[CVE-2026-64413][MODERATE REGULAR] netfilter: ebtables: zero chainstack array [ Upstream

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

CVE: CVE-2026-64413
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: ebtables: zero chainstack array [ Upstream
Commit: 2ade612967e2cdfb9290ebcb773f302c82f311fa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2ade612967e2cdfb9290ebcb773f302c82f311fa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64413
Analysis date: Sat, 25 Jul 2026 11:52:31 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
An ebtables table translation error path can free an uninitialized `chainstack` pointer on sparse CPU topologies after a rare allocation failure, making this primarily a local privileged DoS with weak memory-corruption concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64413 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64413

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64413	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-457;CWE-824;CWE-590;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An uninitialized pointer free can occur in the ebtables table translation error path because newinfo chainstack was allocated without zeroing. On systems with a sparse cpu_possible_mask, some chainstack entries may never be initialized, and a later allocation failure can make cleanup call vfree on such an uninitialized entry. For the CVSS the PR:L is used for the paranoid score because a less privileged but delegated network administration context may be able to update ebtables rules, although a normal deployment usually requires CAP_NET_ADMIN and is closer to PR:H. The issue is local only and is not reachable through packet traffic. Impact is at least local denial of service via kernel crash, with limited confidentiality or integrity impact kept only for the paranoid score because the bug class is an invalid free candidate rather than a proven arbitrary write or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	SIMPLEFIX NETFILTER HARDWARE LINUS 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Weak/indirect corruption candidate: +1. `newinfo->chainstack` was allocated without zeroing, and an uninitialized entry may later be passed to `vfree()`.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Updating ebtables rules normally requires network administration privileges.
* Rare and difficult-to-reach condition: -1. Trigger requires sparse `cpu_possible_mask` plus failure of a later `vmalloc_node()` allocation in the unwind path.
* Reliable crash not fully awarded conservatively. The invalid `vfree()` could crash if reached, but the commit explicitly states this is not expected as a real-world failure path.

Paranoid additional signals:

* Reliable kernel crash / strong DoS: +1. If the rare path is reached, freeing an uninitialized pointer can plausibly cause an Oops or panic.
* Weak LPE concern: +1. Invalid free is a memory-safety candidate, but there is no demonstrated reclaim, controlled object replacement, arbitrary write, or type confusion.
* Reduced privilege interpretation: no full -2 penalty in the paranoid view. In container or delegated administration setups, CAP_NET_ADMIN may be held by a less trusted actor rather than full host root.
* Constrained/no attacker-controlled payload: -1. The freed pointer value is not shown to be attacker-controlled.

## 3. Reachability analysis

The bug is local control-plane only. It is not reachable through bridged packet traffic. A trigger requires creating or replacing ebtables rules so typical systems require CAP_NET_ADMIN. Namespace or container delegation can reduce the practical privilege barrier if a less trusted workload receives network administration rights. The required runtime conditions are unusual: a sparse possible CPU mask and an allocation failure at a specific point in table translation cleanup. This makes practical exploitation unlikely.

Call-site confidence: medium. The relevant allocation and cleanup behavior is sufficiently described by the commit message, but the full unwind loop is not included in the provided diff.

## 4. Severity interpretation

This behaves more like a low-end Moderate or borderline manual-review issue than an Important vulnerability. The theoretical concern is memory safety because an uninitialized pointer can be passed to `vfree()`. Realistic exploitation evidence is weak because the path depends on an unlikely allocation failure and does not show attacker-controlled reclaim, pointer control, write-after-free, or privilege escalation. The main realistic impact is local DoS if the invalid free is reached.

## 5. One-sentence report phrase

An ebtables table translation error path can free an uninitialized `chainstack` pointer on sparse CPU topologies after a rare allocation failure, making this primarily a local privileged DoS with weak memory-corruption concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not a strong Important candidate, but it is still an invalid-free memory-safety candidate in kernel netfilter code. The practical risk is low, yet it should not be auto-closed without confirming the cleanup path and whether delegated CAP_NET_ADMIN environments are in scope.

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

Patch: netfilter: ebtables: zero chainstack array [ Upstream
Commit: 2ade612967e2cdfb9290ebcb773f302c82f311fa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2ade612967e2cdfb9290ebcb773f302c82f311fa

Changed files:
  net/bridge/netfilter/ebtables.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=2ade612967e2cdfb9290ebcb773f302c82f311fa

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

The original Linux kernel CVE announcement for CVE-2026-64413 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64413

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: 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).