[CVE-2026-72336][MODERATE REGULAR] Bluetooth: 6lowpan: hold L2CAP conn across debugfs control [ Upstream

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

CVE: CVE-2026-72336
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: Bluetooth: 6lowpan: hold L2CAP conn across debugfs control [ Upstream
Commit: ba1f1ef6522e63aa5dd29805b4390ea2ccadf05e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ba1f1ef6522e63aa5dd29805b4390ea2ccadf05e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72336
Analysis date: Sat, 15 Aug 2026 21:11:49 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Bluetooth 6LoWPAN debugfs control can race with HCI disconnect or device close and dereference a freed L2CAP connection, causing a confirmed slab use after free and local kernel crash, with practical exposure depending on local debugfs or Bluetooth management privileges.

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

The original Linux kernel CVE announcement for CVE-2026-72336 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72336

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-72336	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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'Bluetooth 6LoWPAN debugfs control can hit a race where get_l2cap_conn returns an L2CAP connection pointer after dropping HCI device protection without holding a reference. A concurrent HCI disconnect or device close can free the same connection before lowpan_control_write later dereferences it, producing a slab use-after-free in KASAN and a kernel crash. For the CVSS the PR:L is used for paranoid scoring because a reduced-capability local process or service with access to the Bluetooth debugfs control file could trigger the write path, while typical default systems may require administrator access to debugfs. The issue is not network reachable by itself because the attacker needs local access to the debugfs control interface. Impact is at least local denial of service and in worst case may allow confidentiality or integrity impact due to use-after-free memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 3)	YES	REMOTE READ OOB DANGER UAF NETWORK HARDWARE DEBUGFS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Paranoid scoring signals:

* Local unprivileged or reduced privilege trigger: +1. The main write path is local debugfs control. Default debugfs access is usually privileged, but a reduced capability service or delegated Bluetooth control context could plausibly reach it.
* Memory corruption, weak race UAF primitive: +1. KASAN confirms slab use after free in `lowpan_control_write`, but the patch shows stale pointer dereference after concurrent disconnect or close, not controlled reclaim, write after free, callback hijack, or type confusion.
* Real lifetime corruption: +1. The bug is a missing reference hold on `struct l2cap_conn` across an unlocked lifetime window.
* Reliable kernel crash / strong DoS: +1. The reproduced KASAN report shows a kernel UAF read and crash path.
* Hard or timing dependent race: normally -1, but not applied to the paranoid score because the commit gives a concrete two path race and validation reproduced the issue.
* Weak LPE concern: +0. Race UAF downgrade rule applies. No attacker controlled reclaim, stale object replacement, write after free, callback dispatch, refcount takeover, or arbitrary write is shown.

Conservative scoring signals:

* Memory corruption, weak race UAF primitive: +1.
* Real lifetime corruption: +1.
* Reliable kernel crash / strong DoS: +1.
* Requires admin or root in typical deployments: -2. debugfs control access and HCI device close paths are commonly privileged.
* Hard or unreliable race: -1. The conservative view treats the timing window as nontrivial.
* Conservative total is floored at 1 because this is confirmed kernel UAF and should not be treated as pure correctness cleanup.

## 3. Reachability analysis

The bug is triggered locally through Bluetooth 6LoWPAN debugfs control while a concurrent HCI disconnect or device close tears down the same L2CAP connection. It is not directly network reachable because the attacker needs local access to the debugfs control interface. In typical deployments debugfs write access and HCI device management are restricted to root or privileged services, so the conservative privilege interpretation is high privilege. A more paranoid interpretation is PR:L style reachability if a reduced capability Bluetooth management service, containerized root with delegated device access, or misconfigured debugfs permissions can issue the control write and influence disconnect timing.

## 4. Severity interpretation

This is not an ordinary low risk cleanup because the patch fixes a real lifetime bug with a reproduced KASAN slab use after free. Realistic demonstrated impact is local kernel crash / DoS. Theoretical privilege escalation is not strongly supported by the patch because only a stale read dereference is shown, with no reclaim control or write primitive. Therefore it is best treated as Actionable Moderate at minimum, with manual review required due to confirmed kernel UAF, but not automatically a Strong Important candidate without additional exploitability evidence.

## 5. One-sentence report phrase

Bluetooth 6LoWPAN debugfs control can race with HCI disconnect or device close and dereference a freed L2CAP connection, causing a confirmed slab use after free and local kernel crash, with practical exposure depending on local debugfs or Bluetooth management privileges.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed kernel UAF plus race and lifetime bug. The current evidence supports DoS reliably, while LPE remains only a paranoid concern unless further analysis shows controlled reclaim, object replacement, write after free, or callback control.

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

Patch: Bluetooth: 6lowpan: hold L2CAP conn across debugfs control [ Upstream
Commit: ba1f1ef6522e63aa5dd29805b4390ea2ccadf05e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ba1f1ef6522e63aa5dd29805b4390ea2ccadf05e

Changed files:
  net/bluetooth/6lowpan.c
  arch/x86/entry/syscall_64.c
  net/bluetooth/l2cap_core.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=ba1f1ef6522e63aa5dd29805b4390ea2ccadf05e

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

The original Linux kernel CVE announcement for CVE-2026-72336 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72336

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:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 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).