[CVE-2026-72392][MODERATE REGULAR] ipv6: fib6: fix NULL deref in fib6_walk_continue() on multi-batch dump [ Upstream

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

CVE: CVE-2026-72392
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: fib6: fix NULL deref in fib6_walk_continue() on multi-batch dump [ Upstream
Commit: 89f9c5fee3c64c5cabc34e65599308fd3c879cf9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=89f9c5fee3c64c5cabc34e65599308fd3c879cf9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72392
Analysis date: Sat, 15 Aug 2026 22:23:20 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A race in IPv6 FIB multi-batch netlink dumps can resume traversal on the wrong routing table after concurrent table insertion, causing `fib6_walk_continue()` to dereference a NULL parent pointer and panic the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-72392 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72392

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-72392	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;**CWE-476;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An IPv6 FIB netlink dump can resume from the wrong table during a multi batch dump because inet6_dump_fib stored a positional index in the hash chain. A concurrent fib6_new_table insertion at the chain head can shift existing entries and make the saved position refer to a different table, leaving the walker root and node inconsistent. This can make fib6_walk_continue() dereference a NULL parent pointer and crash the kernel. For the CVSS the PR:L is used because reliable triggering needs local code that can drive IPv6 FIB dumps and routing table changes, for example through CAP_NET_ADMIN in a network namespace. The issue is not directly network reachable through packet traffic. Impact is at least local denial of service via kernel panic and in the paranoid case may justify limited confidentiality or integrity concern due to race based inconsistent kernel state, but no proven arbitrary write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	REMOTE DANGER NULLPTR NETWORK IPV6  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit shows a concrete NULL pointer dereference in `fib6_walk_continue()` leading to kernel panic.
* Weak/indirect corruption candidate: +1. The bug creates an inconsistent walker state where `w->root` and `w->node` refer to different logical tables, but the demonstrated primitive is a NULL dereference, not UAF or overwrite.
* Broad/default/common subsystem exposure: +1. IPv6 FIB and rtnetlink route dump code are common networking control-plane paths.
* Hard or unreliable race / special timing required: -1. Triggering requires a multi-batch FIB dump racing with concurrent table insertion.
* Requires CAP_NET_ADMIN in typical deployments: -1 or -2. Creating new IPv6 routing tables normally requires CAP_NET_ADMIN. I use -1 for the conservative lower score because network namespaces may reduce the practical privilege barrier.

Paranoid interpretation:

* Local unprivileged trigger: +1. If unprivileged user namespaces are enabled, a local user may create a network namespace, obtain CAP_NET_ADMIN inside it, and race IPv6 FIB dumps with route-table creation.
* Reliable kernel crash / strong DoS: +1.
* Weak/indirect corruption candidate: +1.
* Broad/default/common subsystem exposure: +1.
* Hard race: -1.

No strong memory corruption, UAF, arbitrary write, callback corruption, or demonstrated LPE primitive is supported by the patch.

## 3. Reachability analysis

The bug is reachable through rtnetlink IPv6 FIB dump operations combined with concurrent creation of IPv6 routing tables. It is not remotely reachable through packet traffic. In the initial network namespace, creating new routing tables normally requires CAP_NET_ADMIN, which lowers conservative exploitability. In systems with unprivileged user namespaces enabled, a local user may be able to create a private network namespace and obtain CAP_NET_ADMIN there, making the trigger closer to local unprivileged in practice. The path is part of common IPv6 networking code, but exploitation still requires racing multi-batch netlink dump state against table insertion.

Call-site confidence: high. The commit includes the relevant `inet6_dump_fib()` state handling and the crash path through `fib6_dump_table()` and `fib6_walk_continue()`.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class vulnerability. The demonstrated impact is a local kernel panic caused by a race-induced inconsistent walker state and NULL pointer dereference. The theoretical concern is that the walker mixes state from different FIB tables, but the patch and trace support NULL dereference rather than UAF, stale freed-object reuse, write-after-free, object replacement, or privilege escalation. Therefore the paranoid score is raised for manual-review sensitivity, but not into Important territory.

## 5. One-sentence report phrase

A race in IPv6 FIB multi-batch netlink dumps can resume traversal on the wrong routing table after concurrent table insertion, causing `fib6_walk_continue()` to dereference a NULL parent pointer and panic the kernel.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a race in common IPv6 routing control-plane code with a demonstrated kernel panic and namespace-dependent privilege assumptions, but the patch supports DoS only and does not show a strong memory corruption or LPE primitive.

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

Patch: ipv6: fib6: fix NULL deref in fib6_walk_continue() on multi-batch dump [ Upstream
Commit: 89f9c5fee3c64c5cabc34e65599308fd3c879cf9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=89f9c5fee3c64c5cabc34e65599308fd3c879cf9

Changed files:
  net/ipv6/ip6_fib.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=89f9c5fee3c64c5cabc34e65599308fd3c879cf9

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

The original Linux kernel CVE announcement for CVE-2026-72392 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72392

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:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 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).