[CVE-2026-53208][MODERATE 7.0] Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig

[CVE-2026-53208][MODERATE 7.0] Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig

[AL-KERNEL]

CVE: CVE-2026-53208
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig
Commit: e05c4ac575b457978a7ef441053394169084869c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e05c4ac575b457978a7ef441053394169084869c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53208
Analysis date: Thu, 25 Jun 2026 06:12:13 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An unauthenticated Bluetooth BR/EDR peer within radio range can send an oversized L2CAP signaling packet with many packed ECHO_REQ commands before pairing, forcing many ECHO_RSP frames and causing Bluetooth service degradation through response amplification.

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

The original Linux kernel CVE announcement for CVE-2026-53208 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53208

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.

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

CVE-2026-53208	MODERATE	CHECK	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-400;CWE-770;*CWE-20;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'Bluetooth BR/EDR L2CAP signaling accepted packets larger than the BR/EDR signaling MTU and processed every command inside the oversized packet. A nearby Bluetooth peer can trigger this before pairing by sending a fixed channel CID 0x0001 signaling packet containing many packed L2CAP_ECHO_REQ commands, causing the target to transmit many ECHO_RSP frames from a single input packet. For the CVSS the PR:N is used because no pairing or local account on the target is required, only Bluetooth radio reachability. The attack is adjacent network only because the peer must be within Bluetooth BR/EDR radio range. Impact is denial of service or Bluetooth service degradation through response amplification and radio airtime consumption. There is no clear evidence of memory corruption, information disclosure, or privilege escalation from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE NETWORK SKB TIMER KPANIC PACKET  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	NO	NO	checked

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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

* Remote reachable / network-triggerable: +2
  Bluetooth BR/EDR is adjacent-network reachable. The attacker only needs radio proximity and does not need pairing.
* Reliable kernel crash / strong DoS: +0 conservative, +1 paranoid
  The patch demonstrates reliable response amplification and airtime consumption, but not a kernel crash.
* Availability impact realistic: +1
  A single 681-byte packet caused 168 ECHO_RSP frames over about 220 ms. Sustained abuse can degrade Bluetooth service availability.
* Broad/default/common subsystem: +1
  Bluetooth BR/EDR is common on laptops, desktops, and embedded systems when Bluetooth is enabled.
* No memory corruption: +0
  The patch adds MTU validation and early rejection. It does not show UAF, OOB write, double-free, type confusion, or stale pointer reuse.
* No confidentiality or integrity impact: +0
  The described primitive is response amplification and resource consumption only.

## 3. Reachability analysis

A nearby Bluetooth BR/EDR peer can trigger the bug before pairing by sending an oversized fixed-channel CID 0x0001 L2CAP signaling packet containing many packed ECHO_REQ commands. No local account, pairing, or target-side privileges are required. Containers and namespaces are not relevant because the attack is over the Bluetooth radio interface. The path is reachable only when Bluetooth BR/EDR is enabled and the attacker is within radio range, so this is adjacent-network exposure rather than Internet-reachable remote exposure.

## 4. Severity interpretation

This behaves like an actionable Moderate adjacent-network DoS issue, not an Important memory-corruption vulnerability. The practical impact is unauthenticated Bluetooth response amplification and airtime/resource consumption. There is no realistic evidence in the patch of privilege escalation, information disclosure, or kernel memory corruption. The paranoid score reaches 5 because the attack is unauthenticated, adjacent-network reachable, and demonstrated on real radio hardware.

## 5. One-sentence report phrase

An unauthenticated Bluetooth BR/EDR peer within radio range can send an oversized L2CAP signaling packet with many packed ECHO_REQ commands before pairing, forcing many ECHO_RSP frames and causing Bluetooth service degradation through response amplification.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is unauthenticated and adjacent-network reachable, with demonstrated real-radio amplification, even though the impact appears limited to DoS and no memory-corruption primitive is shown.

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

Patch: Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig
Commit: e05c4ac575b457978a7ef441053394169084869c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e05c4ac575b457978a7ef441053394169084869c

Commit description:

net/bluetooth/l2cap_core.c:l2cap_sig_channel() accepts BR/EDR
signaling packets up to the channel MTU and dispatches each command
without enforcing the signaling MTU (MTUsig). A Bluetooth BR/EDR peer
within radio range can send a fixed-channel CID 0x0001 packet that is
larger than MTUsig and contains many L2CAP_ECHO_REQ commands before
pairing. In a real-radio stock-kernel run, one 681-byte signaling
packet containing 168 zero-length ECHO_REQ commands made the target
transmit 168 ECHO_RSP frames over about 220 ms.

Impact: a Bluetooth BR/EDR peer within radio range, before pairing, can
force 168 ECHO_RSP frames from one 681-byte fixed-channel signaling
packet containing packed ECHO_REQ commands.

Define Linux's BR/EDR signaling MTU as the spec minimum of 48 bytes and
reject any larger signaling packet with one L2CAP_COMMAND_REJECT_RSP
carrying L2CAP_REJ_MTU_EXCEEDED before any command is dispatched.

The Bluetooth Core spec wording for MTUExceeded says the reject
identifier shall match the first request command in the packet, and
that packets containing only responses shall be silently discarded.
Linux intentionally deviates from that prescription: silently
discarding desynchronizes the peer because the remote stack never
learns its responses were dropped, and locating the first request
command requires walking command headers past MTUsig, i.e. processing
bytes from a packet we have already decided is too large to process.
We therefore always emit one reject and use the identifier from the
first command header, a single fixed-offset byte read.

The unrestricted BR/EDR signaling parser and ECHO_REQ response path both
trace to the initial git import; no later introducing commit is
available for a Fixes tag.

Cc: stable@vger.kernel.org
Suggested-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Link: https://lore.kernel.org/r/20260518002800.1361430-1-michael.bommarito@gmail.com
Link: https://lore.kernel.org/r/20260520135034.1060859-1-michael.bommarito@gmail.com
Link: https://lore.kernel.org/r/20260521000555.3712030-1-michael.bommarito@gmail.com
Assisted-by: Claude:claude-opus-4-7
Assisted-by: Codex:gpt-5-5-xhigh
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
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/l2cap_core.c
  include/net/bluetooth/l2cap.h

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=e05c4ac575b457978a7ef441053394169084869c

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

The original Linux kernel CVE announcement for CVE-2026-53208 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53208

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:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  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).