[CVE-2026-64542][MODERATE REGULAR] ipv6: ndisc: fix NULL deref in accept_untracked_na() [ Upstream

[CVE-2026-64542][MODERATE REGULAR] ipv6: ndisc: fix NULL deref in accept_untracked_na() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64542
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: ndisc: fix NULL deref in accept_untracked_na() [ Upstream
Commit: 62c719203cb521b64fab74da94a81bdde5c18808
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=62c719203cb521b64fab74da94a81bdde5c18808
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64542
Analysis date: Mon, 27 Jul 2026 18:03:30 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Moderate. Manual review recommended
Manual review required: YES

Summary:
A race in IPv6 Neighbor Discovery allows a local unprivileged user with network namespace access to trigger a NULL pointer dereference in `accept_untracked_na()`, causing a kernel crash without evidence of confidentiality impact, integrity impact, or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-64542 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64542

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-2026-64542	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:N/I:N/A:H';**CWE-476;CWE-362;CWE-367;BEST CVSS score: '4.7';DESCR 'IPv6 Neighbor Discovery can hit a NULL pointer dereference because accept_untracked_na() re-fetches inet6_dev from dev ip6_ptr and dereferences idev cnf accept_untracked_na without a second NULL check. A concurrent addrconf_ifdown() can clear dev ip6_ptr between the validated lookup in ndisc_recv_na and the later re-fetch, so an incoming Neighbor Advertisement can reach a NULL idev during the same RCU read-side section. For the CVSS the PR:L is used because reliable triggering requires a local unprivileged user context such as a network namespace rather than a purely remote packet sender. The issue is not considered directly Internet reachable because the race also depends on local interface state changes such as MTU driven addrconf_ifdown behavior. Impact is denial of service via kernel crash. No confidentiality or integrity impact is assigned because the primitive shown by the code is a NULL pointer dereference, not UAF or controlled memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate. Manual review recommended (with actual score 4)  SKIP CVE-2026-64542 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: aaa5f515b16b6b3e137779ffb4c9558bb58c1e75 YES  NO NO unknown 	MAYBE	REMOTE READ KASAN OOB NULLPTR NETWORK HARDWARE LINUS IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **4**
* Final recommended bucket: **Borderline Moderate. Manual review recommended**

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**. The commit explicitly states reachability by an unprivileged user via a network namespace.
* **Reliable kernel crash / strong DoS: +1**. The described failure is a NULL pointer dereference in `ndisc_recv_na()` leading to an Oops or panic depending on kernel policy.
* **Broad/default/common subsystem: +1**. IPv6 Neighbor Discovery is part of a broadly deployed networking stack.
* **Hard or unreliable race / special timing required: -1**. Triggering requires a race between `ndisc_recv_na()` and `addrconf_ifdown()` caused by MTU or interface state changes.

Paranoid additional signal:

* **Common networking core path or packet-processing path: +1**. The crash occurs in the ICMPv6 / IPv6 Neighbor Discovery receive path, even though the full trigger also needs local interface-state manipulation.

Not awarded:

* No **remote reachable +2**. A remote Neighbor Advertisement alone is not enough because the race requires local namespace or interface manipulation.
* No **generic memory corruption**. The primitive shown is a NULL pointer dereference, not UAF, OOB write, type confusion, or controlled stale-object reuse.
* No **privilege escalation plausible**. The patch and call path do not show reclaim, overwrite, callback control, or object confusion.
* No **confidentiality or integrity impact**. There is no disclosure or write primitive.

## 3. Reachability analysis

The bug is reachable by a local unprivileged user if unprivileged user and network namespaces are available, because the user can create or control a network namespace and manipulate interface state sufficiently to race IPv6 Neighbor Advertisement processing against `addrconf_ifdown()`. If unprivileged namespaces are disabled, practical triggering may require `CAP_NET_ADMIN`, which lowers exposure.

This is not realistically Internet-reachable as a standalone remote packet bug. Packet processing is involved, but the necessary concurrent `dev->ip6_ptr` clearing depends on local device or namespace state changes.

Call-site confidence: **high**, because the caller `ndisc_recv_na()` and the changed helper `accept_untracked_na()` are both visible in the provided patch.

## 4. Severity interpretation

This behaves like a **borderline Moderate with manual review recommended**, not an Important-class vulnerability. The realistic impact is local denial of service via kernel crash. Theoretical escalation is not supported by the patch because the stale value is not a freed object being reused by an attacker, but a second lookup returning NULL and then being dereferenced.

The paranoid score reaches 4 because the issue combines unprivileged namespace reachability, a race, and a common IPv6 packet-processing path. It should not be auto-closed purely as a low-risk NULL dereference, but current evidence does not justify Actionable Moderate or Important handling.

## 5. One-sentence report phrase

A race in IPv6 Neighbor Discovery allows a local unprivileged user with network namespace access to trigger a NULL pointer dereference in `accept_untracked_na()`, causing a kernel crash without evidence of confidentiality impact, integrity impact, or privilege escalation.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Reason: unprivileged namespace reachability plus IPv6 packet-path involvement makes this worth review, but the demonstrated primitive is only a race-dependent NULL dereference and does not show memory corruption or LPE evidence.

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

Patch: ipv6: ndisc: fix NULL deref in accept_untracked_na() [ Upstream
Commit: 62c719203cb521b64fab74da94a81bdde5c18808
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=62c719203cb521b64fab74da94a81bdde5c18808

Changed files:
  net/ipv6/ndisc.c
  net/ipv6/icmp.c
  net/ipv6/ip6_input.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=62c719203cb521b64fab74da94a81bdde5c18808

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

The original Linux kernel CVE announcement for CVE-2026-64542 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64542

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: Not available
  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: 3
  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).