[CVE-2026-63898][MODERATE 7.0] USB: serial: mct_u232: fix memory corruption with small endpoint

[CVE-2026-63898][MODERATE 7.0] USB: serial: mct_u232: fix memory corruption with small endpoint

[AL-KERNEL]

CVE: CVE-2026-63898
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: USB: serial: mct_u232: fix memory corruption with small endpoint
Commit: 94edbbc5fe00d03cfe1d4e690d7d2cd36317a935
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94edbbc5fe00d03cfe1d4e690d7d2cd36317a935
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63898
Analysis date: Sun, 19 Jul 2026 18:40:42 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious USB serial device can report a smaller bulk-out endpoint size and cause the mct_u232 driver to use an increased transfer size, leading to kernel slab corruption and likely DoS, with paranoid concern for broader kernel heap corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63898 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63898

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-2026-63898	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:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-122;CWE-119;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.8';DESCR 'A malicious USB serial device that matches the mct_u232 Sitecom PID could report a bulk out endpoint max packet size smaller than 16 bytes. The driver previously forced port bulk_out_size to 16 during open, which could increase the transfer size above the buffer size derived from the endpoint and cause slab corruption in the kernel. For the CVSS the PR:N is used because the attacker does not need an account on the host and only needs the ability to present a malicious USB device to the victim system. The issue is not network reachable and requires physical USB access or a supply chain style device substitution. Impact is at least local denial of service via kernel memory corruption, and in the paranoid case may include confidentiality and integrity impact due to a kernel heap corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	USB PRINTF-TTY SIMPLEFIX ERRORPATH HARDWARE LINUS PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Firmware-mediated external influence: +1. A malicious or nonconforming USB device can influence endpoint descriptors and reported packet sizes.
* Constrained kernel buffer overwrite: +1. The driver could increase `bulk_out_size` above the size implied by the endpoint, causing slab corruption, but the overwrite appears tied to a driver transfer buffer rather than an arbitrary target.
* Memory layout invariant restoration: +1. The patch restores consistency between endpoint-derived buffer sizing and the driver override by using `min(16, port->bulk_out_size)`.
* Reliable kernel crash / strong DoS: +1. The commit explicitly states slab corruption from a malicious device, which can realistically crash the kernel.
* Privileged kernel/device-management memory corruption path: +1. The corruption occurs inside a trusted USB serial driver path influenced by device-reported metadata.
* Physical-only or rare hardware-only condition: -2. Exploitation requires physical USB access or malicious device substitution, not network reachability.

Paranoid additions / interpretation:

* Generic memory corruption: +2 instead of constrained +1. The commit explicitly says slab corruption, so a paranoid review can treat this as kernel heap corruption.
* Confidentiality impact plausible: +1. Kernel heap corruption can theoretically expose or affect adjacent kernel data, though no leak primitive is shown.
* Integrity impact plausible: +1. Kernel heap corruption can theoretically corrupt adjacent objects, though no controlled overwrite target is demonstrated.

## 3. Reachability analysis

The bug is triggerable by a malicious USB serial device that matches the affected `mct_u232` Sitecom PID and reports a smaller bulk-out endpoint max packet size than expected. No local user account is required in the CVSS sense if the attacker can physically attach or supply the USB device. Namespaces and containers do not materially lower or raise the triggering requirement because the key input is USB device enumeration and driver binding on the host. The path is not network reachable and is limited to systems where this USB serial driver can bind to the malicious device. Realistic exploitation requires physical access, malicious peripheral insertion, or supply-chain style device replacement.

## 4. Severity interpretation

This behaves above an ordinary Moderate because the patch explicitly prevents kernel slab corruption caused by malicious device-controlled endpoint metadata. The realistic impact is likely host crash or unstable kernel behavior, with physical access limiting exposure. The theoretical memory corruption potential is stronger than a simple NULL dereference or warning, but the patch does not show a controlled arbitrary write, reclaim primitive, function pointer control, or demonstrated privilege escalation. Therefore the conservative handling is borderline/manual-review, while the paranoid handling is Actionable Moderate and should not be auto-closed.

## 5. One-sentence report phrase

A malicious USB serial device can report a smaller bulk-out endpoint size and cause the mct_u232 driver to use an increased transfer size, leading to kernel slab corruption and likely DoS, with paranoid concern for broader kernel heap corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is physical-only, but it is explicitly kernel slab corruption from malicious device input. This is a concrete memory corruption class, so it should not be auto-closed despite the limited physical USB attack vector.

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

Patch: USB: serial: mct_u232: fix memory corruption with small endpoint
Commit: 94edbbc5fe00d03cfe1d4e690d7d2cd36317a935
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94edbbc5fe00d03cfe1d4e690d7d2cd36317a935

Commit description:

The driver overrides the maximum transfer size for a specific device
which only accepts 16 byte packets for its 32 byte bulk-out endpoint.

Make sure to never increase the maximum transfer size to prevent slab
corruption should a malicious device report a smaller endpoint max
packet size than expected.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  drivers/usb/serial/mct_u232.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=94edbbc5fe00d03cfe1d4e690d7d2cd36317a935

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

The original Linux kernel CVE announcement for CVE-2026-63898 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63898

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.8

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: 4
  Paranoid ActionableScore: 6

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).