[CVE-2026-52920][MODERATE 7.0] netfilter: xt_policy: fix strict mode inbound policy matching [ Upstream

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

CVE: CVE-2026-52920
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: xt_policy: fix strict mode inbound policy matching [ Upstream
Commit: eb323f7b82d2e2f638de0cc2a177803eb20e0707
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eb323f7b82d2e2f638de0cc2a177803eb20e0707
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52920
Analysis date: Wed, 24 Jun 2026 03:56:56 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A strict inbound `xt_policy` rule can consume IPsec policy elements in the wrong order for packets with multiple transforms, causing network-reachable policy mismatch or bypass under specific IPsec and netfilter configurations.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-52920 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52920

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
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CVE-2026-52920	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L';CWE-697;CWE-693;CWE-125;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '6.5';DESCR 'xt_policy strict inbound matching can consume policy rule elements in the wrong order for packets with multiple IPsec transforms. The old index calculation can select an invalid or out-of-bounds policy element during match_policy_in, which may cause inbound traffic to be accepted or rejected contrary to the configured x_tables policy. For the CVSS the PR:N is used because a remote packet sender does not need a local account on the target once the vulnerable xt_policy rule and IPsec path are configured. The issue is network reachable only on systems that use IPsec with strict multi-element xt_policy inbound rules, so AC:H is used. Impact is mainly integrity policy bypass, with possible limited availability impact if the invalid element access hits unsafe memory under debug or hardened builds.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)	YES	REMOTE OOB SIMPLEFIX NETFILTER IPV6 KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Inbound packets can reach `xt_policy` matching once the target system has IPsec and strict multi-element `xt_policy` rules configured.

* Cross-boundary isolation/policy bypass: +1
  The bug can make inbound IPsec policy matching consume rule elements in the wrong order, so traffic may be accepted or rejected contrary to the configured firewall/IPsec policy.

* Integrity impact plausible: +1
  The main realistic impact is policy enforcement bypass or incorrect packet classification, which is an integrity impact on the network security policy.

* Common networking / packet-processing path: +1
  This is in netfilter packet matching, not a local-only administrative cleanup path. The trigger is configuration-dependent, but it remains in the packet-processing path.

Paranoid additional signal:

* Memory corruption, weak/indirect corruption candidate: +1
  The old strict-mode index can become negative and select memory before `info->pol[]`, causing adjacent memory to be interpreted as an `xt_policy_elem`. This is read-only / match-logic misuse, not an OOB write or strong corruption primitive.

No strong memory-corruption or LPE signal is counted. There is no demonstrated write primitive, reclaim primitive, callback corruption, type confusion with control-flow impact, or information disclosure path.

## 3. Reachability analysis

A remote sender can potentially trigger the affected path by sending packets that traverse an inbound IPsec path and hit a strict multi-element `xt_policy` rule. No local account on the target is required once the vulnerable configuration is deployed, so the practical PR interpretation is PR:N for the packet sender.

The path is not universally enabled by default. It requires IPsec/sec_path processing and specific strict inbound `xt_policy` rules with multiple policy elements or transforms. This makes exploitation configuration-dependent and justifies high attack complexity, but not auto-closure, because the trigger is still packet-driven in a security policy path.

Namespaces or containers mostly affect who can install the vulnerable rules, not who can trigger packet matching after deployment. If untrusted tenants can influence firewall/IPsec configuration through delegated network administration, operational risk increases.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Moderate. The realistic issue is network policy bypass or incorrect inbound IPsec policy enforcement, not kernel code execution.

The theoretical memory-safety concern is limited to an invalid/negative index causing adjacent rule memory to be read as a policy element. That supports manual review and a paranoid score increase, but not Important-class scoring by itself because there is no write primitive, no leak path, and no demonstrated privilege escalation path.

## 5. One-sentence report phrase

A strict inbound `xt_policy` rule can consume IPsec policy elements in the wrong order for packets with multiple transforms, causing network-reachable policy mismatch or bypass under specific IPsec and netfilter configurations.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is network-triggerable under realistic but specific IPsec/netfilter configurations and affects security policy enforcement. The weak OOB-read style concern should be reviewed, but the available evidence supports Actionable Moderate rather than Strong Important.

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

Patch: netfilter: xt_policy: fix strict mode inbound policy matching [ Upstream
Commit: eb323f7b82d2e2f638de0cc2a177803eb20e0707
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eb323f7b82d2e2f638de0cc2a177803eb20e0707

Changed files:
  net/netfilter/xt_policy.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=eb323f7b82d2e2f638de0cc2a177803eb20e0707

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

The original Linux kernel CVE announcement for CVE-2026-52920 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52920

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L
  The Best / paranoid CVSS score: 6.5

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: 6

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).