[CVE-2026-74388][MODERATE 7.0] ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data [ Upstream

[CVE-2026-74388][MODERATE 7.0] ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74388
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data [ Upstream
Commit: 6dc781778b595be94c31395b2cb167f65145d91f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6dc781778b595be94c31395b2cb167f65145d91f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74388
Analysis date: Sat, 15 Aug 2026 10:47:00 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local ALSA OSS sequencer user can race MIDI SysEx event dispatch with port close, leaving a stale `data.ext.ptr` that is later read in `snd_seq_event_dup()`, causing a kernel UAF and likely local DoS with limited but review-worthy escalation concern.

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

The original Linux kernel CVE announcement for CVE-2026-74388 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74388

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74388	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'ALSA OSS sequencer can hit a use after free when MIDI SysEx bytes are converted into a pending sequencer event and the sequencer port is closed concurrently before the event is dispatched. The event may keep data ext ptr pointing to storage that is released by snd_seq_oss_midi_check_exit_port, and later snd_seq_event_dup copies from this stale pointer. For the CVSS the PR:L is used because a local user or local process needs access to the OSS sequencer device and must race event submission with port close. The issue is not network reachable. Impact is at least local denial of service via kernel crash, and the UAF read primitive can justify manual review for possible limited information disclosure or privilege escalation risk in hardened scoring.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	READ INIT UAF HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user or process with access to the ALSA OSS sequencer device can write MIDI events and race port close.
* Memory corruption, weak primitive: +1. The bug is a real UAF, but the demonstrated access is a stale pointer read in `snd_seq_event_dup()`, not a write-after-free, callback hijack, type confusion, or controlled reclaim.
* Real lifetime corruption: +1. The patch extends `snd_use_lock_t` lifetime until after event dispatch, confirming a missing lifetime reference.
* Reliable kernel crash / strong DoS: +1. The report includes KASAN `slab-use-after-free` in a syscall-triggered path.
* Race-UAF downgrade cap applied. Conservative score capped at 4 because this is a race-dependent stale pointer surviving concurrent close/free without demonstrated reclaim, overwrite, callback control, or refcount takeover.

Paranoid additional signal:

* Local device-object lifetime control: +1. The ALSA OSS sequencer interface lets local users influence event construction, deferred dispatch, and close timing around the affected object lifetime.
* Paranoid score capped at 5 by the race-UAF downgrade rule.

Not awarded:

* Remote reachable: +0. No network path.
* Strong LPE plausibility: +0. No demonstrated attacker-controlled reclaim, write primitive, type confusion, or callback dispatch.
* Confidentiality / integrity impact: +0 conservatively. The UAF read may justify review, but the patch does not show a reliable information leak or modification primitive.

## 3. Reachability analysis

The bug is locally reachable through the ALSA OSS sequencer write/ioctl paths when OSS sequencer support is enabled and the device node is accessible. Typical triggering requires local code execution and access to audio/sequencer devices, often via user session permissions or an audio-related group. Containers do not normally gain this path unless the relevant sound device is passed through. The issue is not network reachable. The race requires concurrent event submission and port close, but this is a realistic local race and the bug was reported with KASAN evidence.

Call-site confidence: high. The patch includes both producer and dispatcher-side callers and shows the lifetime reference being carried until after dispatch.

## 4. Severity interpretation

This is not an ordinary low-priority Moderate because it is a confirmed kernel UAF in a local userspace-accessible path. It also should not be treated as a strong Important candidate by default because the demonstrated primitive is a stale read during event duplication, with no evidence of controlled reclaim, overwrite, callback control, or object replacement. Practically, it behaves as Actionable Moderate requiring manual review, with paranoid handling due to kernel UAF class.

## 5. One-sentence report phrase

A local ALSA OSS sequencer user can race MIDI SysEx event dispatch with port close, leaving a stale `data.ext.ptr` that is later read in `snd_seq_event_dup()`, causing a kernel UAF and likely local DoS with limited but review-worthy escalation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.
This is a confirmed UAF with local userspace reachability, even though the demonstrated primitive is weak and race-dependent.

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

Patch: ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data [ Upstream
Commit: 6dc781778b595be94c31395b2cb167f65145d91f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6dc781778b595be94c31395b2cb167f65145d91f

Changed files:
  arch/x86/entry/syscall_64.c
  sound/core/seq/oss/seq_oss_event.c
  sound/core/seq/oss/seq_oss_event.h
  sound/core/seq/oss/seq_oss_ioctl.c
  sound/core/seq/oss/seq_oss_midi.c
  sound/core/seq/oss/seq_oss_midi.h
  sound/core/seq/oss/seq_oss_rw.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=6dc781778b595be94c31395b2cb167f65145d91f

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

The original Linux kernel CVE announcement for CVE-2026-74388 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74388

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 4
  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).