[CVE-2026-64366][IMPORTANT] HID: wacom: fix slab-out-of-bounds write in wacom_wac_queue_insert

[CVE-2026-64366][IMPORTANT] HID: wacom: fix slab-out-of-bounds write in wacom_wac_queue_insert

[AL-KERNEL]

CVE: CVE-2026-64366
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: HID: wacom: fix slab-out-of-bounds write in wacom_wac_queue_insert
Commit: ca899a926c11a59211b764b0155d9a1cdcc32b81
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ca899a926c11a59211b764b0155d9a1cdcc32b81
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64366
Analysis date: Sat, 25 Jul 2026 11:03:03 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A slab out-of-bounds write in the HID Wacom report queue can be triggered by malformed HID reports when kfifo_skip() is called on an empty fifo, causing kernel memory corruption with at least DoS impact and plausible local privilege escalation if HID emulation is available.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64366 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64366

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-2026-64366	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-20;CWE-908;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A slab out-of-bounds write in the HID Wacom input path can occur when wacom_wac_queue_insert() calls kfifo_skip() on an empty fifo. Stale data in the kmalloc buffer can be interpreted as a record length, corrupting the internal kfifo indexes and making kfifo_unused() report more space than the real 256 byte buffer has. This can bypass the normal kfifo_in() size guard and lead to an out-of-bounds memcpy of attacker controlled HID report data. For the CVSS the PR:L is used for the paranoid score because a local user with access to HID emulation such as uhid may be able to trigger the path without full administrative privileges. The physical device path can also be triggered by a malicious Wacom compatible HID device. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality and integrity impact due to kernel memory corruption and possible privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	WRITE OOB USB SIMPLEFIX IMPROVEONLY LINUS KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* +1 Firmware-mediated external influence: a malicious or malformed Wacom-compatible HID device can influence the report data processed by the driver.
* +2 Memory corruption, strong corruption primitive: the commit explicitly describes a slab out-of-bounds write, with kfifo_copy_in() writing up to 3842 bytes past a 256-byte buffer.
* +2 Privilege escalation plausible: this is attacker-influenced kernel heap corruption, not just a NULL dereference or warning. No full exploit primitive is demonstrated, but LPE is technically plausible.
* +1 Reliable kernel crash / strong DoS: an OOB write in slab memory is a credible crash path.
* +1 Confidentiality impact plausible: kernel heap corruption may permit adjacent object manipulation or disclosure in worst practical cases.
* +1 Integrity impact plausible: attacker-influenced OOB write may corrupt adjacent kernel objects.
* -2 Physical-only or rare hardware-only condition: conservative path assumes a malicious physical or Wacom-compatible HID device.

Paranoid delta:

* +1 Local unprivileged trigger: if /dev/uhid or a similar HID emulation path is available to an untrusted local user, the bug may be reachable without physical hardware.
* Physical-only penalty is removed in that model.

Call-site confidence: medium-high. The affected function and failure mode are directly described, but broader allocator adjacency and exact exploitability depend on runtime heap layout.

## 3. Reachability analysis

The conservative trigger is a malicious or malformed Wacom-compatible HID device sending reports that force the queue handling path into the empty-fifo kfifo_skip() case. This is not network reachable.

The more concerning model is local HID emulation through uhid or similar interfaces. If an untrusted local user can create virtual HID devices or inject Wacom-like reports, the practical privilege requirement drops to local user level. In many deployments /dev/uhid is not universally available to unprivileged users, so this depends on device-node permissions, desktop policy, container device passthrough, or service configuration.

Namespaces alone do not usually make this reachable unless the HID emulation device is exposed into the namespace. Physical-device exploitation requires device attachment or control over an already connected HID path.

## 4. Severity interpretation

This behaves closer to an Important-class kernel memory corruption issue than an ordinary Moderate DoS. The patch and commit message provide a concrete corruption primitive: stale kfifo length interpretation corrupts fifo state, bypasses the normal kfifo_in() guard, and causes an out-of-bounds memcpy beyond a small slab buffer.

Realistic exploitation beyond crash is not demonstrated, and target selection is heap-layout dependent. However, because the payload is HID report data and the overwrite size can be far beyond the object boundary, privilege escalation is plausible enough to require manual review.

## 5. One-sentence report phrase

A slab out-of-bounds write in the HID Wacom report queue can be triggered by malformed HID reports when kfifo_skip() is called on an empty fifo, causing kernel memory corruption with at least DoS impact and plausible local privilege escalation if HID emulation is available.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: concrete slab out-of-bounds write, attacker-influenced input, plausible LPE path, and possible local reachability through HID emulation make this unsuitable for auto-closure.

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

Patch: HID: wacom: fix slab-out-of-bounds write in wacom_wac_queue_insert
Commit: ca899a926c11a59211b764b0155d9a1cdcc32b81
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ca899a926c11a59211b764b0155d9a1cdcc32b81

Commit description:

wacom_wac_queue_insert() calls kfifo_skip() in a loop when the kfifo
doesn't have enough space for the incoming report. If the kfifo is
empty, kfifo_skip() reads stale data left in the kmalloc'd buffer
via __kfifo_peek_n() and interprets it as a record length, advancing
fifo->out by that garbage value. This corrupts the internal kfifo
state, causing kfifo_unused() to return a value much larger than the
actual buffer size, which bypasses __kfifo_in_r()'s guard:

  if (len + recsize > kfifo_unused(fifo))
      return 0;

kfifo_copy_in() then performs an out-of-bounds memcpy, writing up to
3842 bytes past the 256-byte buffer.

Add a !kfifo_is_empty() condition to the while loop so kfifo_skip()
is never called on an empty fifo, and check the return value of
kfifo_in() to reject reports that are too large for the fifo.

Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Fixes: 5e013ad ("HID: wacom: Remove static WACOM_PKGLEN_MAX limit")
Cc: stable@vger.kernel.org
Signed-off-by: Jinmo Yang <jinmo44.yang@gmail.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/hid/wacom_sys.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=ca899a926c11a59211b764b0155d9a1cdcc32b81

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

The original Linux kernel CVE announcement for CVE-2026-64366 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64366

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: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: 6
  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).