stale RCU object access may allow limited confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	REMOTE NETFILTER ERRORPATH NETWORK IPV6 KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The fix changes `kfree()` to `kfree_rcu()` for objects that may still be observed by RCU readers, which is a UAF-class lifetime bug.
* Real lifetime corruption: +1. This is specifically an RCU lifetime issue involving NAT hook ops and private data.
* Reliable kernel crash / strong DoS: +1. A stale pointer read from freed hook metadata can plausibly crash or warn in kernel paths.
* Broad/default/common subsystem: +1. Netfilter and NAT are common kernel networking subsystems.
* Hard or unreliable race / special timing required: -1. Triggering requires overlap between unregister or partial registration error handling and nfnetlink hook inspection.
* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1. Not full -2 because user namespaces, container root, delegated CAP_NET_ADMIN, or service accounts may expose this without full host root.

Paranoid additional signals:

* Local unprivileged or reduced-privilege trigger: +1. In systems with unprivileged user namespaces or delegated network administration, this may be reachable by a reduced-privilege actor.
* Confidentiality impact plausible: +1. Hook dump readers may inspect stale freed objects, so limited stale kernel memory exposure is plausible.
* Integrity impact plausible: +1. Stale object interpretation in a trusted netfilter control-plane path gives a weak but defensible integrity concern, although no arbitrary write primitive is shown.

Not counted:

* Remote reachable: +0. This is not packet-triggered over the network.
* Strong LPE plausibility: +0. The patch supports UAF concern, but does not show reclaim control, arbitrary write, or reliable object confusion.
* Common packet-processing path: +0. The affected exposure is nfnetlink hook dump and NAT hook registration lifetime, not direct packet receive parsing.

## 3. Reachability analysis

The realistic trigger is local control-plane activity involving NAT hook registration or unregistration racing with nfnetlink hook dumping. In ordinary host deployments this typically requires `CAP_NET_ADMIN`, module or namespace setup, or firewall management privileges. In containerized systems or systems with unprivileged user namespaces, the practical privilege requirement may be lower because a container root or delegated network administrator can manipulate netfilter state without being full host root.

The path is not directly network reachable and ordinary packets do not trigger the bug by themselves. The affected subsystem is common, but the exact vulnerable condition depends on hook dump visibility and timing against NAT hook teardown or partial registration failure.

## 4. Severity interpretation

This is stronger than an ordinary Moderate cleanup because replacing `kfree()` with `kfree_rcu()` around objects visible to RCU readers is a classic UAF-lifetime indicator. Realistic impact is most likely local DoS or warning/crash under a race. Theoretical impact includes limited stale memory disclosure or weak integrity effects through stale object interpretation, but the patch does not demonstrate a strong privilege escalation primitive.

Overall, this should be treated as Actionable Moderate at minimum, with a paranoid Strong Important candidate bucket for environments where netfilter administration is delegated to containers or reduced-privilege service accounts.

## 5. One-sentence report phrase

A netfilter NAT RCU lifetime bug can leave hook ops or private data visible to nfnetlink hook dump readers after immediate free, creating a local UAF candidate with likely DoS impact and limited confidentiality or integrity concern in reduced-privilege CAP_NET_ADMIN deployments.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an RCU use-after-free candidate in a common netfilter control-plane path, and namespace or delegated `CAP_NET_ADMIN` deployments may make the practical privilege boundary weaker than full host root.

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

Patch: netfilter: nat: use kfree_rcu to release ops [ Upstream
Commit: 32fdd2e38e7435a368d88f5977a7d6585ebc8b0e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=32fdd2e38e7435a368d88f5977a7d6585ebc8b0e

Changed files:
  net/ipv4/netfilter/iptable_nat.c
  net/ipv6/netfilter/ip6table_nat.c
  net/netfilter/nf_nat_core.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=32fdd2e38e7435a368d88f5977a7d6585ebc8b0e

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

The original Linux kernel CVE announcement for CVE-2026-53000 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53000

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:L/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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).