[CVE-2026-98031][MODERATE REGULAR] nexthop: Initialize extack in remove_nh_grp_entry() [ Upstream

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

CVE: CVE-2026-98031
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nexthop: Initialize extack in remove_nh_grp_entry() [ Upstream
Commit: 030c878eaedf0449e2b5401a0a0146d0afcc645e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=030c878eaedf0449e2b5401a0a0146d0afcc645e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98031
Analysis date: Sat, 26 Sep 2026 15:41:47 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98031 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98031

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-98031	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98031 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:N/A:H';*CWE-457;CWE-824;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'An uninitialized netlink_ext_ack stack object in remove_nh_grp_entry can be used after a nexthop listener rejects replacement of a reduced nexthop group without setting an error message. The error path may dereference a stale stack pointer while reporting the extack message and can crash the kernel. For the CVSS the PR:L rating reflects that an attacker normally needs local network administration capabilities to manage nexthop objects, but does not necessarily need full host root access. Reliable triggering also requires a listener error such as an allocation failure, therefore AC:H is appropriate. The issue is not directly network reachable because it is triggered through local netlink control plane operations. Impact is at least a local denial of service via kernel crash. The paranoid score includes limited confidentiality impact because an uninitialized pointer could theoretically reference readable memory in a diagnostic path, but no controlled information leak or write primitive is demonstrated. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	REMOTE SIMPLEFIX NETWORK  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap applies. The paranoid interpretation does not add a point because the patch establishes an invalid uninitialized-pointer dereference and possible crash, but does not establish a user-visible disclosure, write primitive, controlled pointer value, or post-crash exploitation mechanism.

2. Signal breakdown

* +1 Reliable kernel crash / strong DoS. The commit explicitly states that an uninitialized stack pointer is dereferenced when a nexthop listener returns an error without setting an extack message.
* +0 Weak invalid-pointer concern. The uninitialized extack message pointer is unsafe, but no attacker-controlled pointer target, write, reuse, callback, or structured-object corruption is established.
* +0 Local unprivileged trigger. Not awarded. The available evidence indicates a nexthop control-plane path, but does not establish that an ordinary unprivileged user can perform the complete trigger sequence.
* +0 Remote reachability. Not awarded. This is not a packet-processing path and requires local nexthop and netdevice control-plane activity.
* +0 Confidentiality. Not awarded. A speculative possibility that a stray pointer references readable memory is not a demonstrated disclosure path to userspace, netlink, logs accessible to the attacker, or the network.
* +0 Integrity. Not awarded. No write, refcount mutation, lifetime violation, active-object overwrite, or authorization bypass is shown.
* +0 Strong or weak exploitation leverage. Not awarded. The patch only establishes dereference of an uninitialized pointer during diagnostic error handling.
* +0 Additional availability. Not awarded. The possible kernel crash is already counted once.
* No race penalty. The supplied evidence describes an ordered error path after a listener failure, not a narrow concurrency race.
* No host-admin penalty. Host-admin-equivalent privilege is not positively established by the supplied call-site context.
* No namespace penalty. It is not established that all required nexthop creation, group reduction, listener invocation, and error triggering steps are usable from an attacker-controlled namespace.

3. Reachability analysis

The affected function is in the IPv4 nexthop control plane. Triggering appears to require manipulation of nexthop groups or a related netdevice event that causes a reduced nexthop group to be replaced.

The supplied evidence indicates that a listener such as netdevsim or mlxsw must reject the replacement operation without setting an extack message. The reported examples require an allocation failure in the listener. This is a meaningful environmental prerequisite, but it is not a timing race and does not itself prove attacker control over allocation failure.

Nexthop operations commonly involve CAP_NET_ADMIN checks, but the supplied patch does not show the exact netlink entry point, the relevant target network namespace, whether the affected device is namespace-owned, or whether the complete sequence can be performed by a container or delegated network-management process. Therefore:

* HostAdminRequired is UNKNOWN.
* NamespaceScopedPrivilege is UNKNOWN.
* Ordinary local-user reachability is not established.
* Remote reachability is not established.
* No special hardware is inherently required by the core bug, although the known listener examples involve netdevsim and mlxsw.
* No prior privileged setup is proven by the patch alone, but an applicable listener and a nexthop-group reduction path must exist.

4. Primitive and severity interpretation

The exact primitive is an invalid dereference through an uninitialized stack-resident extack message pointer. The vulnerable object is not a freed object and there is no actual or implied use-after-free.

This is not an active structured-object lifetime issue. There is no first release, surviving dangling reference, second put or release, refcount mutation, controlled reclaim, or post-free object reuse. The stale value is an uninitialized pointer field used by an error-reporting path.

The demonstrated consequence is a possible kernel fault or crash when the listener returns an error without initializing extack. A theoretical non-crashing pointer dereference could read unintended kernel memory, but the patch provides no concrete path that returns such bytes to the attacker. It also provides no kernel write, privilege escalation, authorization bypass, or controlled corruption primitive.

PrimitiveClass is CLASS_C because the demonstrated behavior is invalid-state handling and crash-oriented DoS rather than a meaningful kernel corruption or lifetime primitive. ImportantCandidate is NO because the remaining uncertainty does not amount to unresolved exploit leverage beyond the crash path. AutoDowngradeAllowed is YES because this is a constrained invalid-pointer error-reporting defect with no established UAF, corruption sink, disclosure, or security-boundary consequence.

Likely operational severity is ordinary Moderate or lower-priority local DoS triage. It should be reviewed for exact exposure in products that permit delegated nexthop administration or expose affected listeners, but the supplied evidence does not support Important-class treatment.

5. One-sentence report phrase

An uninitialized netlink extack pointer in nexthop group error handling can be dereferenced after a listener failure, causing a local kernel crash, but no controlled memory corruption, disclosure, or privilege-escalation primitive is established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm the exact rtnetlink capability checks, namespace scope, affected listener deployments, and whether an attacker can realistically cause the required listener allocation-failure path.

7. Call-site confidence

Call-site confidence: medium

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

Patch: nexthop: Initialize extack in remove_nh_grp_entry() [ Upstream
Commit: 030c878eaedf0449e2b5401a0a0146d0afcc645e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=030c878eaedf0449e2b5401a0a0146d0afcc645e

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=030c878eaedf0449e2b5401a0a0146d0afcc645e

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

The original Linux kernel CVE announcement for CVE-2026-98031 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98031

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:N/A:H
  The Best / paranoid CVSS score: 5.3

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: unknown
  Paranoid ActionableScore: 1

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).