[CVE-2026-63903][MODERATE REGULAR] USB: serial: belkin_sa: validate interrupt status length

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

CVE: CVE-2026-63903
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: USB: serial: belkin_sa: validate interrupt status length
Commit: f1617539ab90e67da788959bfd314076f093a11a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f1617539ab90e67da788959bfd314076f093a11a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63903
Analysis date: Sun, 19 Jul 2026 20:32:32 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A malformed Belkin USB serial device can return a short interrupt packet that causes belkin_sa_read_int_callback to read status bytes beyond the completed URB data, resulting in a kernel out-of-bounds read with mainly physical DoS exposure.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63903 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63903

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 REGULAR
Manual review required: NO

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-2026-63903	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-130;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.2';DESCR 'The Belkin USB serial interrupt callback can read beyond the completed interrupt packet because it parses status bytes at fixed offsets 2 and 3 without first checking urb actual_length. A malicious or malformed USB serial device can return a short successful interrupt transfer or advertise a short interrupt endpoint, causing an out of bounds or stale status byte read in kernel context. For the CVSS the PR:N is used because no authenticated user privileges on the target are required once the attacker can present the USB device to the system. The attack is physical rather than network reachable, so AV:P is used. Impact is mainly denial of service via kernel fault or corrupted status handling. Confidentiality is at most limited because this is an invalid read without a demonstrated data disclosure path, and integrity impact is not supported by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	READ KASAN OOB USB INIT PRINTF-TTY SIMPLEFIX LINUS PACKET 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Firmware-mediated or device-mediated external influence: +1
  A malicious or malformed USB serial device can influence interrupt URB contents and actual_length.

* Weak memory safety issue: +0 to +1
  The bug is a read-only out-of-bounds or stale status-byte read. It is not an OOB write, UAF, double free, type confusion, or controlled overwrite.

* Reliable kernel crash / strong DoS: +0 conservative, +1 paranoid
  KASAN reports slab-out-of-bounds read. On production kernels this may only read stale adjacent data or corrupt status handling, but a crash is plausible in hardened/debug configurations or unlucky layouts.

* Confidentiality impact plausible: +0 conservative, +1 paranoid
  A one-byte invalid kernel read exists, but there is no demonstrated disclosure path back to the attacker. Limited confidentiality concern is only a paranoid interpretation.

Subtracted signals:

* Physical-only or rare hardware-only condition: -2
  Triggering requires presenting a malicious or malformed Belkin-compatible USB serial device or equivalent USB emulation. This is not remotely network reachable.

No signals awarded:

* No local unprivileged trigger in the normal syscall sense.
* No privilege escalation primitive.
* No generic strong memory corruption.
* No integrity impact.
* No broad network or common packet-processing exposure.

## 3. Reachability analysis

The realistic attacker is someone who can attach or emulate a malicious USB serial device. No authenticated local user privileges are needed once the attacker can present the device, but the attack vector is physical or USB-device-mediated rather than remote. Namespaces and containers do not materially lower the requirement, because the vulnerable path is driven by kernel USB interrupt completion handling. The path is reachable only when the belkin_sa USB serial driver binds to a compatible device, so exposure is hardware-dependent and not broadly default-triggerable on all systems.

Call-site confidence: high. The affected callback and the exact invalid access condition are visible in the patch context and KASAN trace.

## 4. Severity interpretation

This behaves more like ordinary Moderate or Low-like physical attack surface than an Important-class kernel vulnerability. The theoretical memory-safety issue is real because KASAN confirms a slab out-of-bounds read, but the primitive is read-only, very small, device-specific, and lacks a demonstrated data disclosure or privilege-escalation path. The paranoid score keeps manual-review sensitivity because kernel OOB reads can hide layout-dependent behavior, but the practical exploitability is constrained by physical USB reachability.

## 5. One-sentence report phrase

A malformed Belkin USB serial device can return a short interrupt packet that causes belkin_sa_read_int_callback to read status bytes beyond the completed URB data, resulting in a kernel out-of-bounds read with mainly physical DoS exposure.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a confirmed kernel out-of-bounds read, but it is not an Important candidate by default because the primitive is read-only, physical/device-mediated, and has no demonstrated LPE or disclosure path.

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

Patch: USB: serial: belkin_sa: validate interrupt status length
Commit: f1617539ab90e67da788959bfd314076f093a11a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f1617539ab90e67da788959bfd314076f093a11a

Commit description:

The Belkin interrupt callback treats interrupt data as a four-byte
status report and reads LSR/MSR fields at offsets 2 and 3. The
interrupt-in buffer length is derived from endpoint wMaxPacketSize, and
short interrupt transfers may complete successfully with a smaller
actual_length.

Check the completed interrupt packet length before parsing status
fields so short interrupt endpoints and short successful packets are
ignored instead of causing out-of-bounds or stale status-byte reads.

KASAN report as below:

BUG: KASAN: slab-out-of-bounds in belkin_sa_read_int_callback()
Read of size 1
Call trace:
  belkin_sa_read_int_callback() (drivers/usb/serial/belkin_sa.c:202)
  __usb_hcd_giveback_urb() (drivers/usb/core/hcd.c:1630)
  dummy_timer() (?:?)

Fixes: 1da177e ("Linux-2.6.12-rc2")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/serial/belkin_sa.c
  drivers/usb/core/hcd.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=f1617539ab90e67da788959bfd314076f093a11a

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

The original Linux kernel CVE announcement for CVE-2026-63903 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63903

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 5.2

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: 1
  Paranoid ActionableScore: 2

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 REGULAR

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).