[CVE-2026-63899][IMPORTANT] USB: serial: mxuport: fix memory corruption with small endpoint

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-63899
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: USB: serial: mxuport: fix memory corruption with small endpoint
Commit: 086b858b5f5125bc9d967ea2bd825f83d9f8f29d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=086b858b5f5125bc9d967ea2bd825f83d9f8f29d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63899
Analysis date: Sun, 19 Jul 2026 19:41:24 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Paranoid handling is justified because the patch explicitly describes user-controlled kernel slab corruption. Conservative handling is Actionable Moderate because exploitation requires physical or local USB-device influence rather than network reachability.
Manual review required: YES

Summary:
A malicious USB serial device can report a too-small bulk-out endpoint size and cause user-controlled kernel slab corruption in the mxuport driver, creating a physical-access DoS and plausible privilege-escalation risk.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63899 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63899

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-2026-63899	IMPORTANT	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-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H';BEST CVSS score: '6.8';DESCR 'A malicious USB serial device can report a bulk out endpoint with a max packet size smaller than the mxuport driver expects. The driver can then use too small bulk out buffers for a four byte header and following data, which is described as attacker controlled slab corruption in kernel memory. For the CVSS the PR:N because no local account on the victim is needed beyond the ability to attach or influence the USB device. The practical vector is physical or local USB bus access, not a normal network path. Impact can include denial of service and plausible privilege escalation because this is kernel memory corruption rather than a simple crash only bug.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	USB PRINTF-TTY SIMPLEFIX HARDWARE LINUS PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Paranoid handling is justified because the patch explicitly describes user-controlled kernel slab corruption. Conservative handling is Actionable Moderate because exploitation requires physical or local USB-device influence rather than network reachability.

## 2. Signal breakdown

* Memory corruption, strong corruption primitive: +2
  The commit explicitly says user-controlled slab corruption can occur when a malicious USB device reports a bulk-out endpoint max packet size smaller than expected.

* Privilege escalation plausible: +2
  Kernel slab corruption from attacker-controlled USB device metadata and transfer behavior is a plausible LPE or kernel-code-execution class issue, even though no full exploit primitive is shown.

* Reliable kernel crash / strong DoS: +1
  Slab corruption in a USB serial driver can realistically crash the kernel.

* Confidentiality impact plausible: +1
  Kernel memory corruption can plausibly lead to disclosure depending on allocator layout and corrupted object type.

* Integrity impact plausible: +1
  User-controlled slab corruption can plausibly corrupt adjacent kernel objects or metadata.

* Privileged kernel/device-management memory corruption path: +1
  The bug is in USB device enumeration / serial-port setup, where attacker-supplied device descriptors influence trusted kernel driver state.

* Physical-only or rare hardware-only condition: -2
  The attacker generally needs to attach or emulate a malicious USB serial device. This is not a normal remote network vector.

Paranoid delta: +1 for treating the corruption as requiring manual Important-class review because the commit explicitly states user-controlled slab corruption, even though the attack vector is physical.

## 3. Reachability analysis

The realistic attacker is someone who can attach, emulate, or otherwise present a malicious USB device matching the mxuport driver. No local user account is required if the attacker has physical USB access, so this is not a PR:H administrative interface issue. Containers and namespaces do not materially help unless they can influence host USB device attachment or USB/IP style device presentation. The path depends on the mxuport driver being present and bound to the device, but malicious USB devices can often present compatible identifiers. This is not network reachable in the ordinary TCP/IP sense.

Call-site confidence: medium. The patch context shows validation in endpoint parsing, and the commit directly states the downstream slab corruption condition.

## 4. Severity interpretation

This behaves above ordinary Moderate because the issue is not just a validation warning or device-specific malfunction. The patch prevents an explicitly described user-controlled slab corruption in kernel memory. Realistic exploitation is constrained by physical or local USB-bus access, so it is not a broad remote threat. However, kernel heap corruption from malicious hardware input is a credible privilege-escalation class and should not be auto-closed as a low-risk crash-only bug.

## 5. One-sentence report phrase

A malicious USB serial device can report a too-small bulk-out endpoint size and cause user-controlled kernel slab corruption in the mxuport driver, creating a physical-access DoS and plausible privilege-escalation risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly fixes user-controlled kernel slab corruption, and even though the vector is physical USB access rather than remote network reachability, the memory-corruption class and possible privilege-escalation impact require human review.

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UPSTREAM PATCH SUMMARY
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Patch: USB: serial: mxuport: fix memory corruption with small endpoint
Commit: 086b858b5f5125bc9d967ea2bd825f83d9f8f29d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=086b858b5f5125bc9d967ea2bd825f83d9f8f29d

Commit description:

Make sure that the bulk-out endpoint max packet size is at least eight
bytes to avoid user-controlled slab corruption should a malicious device
report a smaller size.

Fixes: ee467a1 ("USB: serial: add Moxa UPORT 12XX/14XX/16XX driver")
Cc: stable@vger.kernel.org	# 3.14
Cc: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/serial/mxuport.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=086b858b5f5125bc9d967ea2bd825f83d9f8f29d

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

The original Linux kernel CVE announcement for CVE-2026-63899 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63899

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:H/I:H/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: 6
  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: 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).