[CVE-2026-97974][MODERATE REGULAR] ipv6: null-check fib6_node before accessing in __ip6_del_rt_siblings() [ Upstream

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

CVE: CVE-2026-97974
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: null-check fib6_node before accessing in __ip6_del_rt_siblings() [ Upstream
Commit: e7ff91f19c1aae039a4257fa86fe849afe185291
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e7ff91f19c1aae039a4257fa86fe849afe185291
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97974
Analysis date: Fri, 25 Sep 2026 11:07:38 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97974 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97974

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.

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

CVE-2026-97974	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97974 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;**CWE-476;CWE-367;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A race in IPv6 route management can occur because ip6_route_del looks up a route before __ip6_del_rt_siblings acquires the routing table lock. A concurrent RTM_NEWROUTE operation with NLM_F_REPLACE can unlink the old ECMP siblings and set rt fib6_node to NULL before the deletion path accesses fn leaf. This can cause a NULL pointer dereference and an Oops or kernel panic, resulting in local denial of service. For the CVSS the PR:L value is used for the paranoid score because an unprivileged local user may be able to create user and network namespaces and obtain CAP_NET_ADMIN inside that isolated network namespace. The base score uses PR:H for deployments where route modification is restricted to a host administrator. AC:H applies because successful triggering depends on winning a concurrency window between route lookup and route table locking. The issue is not remotely reachable through ordinary IPv6 traffic because the trigger requires local rtnetlink route configuration operations. The analyzed primitive is a NULL pointer dereference rather than a demonstrated UAF or memory corruption primitive, so confidentiality and integrity remain unaffected.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)  SKIP CVE-2026-97974 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE KASAN NULLPTR NETWORK SKB HARDWARE IPV6 SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Local unprivileged trigger. On systems permitting unprivileged user and network namespace creation, an ordinary user can obtain namespace-scoped CAP_NET_ADMIN and perform the required RTM_NEWROUTE and RTM_DELROUTE operations inside that network namespace.
* +1 Reliable kernel crash or strong DoS. A syzbot reproducer reaches the documented NULL dereference, which can cause an Oops or panic depending on kernel configuration.
* +1 Common networking routing path. The defect is in IPv6 routing and rtnetlink route management rather than an unusual hardware-dependent subsystem.
* 0 Race penalty. The attacker can repeatedly issue both route replacement and deletion operations. The evidence does not establish a narrow, one-shot, or externally uncontrollable race.
* 0 Extra Availability. The same NULL dereference is not counted again as a separate availability consequence.
* 0 UAF or lifetime corruption. The competing route replacement explicitly sets `fib6_node` to NULL. No freed object is subsequently dereferenced.
* 0 Structured exploitation leverage. The stale condition terminates in a NULL dereference rather than processing reclaimed or attacker-shaped object contents.
* 0 Confidentiality or Integrity. No disclosure, write primitive, controlled reclaim, authorization bypass, or protected-state modification is established.
* 0 Remote reachability. Ordinary IPv6 packets do not perform the required rtnetlink route replacement and deletion operations.

3. Reachability analysis

The trigger requires concurrent IPv6 route-control operations, specifically deletion and replacement with `NLM_F_REPLACE`. These operations require CAP_NET_ADMIN for the affected network namespace.

The complete operation can be performed inside an attacker-created user and network namespace when unprivileged namespace creation is enabled. The attacker then holds CAP_NET_ADMIN only for that namespace, not host-administrator authority. No physical device, unusual hardware, boot option, or external network traffic is required.

Systems that disable unprivileged user namespaces restrict practical reachability to a delegated network-management actor or host administrator. This deployment restriction does not change the established namespace-scoped kernel interface. Repeated attempts are inexpensive, and the supplied reproducer demonstrates that the race is reachable.

4. Primitive and severity interpretation

The exact primitive is a race-induced NULL pointer dereference. `ip6_route_del()` obtains a route before taking `tb6_lock`. A concurrent replacement can unlink the old ECMP siblings and set `rt->fib6_node` to NULL. After acquiring the lock, the deletion path previously dereferenced `fn->leaf` without revalidating `fib6_node`.

This is not a real UAF. The evidence does not show that `fib6_node` points to freed or reclaimed storage, and the fix checks for NULL rather than extending object lifetime. There is no repeated release, refcount mutation, active-state replacement corruption, arbitrary write, or useful stale-object processing.

The issue is therefore CLASS_C. Its demonstrated consequence is local denial of service through an Oops or kernel panic. Privilege escalation and confidentiality or integrity effects are unsupported. ImportantCandidate is NO because no independently useful corruption or boundary-bypass primitive remains after excluding the crash. AutoDowngradeAllowed is YES because this is a constrained crash-only race with no unresolved Class A or Class B primitive.

5. One-sentence report phrase

A namespace-local attacker with CAP_NET_ADMIN can race IPv6 route replacement against deletion and trigger a NULL pointer dereference in `__ip6_del_rt_siblings()`, causing a kernel Oops or panic without a demonstrated memory-corruption or privilege-escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm the product policy for unprivileged user namespaces and panic-on-Oops behavior, but the supplied patch supports only a local crash primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: ipv6: null-check fib6_node before accessing in __ip6_del_rt_siblings() [ Upstream
Commit: e7ff91f19c1aae039a4257fa86fe849afe185291
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e7ff91f19c1aae039a4257fa86fe849afe185291

Changed files:
  net/ipv6/route.c
  net/core/rtnetlink.c
  net/netlink/af_netlink.c
  net/socket.c
  arch/x86/entry/syscall_64.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=e7ff91f19c1aae039a4257fa86fe849afe185291

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

The original Linux kernel CVE announcement for CVE-2026-97974 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97974

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:H/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:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  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).