[CVE-2026-53209][MODERATE 7.0] Bluetooth: hci_sync: reject oversized Broadcast Announcement prepend

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

CVE: CVE-2026-53209
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Bluetooth: hci_sync: reject oversized Broadcast Announcement prepend
Commit: 10b0e832cc05d7aef4b92bed912cbd4a395d0862
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10b0e832cc05d7aef4b92bed912cbd4a395d0862
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53209
Analysis date: Thu, 25 Jun 2026 06:26:30 -0400
ActionableScore: 6
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local Bluetooth advertising configuration can cause `hci_adv_bcast_annoucement()` to prepend Broadcast Announcement data to an already full extended advertising payload and overrun a temporary kernel buffer, creating a kernel OOB write with DoS impact and possible limited privilege escalation concern.

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

The original Linux kernel CVE announcement for CVE-2026-53209 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53209

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: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-53209	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-120;*CWE-20;*CWE-787;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'An out-of-bounds write can occur in the Bluetooth hci_sync Broadcast Announcement path because hci_adv_bcast_annoucement prepends service data to an existing advertising payload and then copies the old payload into a temporary kernel buffer. If the existing advertising instance already holds the maximum extended advertising payload, the combined length can exceed the buffer size and memcpy can corrupt kernel memory. For the CVSS the PR:L is used because triggering requires local ability to configure Bluetooth advertising state, for example through a Bluetooth management interface, BlueZ policy, or delegated network administration capability, but it does not normally require full root level control. The issue is not directly network reachable from a remote Bluetooth peer because the attacker must influence local advertising configuration rather than only send radio traffic. Impact is at least local denial of service via kernel crash and in the paranoid case may allow confidentiality or integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE SIMPLEFIX ERRORPATH NETWORK HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Generic memory corruption, strong primitive: +2
  The patch prevents `memcpy(ad + len, adv->adv_data, adv->adv_data_len)` from overrunning the temporary kernel buffer `ad`.

* Weak LPE concern: +1
  This is a kernel OOB write with attacker-influenced advertising data, but the patch does not show a demonstrated reclaim primitive, object confusion, function pointer overwrite, or controlled target selection.

* Reliable kernel crash / strong DoS: +1
  A kernel buffer overrun in the Bluetooth advertising path can realistically crash or corrupt kernel state.

* Corruption primitive constrained: -1
  The overwrite target is a temporary internal buffer and the overrun size appears bounded by the Broadcast Announcement prepend length.

* Requires privileged Bluetooth advertising control in typical deployments: -2
  Direct triggering usually requires Bluetooth management access, commonly gated by `CAP_NET_ADMIN` or by BlueZ policy.

Conservative total: 3

Paranoid scoring differences:

* Local unprivileged or reduced-privilege trigger possible: +1
  Some systems expose Bluetooth advertising configuration through BlueZ or delegated policies, so full host root should not be assumed in all deployments.

* Confidentiality impact plausible: +1
  Kernel memory corruption can plausibly affect sensitive kernel data, although no direct leak is demonstrated.

* Integrity impact plausible: +1
  OOB write may corrupt adjacent kernel state, although no controlled write target is demonstrated.

* Reduced privilege penalty instead of full CAP penalty: +0 to -1
  In desktop or delegated-service deployments, the attacker may be a local user or reduced-privilege service account rather than full root.

Paranoid total: 6

## 3. Reachability analysis

The bug is triggered through local configuration of Bluetooth advertising state, not by a remote Bluetooth peer sending radio traffic. The attacker must be able to create or modify an advertising instance that already holds a large extended advertising payload and then cause the Broadcast Announcement prepend path to run.

In typical low-level deployments this requires privileged Bluetooth management access, often `CAP_NET_ADMIN`. In some real systems, BlueZ D-Bus policy or a service account may allow reduced-privilege local users to influence advertising configuration, which justifies the paranoid local PR:L style interpretation.

The path is not a core network packet receive path and is not Internet reachable. Exposure depends on Bluetooth support, relevant advertising features, and local policy.

## 4. Severity interpretation

This is more than an ordinary Moderate issue because the patch explicitly prevents a kernel buffer overrun before `memcpy`. The realistic impact may be local DoS or constrained kernel memory corruption, but the bug class is an OOB write in kernel context and deserves manual review.

Theoretical privilege escalation is plausible enough for triage because attacker-influenced advertising data can be copied past the end of a temporary kernel buffer. However, the patch does not demonstrate a reliable LPE primitive, controlled overwrite target, type confusion, or function pointer corruption. Therefore the conservative result stays borderline, while the paranoid score reaches Actionable Moderate.

## 5. One-sentence report phrase

A local Bluetooth advertising configuration can cause `hci_adv_bcast_annoucement()` to prepend Broadcast Announcement data to an already full extended advertising payload and overrun a temporary kernel buffer, creating a kernel OOB write with DoS impact and possible limited privilege escalation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a kernel OOB write, not a pure validation cleanup or resource leak. The realistic exploitability depends on Bluetooth management permissions and overwrite constraints, but the memory corruption class is sufficient to require manual triage.

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

Patch: Bluetooth: hci_sync: reject oversized Broadcast Announcement prepend
Commit: 10b0e832cc05d7aef4b92bed912cbd4a395d0862
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10b0e832cc05d7aef4b92bed912cbd4a395d0862

Commit description:

Existing advertising instances can already hold the maximum extended
advertising payload. When hci_adv_bcast_annoucement() prepends the
Broadcast Announcement service data to that payload, the combined data
may no longer fit in the temporary buffer used to rebuild the
advertising data.

Reject that case before copying the existing payload and report the
failure through the device log. This keeps the existing advertising
data intact and avoids overrunning the temporary buffer.

Fixes: 5725bc6 ("Bluetooth: hci_sync: Fix broadcast/PA when using an existing instance")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Assisted-by: Codex:GPT-5.4
Signed-off-by: Yuqi Xu <xuyq21@lenovo.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bluetooth/hci_sync.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=10b0e832cc05d7aef4b92bed912cbd4a395d0862

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

The original Linux kernel CVE announcement for CVE-2026-53209 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53209

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:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 6

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).