[CVE-2026-53012][MODERATE REGULAR] nexthop: fix IPv6 route referencing IPv4 nexthop [ Upstream

[CVE-2026-53012][MODERATE REGULAR] nexthop: fix IPv6 route referencing IPv4 nexthop [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53012
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nexthop: fix IPv6 route referencing IPv4 nexthop [ Upstream
Commit: ceffe81a0be92afc0cd1340bc8ca46559cce9bb4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ceffe81a0be92afc0cd1340bc8ca46559cce9bb4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53012
Analysis date: Thu, 25 Jun 2026 14:26:41 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A stale `has_v4` flag in nexthop groups can allow invalid IPv6 routes to reference IPv4 nexthop members, causing a NULL pointer dereference and kernel panic during IPv6 route lookup when route configuration is controlled locally or through delegated network namespaces.

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

The original Linux kernel CVE announcement for CVE-2026-53012 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53012

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
======================================================================

CVE-2026-53012	MODERATE	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:N/A:H';**CWE-476;CWE-754;*CWE-20;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 nexthop route lookup path because a nexthop group can keep a stale has_v4 false state after an IPv6 nexthop is replaced with an IPv4 nexthop. This stale state can allow an IPv6 route to reference a group that effectively contains only IPv4 nexthop members, and a later IPv6 lookup can call nexthop_fib6_nh() and receive NULL. A local actor can trigger the crash by configuring nexthops and an IPv6 route and then causing traffic for that route. For the CVSS the PR:L is used in the paranoid score because reliable triggering requires local route or nexthop configuration rights, and on systems with user namespaces or delegated network administration this may be reachable by a non root local user with CAP_NET_ADMIN in a network namespace. The issue is not directly network reachable as a standalone remote attack because the unsafe state is created through control plane route configuration. Impact is denial of service via kernel crash, with no concrete evidence of information disclosure or privilege escalation from this NULL dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3) 	MAYBE	REMOTE DANGER NULLPTR NETWORK IPV6 SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit includes a concrete reproducer ending in a panic through `nexthop_fib6_nh()` returning NULL.
* Requires CAP_NET_ADMIN in typical deployments: -2. Creating and replacing nexthops and adding IPv6 routes normally requires network administration privileges.
* Common networking route-management path: +1. This is in the kernel nexthop and IPv6 routing infrastructure, but the trigger is control-plane configuration, not ordinary remote packet input.
* Availability impact realistic: +1. The impact is a host kernel panic after route lookup, but this mostly overlaps with the DoS signal.

Paranoid additions / interpretation:

* Local unprivileged trigger: +1. On systems with unprivileged user namespaces or delegated network namespaces, a local user may obtain CAP_NET_ADMIN inside a netns and exercise nexthop configuration.
* CAP penalty reduced: 0 instead of -2. Namespace delegation can make this reachable below full host-root privilege.
* No generic memory corruption score. This is a NULL pointer dereference from stale route state, not UAF, OOB write, double-free, or controlled overwrite.
* No privilege escalation score. The patch supports crash/DoS, not an LPE primitive.

## 3. Reachability analysis

The direct trigger is local route-control configuration: create an IPv6 nexthop, create a group, replace the nexthop with IPv4, attach an IPv6 route to the stale group, then trigger lookup. In ordinary host deployments this requires CAP_NET_ADMIN, so the conservative score is low.

The more actionable case is namespace-dependent. If unprivileged users can create user and network namespaces, or if containers receive CAP_NET_ADMIN in their own netns, the same kernel code path may become reachable by a non-root local actor and still crash the host kernel. The bug is not remotely network-triggerable by packet traffic alone because the invalid state must first be created through nexthop and route configuration.

## 4. Severity interpretation

This behaves like a local or namespace-reachable DoS, not an Important-class memory corruption issue. The demonstrated primitive is a NULL pointer dereference caused by stale nexthop group bookkeeping. There is no evidence of UAF reclaim, arbitrary write, object confusion, data disclosure, or privilege escalation.

Because the crash is reliable and the PR level depends on namespace policy, it should not be treated as a trivial auto-close in environments allowing unprivileged netns usage. It is best classified as Borderline / actionable Moderate for manual triage, not a Strong Important candidate.

## 5. One-sentence report phrase

A stale `has_v4` flag in nexthop groups can allow invalid IPv6 routes to reference IPv4 nexthop members, causing a NULL pointer dereference and kernel panic during IPv6 route lookup when route configuration is controlled locally or through delegated network namespaces.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because exploitability depends heavily on namespace and CAP_NET_ADMIN exposure. The issue is likely DoS-only, but systems with unprivileged user namespaces or delegated container networking may make the trigger reachable by non-root local users.

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

Patch: nexthop: fix IPv6 route referencing IPv4 nexthop [ Upstream
Commit: ceffe81a0be92afc0cd1340bc8ca46559cce9bb4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ceffe81a0be92afc0cd1340bc8ca46559cce9bb4

Changed files:
  net/ipv4/nexthop.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=ceffe81a0be92afc0cd1340bc8ca46559cce9bb4

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

The original Linux kernel CVE announcement for CVE-2026-53012 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53012

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: 4

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).