[CVE-2026-74682][MODERATE 7.0] ALSA: usb-audio: fix OOB write on Type II inbound URBs

[CVE-2026-74682][MODERATE 7.0] ALSA: usb-audio: fix OOB write on Type II inbound URBs

[AL-KERNEL]

CVE: CVE-2026-74682
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ALSA: usb-audio: fix OOB write on Type II inbound URBs
Commit: 6af5f29af7711233ae68d3b25c15d67478468900
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6af5f29af7711233ae68d3b25c15d67478468900
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74682
Analysis date: Sat, 22 Aug 2026 13:53:06 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or emulated USB audio device advertising a Type II capture format can cause `snd-usb-audio` to build an inbound URB whose final iso frame points past the allocated transfer buffer, resulting in a device-controlled slab OOB write and likely kernel crash, with worst-case memory corruption impact requiring manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74682 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74682

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-2026-74682	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H';*CWE-787;CWE-131;CWE-119;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '6.6';DESCR 'A device controlled OOB write exists in the ALSA usb-audio Type II capture path because buffer_size is computed before the extra transfer delimiter packet is added. When prepare_inbound_urb builds iso frame descriptors, the final descriptor can point one packet past the allocated transfer buffer and the USB host controller writes inbound device data there on each transfer. For the CVSS the PR:N is used because the malicious USB device does not need host privileges once it can be attached and the capture stream is opened. The issue is not network reachable and normally requires physical USB access or an equivalent USB device emulation path. Impact is at least denial of service via kernel memory corruption. In the paranoid score, confidentiality and integrity are rated high because this is a real OOB write primitive with device supplied data, even though reliable privilege escalation is not proven from the patch alone.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	WRITE KASAN OOB USB HARDWARE LINUS KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Firmware-mediated external influence: +1. The attacker influence comes from a malicious or emulated USB audio device descriptor and inbound device data, not from a normal local syscall alone.
* Generic memory corruption, strong corruption primitive: +2. The patch and KASAN report show a real slab OOB write caused by an inbound URB descriptor pointing one packet past the allocated transfer buffer.
* Memory layout invariant restoration: +1. The fix restores consistency between `packets`, `buffer_size`, URB packet count, and inbound iso frame layout.
* Reliable kernel crash / strong DoS: +1. KASAN reports one slab OOB write per inbound transfer, so the crash condition is reliable once the affected stream is active.
* Broad/common subsystem exposure: +1. USB audio is a common kernel subsystem, although the vulnerable path requires Type II capture format.
* Physical-only or rare hardware-only condition: -2. Realistic triggering requires physical USB access or equivalent USB device emulation such as raw-gadget or dummy_hcd.

Paranoid additions:

* Confidentiality impact plausible: +1. Kernel memory corruption may expose sensitive state in worst-case exploitation, although no direct leak is demonstrated.
* Integrity impact plausible: +1. Device-supplied inbound data is written out of bounds into kernel memory, so corruption of adjacent kernel objects is plausible.
* Weak LPE concern: +1. The OOB write is real and device-controlled, but there is no demonstrated target selection, reclaim strategy, callback control, or reliable privilege escalation primitive.

Call-site confidence: high. The relevant allocation, packet count update, inbound URB descriptor setup, and corrected bounds check are all visible in the supplied patch context.

## 3. Reachability analysis

The bug can be triggered by a malicious USB audio device or by an equivalent USB gadget emulation setup that advertises a Type II capture format. Userspace must set `hw_params` and start or prepare the affected capture stream, so there is a practical user-interaction or local audio-stack activation requirement. The issue is not network reachable. Namespaces and containers do not materially reduce the physical-device requirement, but systems that pass USB devices into guests or containers may expose the host or delegated environment depending on device ownership and USB passthrough policy.

The path is not exotic in the sense that `snd-usb-audio` is widely deployed, but the exact vulnerable condition depends on a Type II inbound capture endpoint. Exploitation conditions are realistic for malicious-device attacks and fuzzing setups, but less broadly reachable than local syscall or network bugs.

## 4. Severity interpretation

This is not an ordinary Moderate correctness bug because the primitive is a confirmed kernel slab OOB write, not merely a warning, NULL dereference, or resource leak. Realistically, the most demonstrated impact is reliable kernel memory corruption leading to DoS when the affected capture stream is used. Theoretical privilege escalation is possible enough to require manual review, but the patch does not demonstrate controlled target selection, object replacement, or a stable LPE chain.

Overall, this should be handled as Actionable Moderate at minimum, with a paranoid Strong Important candidate interpretation due to device-controlled OOB write into kernel memory.

## 5. One-sentence report phrase

A malicious or emulated USB audio device advertising a Type II capture format can cause `snd-usb-audio` to build an inbound URB whose final iso frame points past the allocated transfer buffer, resulting in a device-controlled slab OOB write and likely kernel crash, with worst-case memory corruption impact requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

The issue has a concrete OOB write primitive with device-controlled data, and although exploitation beyond DoS is not proven, the memory corruption class and USB device attack surface make auto-closure unsafe.

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

Patch: ALSA: usb-audio: fix OOB write on Type II inbound URBs
Commit: 6af5f29af7711233ae68d3b25c15d67478468900
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6af5f29af7711233ae68d3b25c15d67478468900

Commit description:

data_ep_set_params() sizes each URB transfer buffer before it adds the
Format Type II transfer delimiter:

	u->packets = urb_packs;
	u->buffer_size = maxsize * u->packets;

	if (fmt->fmt_type == UAC_FORMAT_TYPE_II)
		u->packets++; /* for transfer delimiter */
	u->urb = usb_alloc_urb(u->packets, GFP_KERNEL);

buffer_size is computed from the pre-increment packet count and never
recomputed, so for a Type II endpoint the buffer is one packet short of
the packet count the URB is built with.

prepare_inbound_urb() then lays out one iso frame per packet and never
consults buffer_size:

	offs = 0;
	for (i = 0; i < urb_ctx->packets; i++) {
		urb->iso_frame_desc[i].offset = offs;
		urb->iso_frame_desc[i].length = ep->curpacksize;
		offs += ep->curpacksize;
	}

	urb->transfer_buffer_length = offs;
	urb->number_of_packets = urb_ctx->packets;

The last descriptor therefore points one packet past the end of the
transfer buffer, where the host controller writes device data on every
inbound transfer.  prepare_silent_urb() and prepare_playback_urb() bound
their fill loops by ctx->buffer_size, so only capture is affected.

fmt_type comes from the device's audio streaming descriptors, so any
device advertising a Type II capture format hits this once userspace sets
hw_params on the stream.

KASAN on 7.2.0-rc5 (arm64) with a dummy_hcd/raw-gadget device, one report
per inbound transfer:

  BUG: KASAN: slab-out-of-bounds in dummy_timer
  Write of size 64 at addr ffff0000186171c0 by task cons02/166
   __asan_memcpy
   dummy_timer
   hrtimer_run_softirq
  Allocated by task 166:
   usb_alloc_coherent
   snd_usb_endpoint_set_params
  The buggy address is located 0 bytes to the right of
   allocated 64-byte region [ffff000018617180, ffff0000186171c0)

Compute buffer_size after the delimiter packet has been accounted for,
and bound the fill loop by buffer_size, as prepare_silent_urb() already
does on the outbound side.  This grows every Type II URB allocation by
one maxsize packet.

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

Fixes: 8fdff6a ("ALSA: snd-usb: implement new endpoint streaming model")
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/20260805013441.38245-1-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/endpoint.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=6af5f29af7711233ae68d3b25c15d67478468900

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

The original Linux kernel CVE announcement for CVE-2026-74682 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74682

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.6

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: 5
  Paranoid ActionableScore: 7

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).