[CVE-2026-53219][MODERATE REGULAR] netfilter: x_tables: avoid leaking percpu counter pointers [ Upstream

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

CVE: CVE-2026-53219
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: x_tables: avoid leaking percpu counter pointers [ Upstream
Commit: b74ba3343eb44b2cbf7e9665918c287df1d52ebb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b74ba3343eb44b2cbf7e9665918c287df1d52ebb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53219
Analysis date: Thu, 25 Jun 2026 06:41:48 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended / Ordinary Moderate info leak
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-53219 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53219

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.

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

CVE-2026-53219	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';*CWE-200;CWE-497;CWE-755;BEST CVSS score: '3.3';DESCR 'x_tables get entries handling can expose an internal percpu counter pointer because the fixed rule entry header was copied to userspace before the counter field was replaced with a sanitized snapshot. A local caller can provide a userspace buffer that faults after the raw pcnt value has been copied but before the sanitized counter copy completes, causing the syscall to return EFAULT while leaving the kernel pointer visible in userspace. For the CVSS the PR:L is used because a local process with CAP_NET_ADMIN in a network namespace can query x_tables entries and control the userspace buffer used for the copy. The issue is not network reachable and does not by itself provide memory corruption or code execution. Impact is low confidentiality loss because a kernel address disclosure may weaken KASLR and can help exploit chaining, while direct integrity or availability impact is not supported by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Ordinary Moderate info leak (with actual score 2)	YES	REMOTE OOB SIMPLEFIX NETFILTER LEAK NETWORK HARDWARE IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

1. ActionableScore
* Conservative score: 1 for host CAP_NET_ADMIN only deployments
* Paranoid score: 3 when unprivileged user namespaces or delegated network namespaces allow local users to obtain CAP_NET_ADMIN in their own netns
* Final recommended bucket: **Borderline manual review recommended / Ordinary Moderate info leak**::

2. Signal breakdown
* Local unprivileged trigger: +1 in namespace-enabled deployments, because a local user may obtain CAP_NET_ADMIN inside a user or network namespace and call x_tables get-entries paths.
* Confidentiality impact plausible: +1, because the bug leaks an internal percpu kernel pointer to userspace and may weaken KASLR.
* Broad/default/common subsystem: +1, because x_tables and iptables compatibility paths are widely deployed, even though this is control plane rather than packet receive path.
* Requires CAP_NET_ADMIN in typical host deployments: lower-score interpretation applies when unprivileged namespaces are disabled or not delegated.
* No memory corruption, UAF, write primitive, crash, or direct LPE primitive is shown by the patch.

3. Reachability analysis
The bug is triggered locally through native or compat x_tables get-entries APIs for IPv4, IPv6, or ARP tables. The caller must be able to query x_tables entries, usually requiring CAP_NET_ADMIN in the relevant network namespace. On systems with unprivileged user namespaces enabled, this may be reachable by an otherwise unprivileged local user inside a new namespace. It is not remotely reachable through packet traffic. The trigger is realistic because the user controls the destination buffer and can arrange a copy_to_user fault after the raw pcnt pointer is copied.

4. Severity interpretation
This behaves like an ordinary Moderate information disclosure, not an Important memory corruption issue. The disclosed value is a raw percpu allocation pointer, which can assist exploit chaining by weakening KASLR, but the patch does not show object lifetime corruption, arbitrary write, type confusion, or privilege escalation. Manual review is still useful because kernel pointer disclosures can be relevant when combined with separate memory corruption bugs.

5. One-sentence report phrase
A local x_tables get-entries caller can fault the userspace copy after a raw percpu counter pointer is copied but before sanitized counters are written, leaking a kernel pointer that may weaken KASLR without directly causing memory corruption or privilege escalation.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED.
The issue is low by standalone impact, but it is a kernel pointer disclosure in a common netfilter compatibility path and may be reachable by local users through namespace-delegated CAP_NET_ADMIN.

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

Patch: netfilter: x_tables: avoid leaking percpu counter pointers [ Upstream
Commit: b74ba3343eb44b2cbf7e9665918c287df1d52ebb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b74ba3343eb44b2cbf7e9665918c287df1d52ebb

Changed files:
  net/ipv4/netfilter/arp_tables.c
  net/ipv4/netfilter/ip_tables.c
  net/ipv6/netfilter/ip6_tables.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=b74ba3343eb44b2cbf7e9665918c287df1d52ebb

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

The original Linux kernel CVE announcement for CVE-2026-53219 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53219

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS score: 3.3

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: 1
  Paranoid ActionableScore: 3

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).