[CVE-2026-90061][MODERATE REGULAR] netfilter: nf_tables: skip double clone set expressions on element insert [ Upstream

[CVE-2026-90061][MODERATE REGULAR] netfilter: nf_tables: skip double clone set expressions on element insert [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90061
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: nf_tables: skip double clone set expressions on element insert [ Upstream
Commit: 1e7fb6801d9cc826adcdb7fa0533c397730657f2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e7fb6801d9cc826adcdb7fa0533c397730657f2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90061
Analysis date: Fri, 18 Sep 2026 02:56:51 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90061 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90061

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-90061	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90061 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-911;*CWE-664;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'An nf_tables set expression handling bug can double clone default set expressions when set elements are inserted through the dynset or newsetelem paths without an override expression. This can create a module reference count imbalance with expressions such as connlimit and can produce a kernel WARNING during use of default set expressions. For the CVSS the PR:L is used in the paranoid interpretation because CAP_NET_ADMIN may be delegated to container root or available through local namespace configurations, even though many production hosts restrict nftables management to administrators. The issue is not directly network reachable and is triggered by local control plane ruleset operations rather than packet traffic. Impact is primarily denial of service or operational disruption through kernel warnings and module lifecycle inconsistency. No direct UAF, OOB access, or arbitrary write primitive is demonstrated by the provided patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE DANGER NOMEMCHK NETFILTER  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2. No floor or cap changed the score.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap changed the score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1. Concrete kernel reference or lifecycle bookkeeping defect. The patch and commit establish a module refcount imbalance involving nf_tables set expressions, especially connlimit.
* +1. Limited availability or operational disruption. The demonstrated effect is a kernel WARNING splat and inconsistent module lifecycle state, not a proven panic or memory corruption.

Paranoid additional signal:

* +1. Persistent module lifecycle impact remains plausible. Repeated nftables set element operations involving default set expressions may leave module reference accounting inconsistent beyond a single warning, but this is still not a demonstrated UAF, arbitrary write, or LPE primitive.

Non-awarded signals:

* RealUAF not awarded. The patch does not establish post-free object use.
* Concrete repeated release or refcount UAF not awarded. There is a refcount imbalance, but the supplied control flow does not establish first release may free object, exact reference survives, and second lifetime mutation occurs on dangling storage.
* Strong corruption not awarded. No controlled reclaim, OOB write, arbitrary write, callback control, or type confusion is demonstrated.
* Reliable kernel crash not awarded. WARNING alone is not counted as a host crash unless a fatal setting such as panic_on_warn is part of the evaluated environment.
* Remote reachability not awarded. This is nftables control plane configuration, not packet-triggered data plane processing.
* Local unprivileged trigger not awarded. CAP_NET_ADMIN or equivalent nftables management rights are needed, and complete below-host-admin reachability is not established from the supplied input.
* Confidentiality and integrity not awarded. The evidence supports module reference accounting inconsistency and warning behavior, not data disclosure or unauthorized modification of protected state.

3. Reachability analysis

The trigger is local nftables control plane activity involving set element insertion through dynset or newsetelem paths with default set expressions and no override expression. The relevant operation normally requires CAP_NET_ADMIN for nftables rule or object management.

Host-admin-equivalent privilege is not positively established because nf_tables is namespace-aware in many paths. Below-host-admin reachability is also not fully established because the complete chain depends on the exact capability namespace, availability of the connlimit expression, module loading policy, and product configuration. Therefore HostAdminRequired is UNKNOWN and NamespaceScopedPrivilege is UNKNOWN.

No direct network trigger is established. Packets may exercise already installed nftables rules, but the vulnerable behavior described here is caused by ruleset or set element configuration, not by ordinary packet traffic alone. No special hardware is required. No race or narrow timing requirement is established.

4. Primitive and severity interpretation

The exact primitive is an nf_tables expression ownership and reference accounting mismatch. The old logic cloned default set expressions in both dynset and newsetelem paths when userspace did not provide override expressions. Those cloned expressions were then handled as if they were independent override expressions, which can cause module refcount imbalance for expressions such as connlimit.

This is not an active structured UAF and not fixed stale storage. It is best classified as CLASS_C invalid lifecycle or reference bookkeeping behavior with operational DoS potential. The demonstrated consequence is a module refcount imbalance WARNING splat. The speculative consequence is broader module lifecycle disruption if the imbalance persists or accumulates, but the patch does not establish controlled memory corruption or privilege escalation.

ImportantCandidate is NO because no Class A primitive and no strong Class B unresolved corruption primitive is established. ManualReview is RECOMMENDED because nf_tables module reference accounting bugs can be subtle and the connlimit expression is explicitly mentioned. AutoDowngradeAllowed is YES because the demonstrated impact is warning or operational disruption without UAF, repeated dangling refcount mutation, authorization bypass, or controlled corruption.

5. One-sentence report phrase

An nf_tables set expression handling bug can double clone default expressions during dynset or newsetelem insertion, causing a connlimit module refcount imbalance and kernel WARNING rather than a proven memory corruption or privilege escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is probably an ordinary Moderate or borderline control-plane DoS, but manual review is useful because module refcount imbalance in nf_tables may have configuration-dependent lifecycle effects.

7. Call-site confidence

Call-site confidence: medium

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

Patch: netfilter: nf_tables: skip double clone set expressions on element insert [ Upstream
Commit: 1e7fb6801d9cc826adcdb7fa0533c397730657f2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e7fb6801d9cc826adcdb7fa0533c397730657f2

Changed files:
  include/net/netfilter/nf_tables.h
  net/netfilter/nf_tables_api.c
  net/netfilter/nft_dynset.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=1e7fb6801d9cc826adcdb7fa0533c397730657f2

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

The original Linux kernel CVE announcement for CVE-2026-90061 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90061

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:L/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: 2
  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).