[CVE-2026-74508][MODERATE 7.0] Bluetooth: HIDP: reject frames without a transaction header

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

CVE: CVE-2026-74508
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: Bluetooth: HIDP: reject frames without a transaction header
Commit: 46ca5ab39737d7c6f9ca77ecf714cdcfa6caaeec
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46ca5ab39737d7c6f9ca77ecf714cdcfa6caaeec
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74508
Analysis date: Sat, 15 Aug 2026 09:09:22 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Bluetooth HIDP receive paths consumed a transaction header byte before verifying that the L2CAP SDU contained one, allowing a connected HIDP peer or malformed adjacent Bluetooth packet to use an uninitialized or out-of-SDU byte and potentially terminate the HIDP session.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74508 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74508

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
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CVE-2026-74508	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L';CWE-908;CWE-125;*CWE-20;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.3';DESCR 'Bluetooth HIDP control and interrupt receive paths read the first byte of skb data before verifying that the L2CAP SDU contains a HIDP transaction header. A connected HIDP peer can send an empty basic mode SDU and make the kernel consume an uninitialized byte from skb tailroom. A malformed lower layer Bluetooth packet can also place an extra byte after a declared zero length L2CAP payload, which may be interpreted as a HIDP virtual cable unplug command and terminate the HIDP session. For the CVSS the PR:N is used because exploitation does not require local privileges on the victim once the attacker can act as a connected Bluetooth HIDP peer or inject the malformed adjacent Bluetooth traffic. The issue is adjacent network reachable rather than Internet reachable. Impact is primarily denial of service against the HIDP session. For the paranoid score, choose the highest still defensible interpretation supported by the bug class and patch context, with preference for manual review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE WRITE KASAN OOB USB NETWORK SKB PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Remote reachable / network-triggerable: +2**
  The bug is reachable from a connected Bluetooth HIDP peer or malformed Bluetooth lower-layer traffic. This is adjacent-network reachable, not Internet remote.
* **Availability impact realistic: +1**
  A controlled malformed packet can be interpreted as `HIDP_CTRL_VIRTUAL_CABLE_UNPLUG` and terminate the HIDP session.

Conservative total: **3**

Paranoid additional signals:

* **Integrity impact plausible: +1**
  The attacker-controlled extra byte can alter HIDP protocol state by forcing a virtual cable unplug action.
* **Broad/default/common subsystem exposure: +1**
  Bluetooth HIDP is a common client-side input-device path on desktops and laptops, although not normally exposed on servers.

Paranoid total: **5**

Not awarded:

* **Generic memory corruption: +0**
  This is an uninitialized or out-of-SDU byte read, not UAF, double-free, OOB write, type confusion, or arbitrary write.
* **Privilege escalation plausible: +0**
  No reclaim, write primitive, callback control, refcount abuse, or object replacement is shown.
* **Reliable kernel crash / strong DoS: +0**
  The demonstrated impact is HIDP session termination, not a host-wide kernel panic.

## 3. Reachability analysis

A connected Bluetooth HIDP peer can trigger the issue by sending an empty basic-mode SDU. A malformed lower-layer HCI ACL packet can also provide an extra byte after a declared zero-length L2CAP payload, which the old HIDP path may consume despite `skb->len` being reduced.

No local user privileges on the victim are required once the attacker can act as a connected Bluetooth HIDP peer or inject adjacent Bluetooth traffic. This is **AV:A-style adjacent exposure**, not general Internet reachability. Containers and namespaces are not the primary factor here because the trigger is through Bluetooth protocol input, not a local syscall-only interface.

Call-site confidence: **high**. The affected reads and the fixed call sites are directly shown in `hidp_recv_ctrl_frame()` and `hidp_recv_intr_frame()`.

## 4. Severity interpretation

This behaves more like **Actionable Moderate** than Important. The bug has an externally reachable Bluetooth protocol trigger and a demonstrated state-changing DoS against the HIDP session, but the patch does not show a strong memory corruption primitive.

Theoretical concern comes from consuming an uninitialized or logically out-of-SDU byte. Realistic evidence supports protocol-state manipulation and session disruption, not kernel code execution or privilege escalation.

## 5. One-sentence report phrase

Bluetooth HIDP receive paths consumed a transaction header byte before verifying that the L2CAP SDU contained one, allowing a connected HIDP peer or malformed adjacent Bluetooth packet to use an uninitialized or out-of-SDU byte and potentially terminate the HIDP session.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

YES REQUIRES MANUAL CHECK because the issue is adjacent-network reachable Bluetooth input and involves uninitialized or out-of-SDU byte consumption, even though the demonstrated impact is limited to HIDP session DoS and protocol-state manipulation rather than proven kernel memory corruption.

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

Patch: Bluetooth: HIDP: reject frames without a transaction header
Commit: 46ca5ab39737d7c6f9ca77ecf714cdcfa6caaeec
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46ca5ab39737d7c6f9ca77ecf714cdcfa6caaeec

Commit description:

hidp_recv_ctrl_frame() and hidp_recv_intr_frame() read skb->data[0]
before checking that the L2CAP SDU contains a transaction header. A
connected HIDP peer can send an empty basic-mode SDU and make both paths
use an uninitialized byte from skb tailroom.

KMSAN reports the use in hidp_session_run(), with the uninitialized value
originating in __alloc_skb() through vhci_write(). The control path
produces two reports and the interrupt path produces one.

The byte can also be controlled by a malformed lower-layer packet. If an
HCI ACL packet contains an L2CAP PDU with a declared zero-length payload
followed by an extra 0x15 byte, l2cap_recv_acldata() reduces skb->len to
the declared PDU length before dispatch. The current HIDP path nevertheless
consumes the extra byte as HIDP_TRANS_HID_CONTROL |
HIDP_CTRL_VIRTUAL_CABLE_UNPLUG and terminates the HIDP session. With this
change, the same packet is discarded and a subsequent feature report
request succeeds.

Pull the transaction header with skb_pull_data() and discard frames that
do not contain it.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Sangho Lee <kudo3228@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/hidp/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=46ca5ab39737d7c6f9ca77ecf714cdcfa6caaeec

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

The original Linux kernel CVE announcement for CVE-2026-74508 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74508

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:L/I:L/A:L
  The Best / paranoid CVSS score: 6.3

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: 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: NO
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).