[CVE-2026-64487][MODERATE REGULAR] ALSA: caiaq: fix out-of-bounds read in the Traktor Kontrol S4 input parser

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

CVE: CVE-2026-64487
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ALSA: caiaq: fix out-of-bounds read in the Traktor Kontrol S4 input parser
Commit: de5f9edc705497b1b2c6b173b22f283486d2fd91
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=de5f9edc705497b1b2c6b173b22f283486d2fd91
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64487
Analysis date: Sat, 25 Jul 2026 10:11:20 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Borderline manual review due kernel OOB read
Manual review required: YES

Summary:
A malformed Traktor Kontrol S4 USB input packet can make the ALSA caiaq parser underflow its length counter and read beyond the endpoint buffer, causing a physical device triggered kernel OOB read with realistic DoS impact but no demonstrated privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-64487 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64487

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-2026-64487	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-125;CWE-191;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '5.7';DESCR 'A malformed Traktor Kontrol S4 USB input packet can make snd_usb_caiaq_tks4_dispatch process a trailing partial block as if it were a full 16 byte message. Because len is unsigned, subtracting TKS4_MSGBLOCK_SIZE from a value between 1 and 15 can underflow and make the loop continue while buf walks beyond the endpoint buffer. This creates an out of bounds read in the kernel and can lead to a kernel crash or other availability impact. For the CVSS the PR:N is used because the attacker does not need an account on the target if they can attach or control a USB device that is handled by this driver. The issue is not network reachable. The base impact is denial of service only, while the paranoid impact keeps C:L and I:L because an invalid kernel read may influence parsed input state, but the patch does not support an OOB write or strong privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Borderline manual review due kernel OOB read (with actual score 2)	YES	WRITE OOB USB HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: **1**
* Paranoid score: **3**
* Final recommended bucket: **Ordinary Moderate / Borderline manual review due kernel OOB read**

## 2. Signal breakdown

Conservative signals:

* Firmware-mediated external influence: **+1**. The malformed length comes from a USB device controlled by external hardware or firmware.
* Reliable kernel crash / strong DoS: **+1**. The unsigned underflow can make the parser walk past the 512 byte endpoint buffer and read out of bounds until invalid memory or a bogus block is hit.
* Physical-only or rare hardware-only condition: **-2**. Triggering requires a malicious or compromised Traktor Kontrol S4 compatible USB device or equivalent physical USB control.
* Rare AND difficult-to-reach subsystem/configuration: **-1**. This is a device-specific ALSA USB caiaq parser, not a broad default kernel data path.
* Minimum memory-safety review relevance: **+2 floor adjustment to 1 effective lower score**. The bug is still a real kernel OOB read, but without write, UAF, reclaim, callback, or disclosure primitive.

Paranoid additions:

* Weak/indirect corruption candidate: **+1**. The parser can interpret out-of-bounds bytes as message block data and may emit unintended input events, but this is not a write primitive.
* Confidentiality impact plausible: **+1**. Limited kernel OOB read exists, but no user-visible disclosure path is shown.
* Integrity impact plausible: **+1**. Parsed OOB data may affect input-event interpretation, but not kernel object integrity.
* Physical-only condition still strongly limits practical exploitability: **-2**.

## 3. Reachability analysis

The bug is triggered by device-supplied USB input length in the Traktor Kontrol S4 endpoint parser. A local Linux user account is not required if the attacker can attach or emulate the affected USB device, but this is not network reachable. Containers and namespaces do not materially lower the requirement because the trigger is at the USB device boundary, not a namespaced syscall or netlink interface. The affected driver is device-specific and not a common default-exposed path unless the relevant USB audio/MIDI hardware is connected and bound to snd-usb-caiaq.

Call-site confidence: **high**, because the vulnerable loop and the fixed loop condition are directly shown in the patch.

## 4. Severity interpretation

This behaves like an **ordinary Moderate** device-triggered kernel memory-safety bug, with **manual review recommended** because it is a real OOB read in kernel context. The realistic impact is kernel crash or device/input misbehavior. Theoretical confidentiality or integrity impact is weak because the patch shows read-only OOB access and unsigned underflow, but does not show OOB write, UAF, attacker-controlled reclaim, callback corruption, or an information disclosure path.

## 5. One-sentence report phrase

A malformed Traktor Kontrol S4 USB input packet can make the ALSA caiaq parser underflow its length counter and read beyond the endpoint buffer, causing a physical device triggered kernel OOB read with realistic DoS impact but no demonstrated privilege escalation primitive.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Reason: this is a kernel out-of-bounds read reachable from device-supplied USB data, but the practical exposure is physical/device-specific and the patch does not support a strong LPE or remote attack path.

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

Patch: ALSA: caiaq: fix out-of-bounds read in the Traktor Kontrol S4 input parser
Commit: de5f9edc705497b1b2c6b173b22f283486d2fd91
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=de5f9edc705497b1b2c6b173b22f283486d2fd91

Commit description:

snd_usb_caiaq_tks4_dispatch() decodes the Traktor Kontrol S4 input
stream in fixed 16-byte (TKS4_MSGBLOCK_SIZE) message blocks. On every
iteration it advances buf and subtracts the block size while looping on
"while (len)".

len is urb->actual_length. That value is supplied by the device and is
not guaranteed to be a multiple of 16. When a final short block leaves
len between 1 and 15, the loop runs once more, reads up to buf[15], and
then does "len -= TKS4_MSGBLOCK_SIZE". As len is unsigned this underflows
to a huge value. The loop then keeps iterating and walking buf far past
the end of the 512-byte ep4_in_buf, reading out of bounds until a bogus
block id happens to be hit.

Iterate only while a full message block is available. This stops the
unsigned underflow and silently drops any trailing partial block, which
carries no complete control value anyway.

The sibling endpoint-4 parsers are not affected. The Traktor Kontrol X1
and Maschine arms in snd_usb_caiaq_ep4_reply_dispatch() floor
urb->actual_length before dispatching.

Fixes: 15c5ab6 ("ALSA: snd-usb-caiaq: Add support for Traktor Kontrol S4")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Link: https://patch.msgid.link/178176259547.3343534.2724779296835237429@maoyixie.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/caiaq/input.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=de5f9edc705497b1b2c6b173b22f283486d2fd91

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

The original Linux kernel CVE announcement for CVE-2026-64487 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64487

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:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.7

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: 3

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).