[CVE-2026-64538][MODERATE 7.0] ipv6: Fix null-ptr-deref in fib6_nh_mtu_change(). [ Upstream

[CVE-2026-64538][MODERATE 7.0] ipv6: Fix null-ptr-deref in fib6_nh_mtu_change(). [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64538
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: Fix null-ptr-deref in fib6_nh_mtu_change(). [ Upstream
Commit: d08d019f2f43a6f9a71e81868bbc326b3afaf37b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d08d019f2f43a6f9a71e81868bbc326b3afaf37b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64538
Analysis date: Mon, 27 Jul 2026 22:10:10 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An adjacent IPv6 attacker may trigger a NULL pointer dereference in `fib6_nh_mtu_change()` by driving RA-based MTU processing during interface teardown, causing a kernel crash but not showing a supported privilege-escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-64538 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64538

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 7.0
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-2026-64538	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-362;CWE-754;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.5';DESCR 'fib6_nh_mtu_change can re fetch the inet6 device with __in6_dev_get and then dereference idev.cnf.mtu6 without checking whether idev is NULL. An adjacent attacker on the same IPv6 link may contribute to the trigger by sending Router Advertisement traffic that drives the MTU update path while the interface is being torn down and dev.ip6_ptr has already been cleared. For the CVSS the PR:N is used because no privileges on the target are needed if the attacker can send IPv6 RA packets on the local link. The issue is not Internet routable in normal IPv6 deployments because Router Advertisements are link local control traffic. Impact is denial of service via kernel oops or crash. The observed primitive is a NULL pointer dereference, so privilege escalation is not supported by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE KASAN NULLPTR NETWORK IPV6 KPANIC 	YES	NO	guess

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  IPv6 Router Advertisement processing is reachable from the local L2 link through `icmpv6_rcv()` and `ndisc_router_discovery()`. This is adjacent-network reachable, not Internet-routable AV:N.
* Reliable kernel crash / strong DoS: +1
  The commit shows a KASAN NULL pointer dereference and Oops in `fib6_nh_mtu_change()`.
* Common networking core path or packet-processing path: +1
  The call path is in IPv6 RA and route MTU update handling.
* Hard or unreliable race / special timing required: -1
  The crash requires overlap between RA-driven MTU handling and interface teardown where `dev->ip6_ptr` has already been cleared.
* Broad/default/common subsystem: +1
  IPv6 is broadly deployed, though this specific path requires RA handling plus nexthop-backed routes surviving teardown.

Conservative total: 4

Paranoid adjustment:

* Remove or reduce the race penalty in environments where an attacker can repeatedly send RA traffic while interface state changes are common or externally induced. This raises practical triage sensitivity to 5.

No memory-corruption or LPE signals are awarded. The demonstrated primitive is a NULL pointer dereference, not UAF, OOB write, type confusion, stale callback, or controlled object reuse.

## 3. Reachability analysis

An attacker on the same IPv6 link may influence the vulnerable path by sending Router Advertisement traffic that triggers MTU update processing. The issue is not normally reachable from the public Internet because IPv6 RA is link-local control traffic. No local privileges on the victim are needed for the network-side RA input, so the practical CVSS-style PR is PR:N for the adjacent attacker model.

The timing condition is important. The crash requires RA-driven route MTU walking to occur while `addrconf_ifdown()` has cleared `dev->ip6_ptr`, and while a nexthop-backed route still reaches `fib6_nh_mtu_change()`. Namespaces or containers may affect exposure if they can create IPv6 interfaces, manipulate teardown timing, or receive RA traffic, but the primary trigger remains adjacent-network plus teardown race.

## 4. Severity interpretation

This behaves like an actionable Moderate DoS issue, not an Important memory-corruption issue. The theoretical impact is limited by the observed primitive: an unchecked NULL `idev` dereference. The patch context does not support privilege escalation, confidentiality impact, integrity impact, attacker-controlled reclaim, or arbitrary write.

The reason it should not be treated as an ordinary low-priority Moderate is that it is in IPv6 packet-triggered networking code and can be influenced from the local network. The practical risk is kernel crash or Oops under race conditions.

## 5. One-sentence report phrase

An adjacent IPv6 attacker may trigger a NULL pointer dereference in `fib6_nh_mtu_change()` by driving RA-based MTU processing during interface teardown, causing a kernel crash but not showing a supported privilege-escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is adjacent-network reachable through IPv6 RA handling and affects a common networking path, but the demonstrated primitive is NULL dereference only and the race condition limits reliability.

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

Patch: ipv6: Fix null-ptr-deref in fib6_nh_mtu_change(). [ Upstream
Commit: d08d019f2f43a6f9a71e81868bbc326b3afaf37b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d08d019f2f43a6f9a71e81868bbc326b3afaf37b

Changed files:
  net/ipv6/route.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=d08d019f2f43a6f9a71e81868bbc326b3afaf37b

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

The original Linux kernel CVE announcement for CVE-2026-64538 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64538

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).