[CVE-2026-72320][IMPORTANT] netfilter: nft_lookup: fix catchall element handling with inverted lookups [ Upstream

[CVE-2026-72320][IMPORTANT] netfilter: nft_lookup: fix catchall element handling with inverted lookups [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72320
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: nft_lookup: fix catchall element handling with inverted lookups [ Upstream
Commit: 0ab8880865f9678eb6174e72c1fc4712e44c745c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ab8880865f9678eb6174e72c1fc4712e44c745c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72320
Analysis date: Sat, 15 Aug 2026 23:06:41 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An nftables logic error can mis-evaluate inverted lookups with catchall interval set elements, allowing remote packets to receive the wrong firewall verdict and potentially bypass intended network filtering policy.

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

The original Linux kernel CVE announcement for CVE-2026-72320 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72320

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: IMPORTANT
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
======================================================================

CVE-2026-72320	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L';CWE-693;CWE-284;CWE-670;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '7.7';DESCR 'An nftables logic error in nft_lookup_eval can evaluate inverted set lookups incorrectly when a direct lookup misses and an interval set catchall element should be used. A remote packet can trigger the wrong verdict if the target system already has a ruleset using NFT_LOOKUP_F_INV with a catchall interval set, which may allow traffic that should be blocked or block traffic that should be allowed. For the CVSS the PR:N is used for the paranoid score because exploitation of an already deployed vulnerable ruleset does not require attacker privileges on the target beyond network reachability. AC:H is used because the issue depends on a specific nftables configuration and matching traffic pattern. Impact is a firewall policy bypass or traffic filtering failure, not kernel memory corruption or local privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE SIMPLEFIX NETFILTER LINUS KPANIC 	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  A remote packet can exercise the affected nftables lookup path if the target already has a vulnerable ruleset using inverted lookup with a catchall interval set.

* Security-boundary bypass with practical sensitive-object access: +2
  nftables is commonly used as a network security boundary. A wrong verdict can allow traffic that policy intended to block.

* Common networking core path or packet-processing path: +1
  This is in netfilter packet evaluation, not only in an admin-only configuration parser.

* Confidentiality impact plausible: +1 paranoid only
  If the wrong verdict permits access to protected network services, confidentiality impact is plausible.

* Integrity impact plausible: +1 paranoid only
  If the wrong verdict permits access to protected services or management endpoints, integrity impact is plausible.

No memory corruption, UAF, OOB write, refcount abuse, or LPE primitive is indicated by the patch.

## 3. Reachability analysis

The bug is triggered by packet processing, not by local rule creation at attack time. The attacker does not need local privileges if the vulnerable nftables ruleset is already installed and reachable over the network. Practical triggering requires a specific nftables configuration: an inverted lookup with `NFT_LOOKUP_F_INV` and a catchall interval set such as rbtree interval sets. Namespaces or containers mainly matter for who can install the affected ruleset, but packet-triggered exploitation against an already configured host is PR:N.

## 4. Severity interpretation

This behaves like an actionable network policy bypass, not memory corruption. Realistic impact depends heavily on the deployed firewall rules and whether the affected verdict protects sensitive services or network boundaries. Conservative handling is Actionable Moderate because it is network-triggerable and affects a packet-filtering decision. Paranoid handling reaches Strong Important candidate because a firewall bypass can plausibly expose confidentiality and integrity of protected services.

## 5. One-sentence report phrase

An nftables logic error can mis-evaluate inverted lookups with catchall interval set elements, allowing remote packets to receive the wrong firewall verdict and potentially bypass intended network filtering policy.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Network-reachable firewall verdict bugs are configuration-dependent but can protect critical security boundaries, so this should not be auto-closed without checking whether affected nftables rulesets are used.

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

Patch: netfilter: nft_lookup: fix catchall element handling with inverted lookups [ Upstream
Commit: 0ab8880865f9678eb6174e72c1fc4712e44c745c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ab8880865f9678eb6174e72c1fc4712e44c745c

Changed files:
  net/netfilter/nft_lookup.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=0ab8880865f9678eb6174e72c1fc4712e44c745c

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

The original Linux kernel CVE announcement for CVE-2026-72320 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72320

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:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L
  The Best / paranoid CVSS score: 7.7

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: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).