[CVE-2026-46101][LOW] netfilter: reject zero shift in nft_bitwise

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

CVE: CVE-2026-46101
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: netfilter: reject zero shift in nft_bitwise
Commit: bffef0acec9c3b837a785248a893137fb7f26c95
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bffef0acec9c3b837a785248a893137fb7f26c95
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46101
Analysis date: Wed, 27 May 2026 11:14:06 -0400
ActionableScore: 3
ActionableScore lower bound: 0
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A malformed nftables bitwise shift rule with shift value 0 can reach the packet path and trigger undefined 32-bit shift behavior, potentially causing incorrect rule evaluation or sanitizer DoS, but rule installation normally requires CAP_NET_ADMIN.

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

The original Linux kernel CVE announcement for CVE-2026-46101 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46101

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: 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-2026-46101	LOW	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:N/I:L/A:H';CWE-758;CWE-682;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '6.1';DESCR 'A malformed nft_bitwise shift expression could allow a zero shift operand to reach the packet path. The carry propagation logic then computes a 32 bit shift on a u32 value, which is undefined behavior and may cause incorrect rule evaluation or sanitizer triggered denial of service. For the CVSS the PR:L is used only in the paranoid score because delegated network administration or privileged container setups may let a less trusted local actor install nftables rules. The typical host case requires CAP_NET_ADMIN and is closer to PR:H. The issue is not directly network reachable because the malformed rule must be installed through the local nftables control plane. Impact is mainly integrity of packet filtering logic or availability on affected builds, with no UAF, out of bounds write, arbitrary write, information disclosure, or privilege escalation primitive shown by the patch.';NO MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	REMOTE SIMPLEFIX NETFILTER PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Broad/default/common subsystem exposure: +1
  nftables/netfilter is a common packet filtering subsystem.

* Integrity impact weakly plausible: +1
  Undefined behavior in a bitwise expression may cause incorrect rule evaluation, but only for a malformed rule installed through the control plane.

* Availability impact weakly plausible: +1
  On sanitizer or debug builds, the undefined shift may trigger warnings or panic.

Subtracting signals:

* Requires CAP_NET_ADMIN in typical deployments: -2
  Installing nftables rules normally requires administrative network privileges.

* Constrained primitive / no memory corruption: -1
  The issue is a zero-shift undefined behavior case, not UAF, OOB write, arbitrary write, or object lifetime corruption.

Paranoid signals:

* Local delegated trigger concern: +1
  In containers, namespaces, or delegated network administration setups, a less trusted local actor may be able to install nftables rules.

* Packet filtering policy impact concern: +1
  Once installed, the malformed expression can affect packet path evaluation.

No signals apply for remote reachability, strong memory corruption, real lifetime corruption, privilege escalation, information disclosure, or sensitive-object access.

## 3. Reachability analysis

The bug is reachable through the nftables control plane by installing a malformed `nft_bitwise` shift expression with shift value 0. It is not directly network-triggerable because packets only exercise the expression after a local privileged or delegated actor installs the rule.

In normal host deployments this requires CAP_NET_ADMIN, so exploitation by an ordinary unprivileged user is unlikely. In delegated network namespaces or privileged containers, the effective privilege requirement may be lower, but impact is usually limited to that network namespace unless broader host firewall control is delegated.

## 4. Severity interpretation

This behaves like a low-to-borderline netfilter validation bug rather than an Important vulnerability. The patch prevents undefined behavior from reaching the packet path, but it does not show memory corruption or privilege escalation.

Realistic impact is malformed-rule rejection, incorrect rule behavior, or sanitizer-triggered DoS. Broader security impact depends on whether less trusted actors can install nftables rules that affect meaningful traffic policy.

## 5. One-sentence report phrase

A malformed nftables bitwise shift rule with shift value 0 can reach the packet path and trigger undefined 32-bit shift behavior, potentially causing incorrect rule evaluation or sanitizer DoS, but rule installation normally requires CAP_NET_ADMIN.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: no memory corruption is shown, but this is netfilter packet-path logic with possible policy impact in delegated network administration environments.

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

Patch: netfilter: reject zero shift in nft_bitwise
Commit: bffef0acec9c3b837a785248a893137fb7f26c95
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bffef0acec9c3b837a785248a893137fb7f26c95

Commit description:

Reject zero shift operands for nft_bitwise left and right shift
expressions during initialization.

The carry propagation logic computes the carry from the adjacent 32-bit
word using BITS_PER_TYPE(u32) - shift. A zero shift operand turns this
into a 32-bit shift, which is undefined behaviour.

Reject zero shift operands in the control plane, alongside the existing
check for values greater than or equal to 32, so malformed rules never
reach the packet path.

Fixes: 567d746 ("netfilter: bitwise: add support for shifts.")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Kai Ma <k4729.23098@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/nft_bitwise.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=bffef0acec9c3b837a785248a893137fb7f26c95

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

The original Linux kernel CVE announcement for CVE-2026-46101 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46101

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:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 6.1

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: 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: 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).