[CVE-2026-63991][LOW] Bluetooth: 6lowpan: check skb_clone() return value in send_mcast_pkt() [ Upstream

[CVE-2026-63991][LOW] Bluetooth: 6lowpan: check skb_clone() return value in send_mcast_pkt() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63991
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: Bluetooth: 6lowpan: check skb_clone() return value in send_mcast_pkt() [ Upstream
Commit: 9afcb5ea080af13aab37930da627db43bd277665
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9afcb5ea080af13aab37930da627db43bd277665
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63991
Analysis date: Sun, 19 Jul 2026 15:02:41 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
Bluetooth 6LoWPAN multicast transmit handling can dereference a NULL skb clone after `skb_clone()` allocation failure, causing a local denial of service under memory pressure when a configured Bluetooth 6LoWPAN interface is used.

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

The original Linux kernel CVE announcement for CVE-2026-63991 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63991

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: LOW
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-2026-63991	LOW	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-252;CWE-703;BEST CVSS score: '4.7';DESCR 'Bluetooth 6LoWPAN multicast transmit handling can dereference a NULL skb clone because send_mcast_pkt() did not check whether skb_clone() failed before passing the clone to send_pkt(). The failure condition depends on memory allocation failure in GFP_ATOMIC context, so reliable triggering requires local ability to send multicast traffic through a configured Bluetooth 6LoWPAN interface and to hit memory pressure at the right time. For the CVSS the PR:L is selected because a local user or local process is required to generate the relevant transmit path, while no administrative privileges are necessarily required once the interface is available. The issue is not directly network reachable based on the patch context. Impact is denial of service via kernel crash only, with no supported evidence of information disclosure, memory corruption, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-63991 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE NULLPTR SIMPLEFIX NETWORK SKB HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process may be able to send multicast traffic through an already configured Bluetooth 6LoWPAN interface.
* Availability impact realistic: +1. If `skb_clone()` fails and the NULL clone reaches `send_pkt()`, the likely result is a kernel NULL pointer dereference and crash.
* Hard or unreliable trigger: -1. The failure depends on `GFP_ATOMIC` allocation failure, so reliable triggering requires memory pressure or timing-sensitive low-memory conditions.

Paranoid adjustment:

* Reliable kernel crash / strong DoS: +1. In the paranoid view, once allocation failure is reached, the crash path is direct because `send_pkt()` dereferences `skb->data`.
* No memory-corruption or LPE bonus. This is a NULL pointer dereference after unchecked allocation failure, not UAF, OOB write, double-free, type confusion, or controlled corruption.

Call-site confidence: high. The commit explicitly states the failing call path: `send_mcast_pkt()` -> `skb_clone()` -> `send_pkt()` -> `skb->data`.

## 3. Reachability analysis

The bug is triggered from the Bluetooth 6LoWPAN multicast transmit path. A realistic attacker needs local ability to generate traffic through a configured Bluetooth 6LoWPAN network device and must also hit an allocation failure in atomic context. Bluetooth 6LoWPAN is not a broad default network path and normally requires specific device and interface setup.

This is not directly network-reachable from ordinary IP networking. A nearby Bluetooth peer may influence the environment only indirectly, but the patch context does not justify scoring it as remote reachable or firmware-mediated external influence. Namespaces do not obviously turn this into a broad unprivileged attack surface unless the product delegates access to the relevant Bluetooth 6LoWPAN interface.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS issue. The theoretical impact is a kernel crash, but the trigger depends on memory allocation failure and a non-default Bluetooth 6LoWPAN configuration. There is no evidence of confidentiality impact, integrity impact, privilege escalation, attacker-controlled reclaim, stale object reuse, or write primitive.

## 5. One-sentence report phrase

Bluetooth 6LoWPAN multicast transmit handling can dereference a NULL skb clone after `skb_clone()` allocation failure, causing a local denial of service under memory pressure when a configured Bluetooth 6LoWPAN interface is used.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed in normal triage because it is a conditional local NULL pointer dereference DoS with no supported memory corruption or privilege escalation primitive.

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

Patch: Bluetooth: 6lowpan: check skb_clone() return value in send_mcast_pkt() [ Upstream
Commit: 9afcb5ea080af13aab37930da627db43bd277665
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9afcb5ea080af13aab37930da627db43bd277665

Changed files:
  net/bluetooth/6lowpan.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=9afcb5ea080af13aab37930da627db43bd277665

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

The original Linux kernel CVE announcement for CVE-2026-63991 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63991

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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).