[CVE-2026-72254][LOW] netfilter: nft_fib: reject fib expression on the netdev egress hook [ Upstream

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

CVE: CVE-2026-72254
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: netfilter: nft_fib: reject fib expression on the netdev egress hook [ Upstream
Commit: 4fee43759b489559a491f7c95f9bfa7a1d0c7a10
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4fee43759b489559a491f7c95f9bfa7a1d0c7a10
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72254
Analysis date: Sun, 16 Aug 2026 07:26:09 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A validation bug in netfilter nft_fib allows a fib expression to be accepted on the netdev egress hook, where evaluating it dereferences a NULL input device pointer and can crash the kernel if an attacker can modify nftables rules.

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

The original Linux kernel CVE announcement for CVE-2026-72254 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72254

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-2026-72254	LOW	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-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in the netfilter nft_fib netdev path because a fib expression can be accepted on a netdev egress base chain even though nft_in(pkt) is NULL on the transmit path. The root cause is a validation mismatch where NF_INET hook masks were applied to the netdev hook namespace and NF_NETDEV_EGRESS aliases an NF_INET hook number. For the CVSS the PR:L is used in the paranoid score because some deployments delegate CAP_NET_ADMIN to containers or service accounts, although the normal host level configuration path is closer to PR:H. The issue is not directly network reachable because an attacker must be able to install or control nftables rules rather than only send packets. Impact is denial of service via kernel crash, with no clear UAF, OOB write, information leak, or privilege escalation primitive visible from the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	REMOTE NULLPTR NETFILTER HARDWARE LINUS IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

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

## 2. Signal breakdown

* Reliable kernel crash / strong DoS: +1
  The patch and commit message describe a concrete NULL pointer dereference when `nft_in(pkt)` is NULL on the netdev egress transmit path.

* Broad/default/common subsystem or broadly deployed exposure: +1
  nf_tables and netfilter are common kernel networking components, although this specific trigger requires a configured netdev egress chain with a problematic fib expression.

* Common networking packet-processing path: +1
  The crash occurs during packet evaluation on the netdev egress path after an invalid ruleset is accepted.

* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2 conservative
  Creating or modifying nftables netdev base chains normally requires CAP_NET_ADMIN.

* Reduced privilege / delegated CAP_NET_ADMIN interpretation: -1 paranoid instead of -2
  Some container, namespace, or service-account deployments delegate CAP_NET_ADMIN without full host-root equivalence, so the practical privilege penalty can be weaker in those environments.

* No generic memory corruption bonus
  This is a NULL pointer dereference, not a UAF, OOB write, double free, type confusion, refcount takeover, or attacker-controlled overwrite.

* No privilege escalation bonus
  The patch does not show a plausible LPE primitive beyond crash.

* No confidentiality or integrity impact
  The described primitive is crash-only.

## 3. Reachability analysis

The bug is triggered by installing an nft_fib expression in a netdev egress base chain where validation incorrectly accepts a hook context that later has no input device. In typical host deployments this requires CAP_NET_ADMIN, so an ordinary remote attacker cannot trigger it by sending packets alone. Namespaces or containers matter only if an untrusted user can obtain CAP_NET_ADMIN in a network namespace with nftables netdev support and trigger egress traffic there. The vulnerable path is not remote-reachable by default, but it can become reachable to reduced-privilege local actors in delegated network-administration setups.

## 4. Severity interpretation

This behaves like an ordinary Moderate kernel DoS, not an Important-class vulnerability. The realistic impact is a local or delegated-admin-triggered kernel crash. Theoretical memory corruption or privilege escalation is not supported by the patch because the concrete failure mode is a NULL dereference of `nft_in(pkt)` rather than stale object reuse, writable UAF, OOB write, or type confusion. The issue should not be treated as remote network exploitation because packet traffic only triggers the already-installed invalid rule.

## 5. One-sentence report phrase

A validation bug in netfilter nft_fib allows a fib expression to be accepted on the netdev egress hook, where evaluating it dereferences a NULL input device pointer and can crash the kernel if an attacker can modify nftables rules.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a concrete NULL pointer dereference DoS with no demonstrated memory corruption, information leak, or privilege escalation primitive. Manual review is only worth considering for products that intentionally delegate CAP_NET_ADMIN or unprivileged network namespace creation to untrusted users.

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

Patch: netfilter: nft_fib: reject fib expression on the netdev egress hook [ Upstream
Commit: 4fee43759b489559a491f7c95f9bfa7a1d0c7a10
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4fee43759b489559a491f7c95f9bfa7a1d0c7a10

Changed files:
  net/netfilter/nft_fib.c
  net/netfilter/nft_fib_netdev.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=4fee43759b489559a491f7c95f9bfa7a1d0c7a10

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

The original Linux kernel CVE announcement for CVE-2026-72254 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72254

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:H
  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: 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).