[CVE-2026-52964][MODERATE REGULAR] ALSA: usb-audio: Bound MIDI 2.0 endpoint descriptor scans

[CVE-2026-52964][MODERATE REGULAR] ALSA: usb-audio: Bound MIDI 2.0 endpoint descriptor scans

[AL-KERNEL]

CVE: CVE-2026-52964
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ALSA: usb-audio: Bound MIDI 2.0 endpoint descriptor scans
Commit: fafc97bd01e4c737eaeafadfdadb1af4bbfa7307
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fafc97bd01e4c737eaeafadfdadb1af4bbfa7307
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52964
Analysis date: Thu, 25 Jun 2026 08:41:13 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A malformed USB MIDI 2.0 device can cause the ALSA usb-audio endpoint descriptor scanner to read past the walked descriptor, primarily creating a physical-access denial-of-service risk with limited memory-safety concern.

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

The original Linux kernel CVE announcement for CVE-2026-52964 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52964

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-2026-52964	MODERATE	CHECK	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-126;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.7';DESCR 'USB MIDI 2.0 endpoint descriptor scanning can accept a malformed endpoint extra descriptor whose bLength exceeds the remaining bytes in the scan. Later MIDI 2.0 parsing may read baAssoGrpTrmBlkID entries past the walked descriptor, which is an out-of-bounds read in kernel USB audio parsing. For the CVSS the PR:N is used because an attacker does not need an account on the target if they can attach a malicious USB audio device or present one through USB redirection to the host. The issue is not network reachable in the normal CVSS sense and usually requires physical or equivalent device-level access. Impact is primarily denial of service through a possible kernel warning or crash. For the paranoid score, limited confidentiality or integrity impact is kept possible because malformed descriptor bytes can influence kernel parsing, but there is no evidence of an out-of-bounds write or arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3) 	MAYBE	USB SIMPLEFIX HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Firmware-mediated external influence: +1. A malformed USB MIDI 2.0 device controls endpoint descriptor contents consumed by kernel USB audio parsing.
* Privileged kernel/device-management parsing path: +1. The bug is in kernel-side USB descriptor parsing for a device-provided structure.
* Physical-only or equivalent device access: -2. Normal triggering requires attaching a malicious USB device or equivalent USB redirection/passthrough.
* Availability impact realistic: +1. The malformed descriptor can cause out-of-bounds reads during parsing and may lead to warning, failed probe, or crash depending on allocation layout.

Paranoid additional signals:

* Weak/indirect corruption candidate: +1. This is an out-of-bounds read caused by accepting an overlong descriptor during descriptor walking, but no write primitive is shown.
* Confidentiality impact plausible: +1. Limited kernel memory over-read is possible in principle, although no user-visible disclosure path is demonstrated.
* Integrity impact plausible: +1. Only weakly plausible because malformed descriptor bytes may influence subsequent parsing state, but there is no OOB write or arbitrary write primitive.

Signals not applied:

* Remote reachable: +0. This is not network-triggerable in the normal CVSS sense.
* Strong LPE plausibility: +0. No UAF reclaim, arbitrary write, object confusion, or controlled overwrite is shown.
* Strong generic memory corruption: +0. The patch supports OOB read, not OOB write or lifetime corruption.
* Reliable strong DoS: +0. Crash is possible, but the patch does not prove a reliable host-wide crash primitive.

## 3. Reachability analysis

A malicious USB MIDI 2.0 device, or a device presented through USB redirection or passthrough, can trigger the parser by supplying malformed endpoint-extra descriptors. No local account is required if the attacker controls the physical or virtual USB device path. Containers and namespaces generally do not reduce the physical/device requirement unless USB device assignment is delegated to an untrusted tenant. The affected code is reachable when USB audio MIDI 2.0 UMP support is enabled and the device is enumerated. This is not a default network exposure.

## 4. Severity interpretation

This behaves more like a low-to-borderline Moderate issue than an Important-class vulnerability. The core bug is a bounded descriptor-walking validation failure leading to an out-of-bounds read. The realistic impact is probe failure, warning, or kernel crash. The theoretical concern is limited confidentiality or integrity impact from reading past the descriptor, but the patch does not show disclosure, write, UAF, object lifetime corruption, or a practical privilege escalation path.

## 5. One-sentence report phrase

A malformed USB MIDI 2.0 device can cause the ALSA usb-audio endpoint descriptor scanner to read past the walked descriptor, primarily creating a physical-access denial-of-service risk with limited memory-safety concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is physical or USB-redirection dependent and not remotely reachable, but it is still malformed external device input causing an out-of-bounds read in kernel parsing code, so it should not be blindly auto-closed without confirming exposure and whether any disclosure or crash path exists.

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

Patch: ALSA: usb-audio: Bound MIDI 2.0 endpoint descriptor scans
Commit: fafc97bd01e4c737eaeafadfdadb1af4bbfa7307
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fafc97bd01e4c737eaeafadfdadb1af4bbfa7307

Commit description:

The USB MIDI 2.0 endpoint parser has the same descriptor walking
pattern as the legacy MIDI parser. It validates bLength against
bNumGrpTrmBlock before reading baAssoGrpTrmBlkID[], but not against the
remaining bytes in the endpoint-extra scan.

A malformed device can therefore make later baAssoGrpTrmBlkID[] reads
consume bytes past the walked descriptor.

Reject zero-length and overlong descriptors while walking endpoint
extras.

Fixes: ff49d1d ("ALSA: usb-audio: USB MIDI 2.0 UMP support")
Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260507-usb-midi-endpoint-scan-bounds-v1-2-329d7348160e@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/midi2.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=fafc97bd01e4c737eaeafadfdadb1af4bbfa7307

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

The original Linux kernel CVE announcement for CVE-2026-52964 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52964

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:N/S:U/C:N/I:N/A:L
  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: 4

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).