[CVE-2026-72350][LOW] netfilter: xt_u32: reject invalid shift counts [ Upstream

[CVE-2026-72350][LOW] netfilter: xt_u32: reject invalid shift counts [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72350
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: netfilter: xt_u32: reject invalid shift counts [ Upstream
Commit: ca7c92d9701249e6daf132087756a88cbf2bb2a3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ca7c92d9701249e6daf132087756a88cbf2bb2a3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72350
Analysis date: Sun, 16 Aug 2026 05:39:18 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A malformed xt_u32 netfilter rule can specify an invalid 32 bit shift count, causing undefined behavior during packet evaluation and potentially leading to limited DoS or incorrect firewall rule matching when a privileged actor can install such a rule.

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

The original Linux kernel CVE announcement for CVE-2026-72350 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72350

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: LOW
Manual review required: NO

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-2026-72350	LOW	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';*CWE-20;CWE-682;CWE-758;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'A malformed netfilter xt_u32 rule can provide a shift count of 32 or more for XT_U32_LEFTSH or XT_U32_RIGHTSH. During packet evaluation u32_match_it() may execute this rule supplied shift on a 32 bit value, which is undefined behavior and may lead to a warning, crash, or incorrect rule matching. For the CVSS the PR:L is used for the paranoid score because some deployments delegate CAP_NET_ADMIN to containers, service accounts, or reduced capability root contexts. The issue is not directly network reachable because packets only trigger evaluation after a malformed local rule has already been installed. Impact is mainly denial of service or limited firewall policy correctness impact, with no clear UAF, OOB write, or arbitrary write primitive visible from the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	REMOTE WRITE OOB SIMPLEFIX NETFILTER LINUS PACKET  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Common packet-processing path: +1. The invalid operation is reached during packet evaluation in netfilter xt_u32.
* Requires CAP_NET_ADMIN in typical deployments: -2. Installing or modifying xt_u32 rules normally requires administrative firewall privileges.
* Availability impact possible: +1. Undefined shift behavior may cause warning, sanitizer report, incorrect evaluation, or crash in some configurations.

Paranoid additional signal:

* Integrity impact plausible: +1. Incorrect rule matching could affect firewall policy behavior, but this is limited because the malformed rule must already be installed by a privileged actor.

Not awarded:

* No remote reachable score. Network packets can only trigger evaluation after a malformed local rule is installed.
* No generic memory corruption score. The patch shows invalid shift validation, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No privilege escalation score. There is no concrete LPE primitive.
* No strong DoS score. The commit describes undefined behavior, not a reliable kernel panic path.

## 3. Reachability analysis

The bug is triggered by creating a malformed xt_u32 netfilter rule with XT_U32_LEFTSH or XT_U32_RIGHTSH shift count 32 or greater, then evaluating traffic against that rule. In normal deployments this requires CAP_NET_ADMIN or equivalent firewall management privileges. In container or delegated network administration setups, a reduced-privilege root or service account may be able to install such rules, but this still is not a true unauthenticated or remote trigger. The packet path is common, but the vulnerable condition depends on a malformed control-plane rule.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like validation bug. The theoretical concern is undefined behavior in kernel packet evaluation, with possible crash or incorrect match logic. Realistic exploitation evidence is weak because the patch does not show memory corruption, attacker-controlled overwrite, stale object lifetime, information disclosure, or privilege escalation. The issue should not be treated as an Important-class kernel vulnerability based on the supplied patch.

## 5. One-sentence report phrase

A malformed xt_u32 netfilter rule can specify an invalid 32 bit shift count, causing undefined behavior during packet evaluation and potentially leading to limited DoS or incorrect firewall rule matching when a privileged actor can install such a rule.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a validation-only fix for a privileged netfilter rule configuration path, with no concrete memory corruption or privilege escalation primitive shown.

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

Patch: netfilter: xt_u32: reject invalid shift counts [ Upstream
Commit: ca7c92d9701249e6daf132087756a88cbf2bb2a3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ca7c92d9701249e6daf132087756a88cbf2bb2a3

Changed files:
  net/netfilter/xt_u32.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=ca7c92d9701249e6daf132087756a88cbf2bb2a3

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

The original Linux kernel CVE announcement for CVE-2026-72350 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72350

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 4.4

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: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).