[CVE-2026-74499][IMPORTANT] ALSA: usb-audio: fix OOB write in snd_usbmidi_akai_output()

[CVE-2026-74499][IMPORTANT] ALSA: usb-audio: fix OOB write in snd_usbmidi_akai_output()

[AL-KERNEL]

CVE: CVE-2026-74499
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ALSA: usb-audio: fix OOB write in snd_usbmidi_akai_output()
Commit: 29a4c29943631301e85f5e9d10f25741bd78e7ba
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=29a4c29943631301e85f5e9d10f25741bd78e7ba
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74499
Analysis date: Sat, 15 Aug 2026 11:02:54 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious USB MIDI device advertising a tiny bulk OUT transfer size can trigger an attacker-controlled kernel heap out-of-bounds write in `snd_usbmidi_akai_output()` when a local process writes to the ALSA MIDI device node, making this a strong manual-review candidate for DoS and possible privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74499 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74499

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: IMPORTANT
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-2026-74499	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-195;CWE-131;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A heap out-of-bounds write in the ALSA usb-audio MIDI path occurs because snd_usbmidi_akai_output computes buf_end as a signed value from a device controlled max_transfer. A malicious USB MIDI device can advertise a tiny bulk OUT transfer size, making buf_end negative and causing the loop bound to wrap when compared with urb transfer_buffer_length. When a local process writes to the created /dev/snd/midiC*D* node, the function can append attacker-controlled SysEx framing and payload bytes past the end of the URB transfer buffer. For the CVSS the PR:L is used for the paranoid vector because a local user or process may be able to write to the exposed MIDI device node after the malicious USB device is present. The issue is not network reachable and requires local or physical USB exposure. Impact is at least denial of service via kernel crash and may plausibly allow privilege escalation because the primitive is controlled kernel heap memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	WRITE OOB USB HARDWARE LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

* Firmware-mediated external influence: +1. A malicious USB device can influence `ep->max_transfer` via advertised endpoint properties.
* Local unprivileged trigger: +1. After the device is present, a local process may trigger the path by writing to the created `/dev/snd/midiC*D*` node, depending on device-node permissions.
* Strong LPE plausibility: +2. The primitive is attacker-length- and content-controlled kernel heap OOB write, which is a plausible privilege-escalation class.
* Memory corruption, strong corruption primitive: +2. This is an explicit heap out-of-bounds write, not merely a NULL dereference or validation-only bug.
* Reliable kernel crash / strong DoS: +1. Heap OOB write in a USB audio driver is realistically crashable.
* Privileged kernel/device-management memory corruption path: +1. The corruption is in a trusted kernel driver path influenced by malicious USB device descriptors and MIDI output behavior.
* Confidentiality impact plausible: +1. Kernel heap corruption can plausibly be shaped into disclosure in worst-case analysis.
* Integrity impact plausible: +1. Kernel heap OOB write gives plausible integrity impact and LPE concern.
* Physical-only or rare hardware-only condition: -2. The realistic entry condition is malicious local USB or equivalent USB gadget exposure, not network reachability.

Conservative total: 7.
Paranoid total: 8 if counting the firmware/device influence signal in addition to local triggering.

## 3. Reachability analysis

The bug is not network reachable. The realistic attacker needs either physical USB access, control of a USB gadget, or another way to present a malicious USB MIDI device to the target. After enumeration, the final trigger requires a process to write to the ALSA MIDI device node. On desktop systems this may be available to a logged-in user through ACLs or audio group policy, while on hardened servers it may require stronger local privileges or no practical exposure at all.

Namespaces and containers do not generally make the physical USB precondition easier unless the device node is passed through into the container. If `/dev/snd/midiC*D*` is exposed to a container or sandboxed service, this can become a reduced-privilege local attack surface.

Call-site confidence: high. The commit directly describes the affected function, the arithmetic wrap, the loop condition, the buffer size relation, and the resulting heap OOB write.

## 4. Severity interpretation

This behaves more like an Important-class kernel vulnerability than an ordinary Moderate. The realistic deployment constraint is significant because the attacker needs malicious USB exposure and MIDI device-node access. However, the underlying primitive is strong: attacker-controlled kernel heap out-of-bounds write with controllable length/content. That is enough for manual review and Important-candidate handling even if no working LPE exploit is shown.

Theoretical impact includes privilege escalation. Realistic demonstrated impact from the patch is kernel memory corruption and likely crash.

## 5. One-sentence report phrase

A malicious USB MIDI device advertising a tiny bulk OUT transfer size can trigger an attacker-controlled kernel heap out-of-bounds write in `snd_usbmidi_akai_output()` when a local process writes to the ALSA MIDI device node, making this a strong manual-review candidate for DoS and possible privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is explicit attacker-controlled kernel heap memory corruption, not a resource leak or validation-only issue. The physical USB requirement lowers exposure, but the primitive is strong enough to avoid auto-close.

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

Patch: ALSA: usb-audio: fix OOB write in snd_usbmidi_akai_output()
Commit: 29a4c29943631301e85f5e9d10f25741bd78e7ba
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=29a4c29943631301e85f5e9d10f25741bd78e7ba

Commit description:

snd_usbmidi_akai_output() computes its fill-loop bound

	buf_end = ep->max_transfer - MAX_AKAI_SYSEX_LEN - 1;

as a signed int, so a small device-advertised bulk-OUT max_transfer
makes buf_end negative.  The loop guard then compares the u32
urb->transfer_buffer_length against that negative int: the usual
arithmetic conversion turns buf_end into a large unsigned value, so the
guard stays true and each iteration keeps appending SysEx framing and
payload bytes past the end of the URB transfer buffer, which is only
max_transfer bytes long.

A USB device that advertises a tiny bulk-OUT endpoint can therefore
trigger an attacker-length- and content-controlled heap out-of-bounds
write when a process writes to the created /dev/snd/midiC*D* node.

Return early when there is no room for even one SysEx, so the loop is
never entered with a bound that would wrap.  The loop is the last
statement of the function, so bailing out is equivalent to it not
running.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 4434ade ("ALSA: usb-audio: add support for Akai MPD16")
Suggested-by: Takashi Iwai <tiwai@suse.de>
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
Link: https://patch.msgid.link/20260726074500.50145-1-baul.lee@xbow.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/midi.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=29a4c29943631301e85f5e9d10f25741bd78e7ba

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

The original Linux kernel CVE announcement for CVE-2026-74499 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74499

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:H/I:H/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: 7
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).