[CVE-2026-72347][LOW] netfilter: xt_connmark: reject invalid shift parameters [ Upstream

[CVE-2026-72347][LOW] netfilter: xt_connmark: reject invalid shift parameters [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72347
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: netfilter: xt_connmark: reject invalid shift parameters [ Upstream
Commit: d8ce63d928b457fba7ed1e302492dfd32293671c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8ce63d928b457fba7ed1e302492dfd32293671c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72347
Analysis date: Sat, 15 Aug 2026 22:45:32 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
CONNMARK revision 2 failed to reject invalid shift parameters, allowing a privileged local rule installer to place malformed rules that can trigger undefined 32 bit shift behavior in the netfilter packet path, with practical impact limited to incorrect behavior or DoS in hardened configurations.

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

The original Linux kernel CVE announcement for CVE-2026-72347 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72347

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
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CVE-2026-72347	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:N/A:H';*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: '5.5';DESCR 'CONNMARK revision 2 accepted user controlled shift_dir and shift_bits values without rejecting invalid combinations. A malformed rule with shift_bits 32 or larger can reach connmark_tg_shift in the packet path and trigger undefined 32 bit shift behavior. This is primarily a local control plane issue because the malformed xtables rule must be installed before packets can exercise the target. For the CVSS the PR:L paranoid value is used only for environments where CAP_NET_ADMIN is delegated to a container root or service account, while the normal host firewall administration case is PR:H. The issue is not directly network reachable by packet traffic alone. Impact is denial of service or incorrect mark handling, with no clear evidence of UAF, OOB access, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	REMOTE SIMPLEFIX NETFILTER LINUS IPV6 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=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Conservative signals:

* Common packet-processing path: +1. The malformed CONNMARK rule is evaluated from the netfilter packet path after installation.
* Requires CAP_NET_ADMIN in typical deployments: -2. Installing xtables or iptables CONNMARK rules normally requires administrative network privileges.
* No generic memory corruption: +0. The patch shows invalid input validation for shift parameters, not UAF, OOB write, double free, refcount misuse, or type confusion.
* No demonstrated reliable crash: +0. The commit describes undefined 32 bit shift behavior, but does not show a panic, Oops, KASAN report, UAF, or controlled corruption primitive.

Paranoid additional signals:

* Possible strong DoS in hardened/debug configurations: +1. Undefined shift may trigger UBSAN or warning based failures on some kernels, especially with panic_on_warn or sanitizer trap style settings.
* No LPE signal: +0. There is no supported path to privilege escalation, information disclosure, object lifetime corruption, or attacker-controlled overwrite.

## 3. Reachability analysis

The bug is triggered by installing a malformed CONNMARK revision 2 rule with shift_bits >= 32 or an invalid shift_dir, then having traffic evaluate that rule. Ordinary remote packets alone cannot install the malformed rule, so this is not directly network reachable despite being exercised in the packet path. In typical deployments, rule installation requires root or CAP_NET_ADMIN. In container or delegated network administration environments, a reduced-trust user with CAP_NET_ADMIN could potentially install the rule, but this is still a local control-plane trigger rather than a remote protocol parser bug. The affected code is in netfilter, a common subsystem, but the vulnerable condition depends on privileged rule configuration.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like validation issue, not an Important-class kernel vulnerability. The theoretical risk is undefined behavior during packet evaluation, but the patch only establishes missing validation of shift parameters and does not show a memory corruption primitive, privilege escalation path, confidentiality impact, or integrity impact. The realistic impact is incorrect mark handling or possible local privileged DoS on configurations where undefined shift diagnostics become fatal.

## 5. One-sentence report phrase

CONNMARK revision 2 failed to reject invalid shift parameters, allowing a privileged local rule installer to place malformed rules that can trigger undefined 32 bit shift behavior in the netfilter packet path, with practical impact limited to incorrect behavior or DoS in hardened configurations.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for urgent triage purposes because it is a validation-only fix requiring CAP_NET_ADMIN in normal deployments, with no demonstrated memory corruption, information disclosure, privilege escalation, or remote trigger.

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

Patch: netfilter: xt_connmark: reject invalid shift parameters [ Upstream
Commit: d8ce63d928b457fba7ed1e302492dfd32293671c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8ce63d928b457fba7ed1e302492dfd32293671c

Changed files:
  net/netfilter/xt_connmark.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=d8ce63d928b457fba7ed1e302492dfd32293671c

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

The original Linux kernel CVE announcement for CVE-2026-72347 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72347

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:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 0
  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).