[CVE-2026-63902][MODERATE REGULAR] USB: serial: cypress_m8: validate interrupt packet headers

[CVE-2026-63902][MODERATE REGULAR] USB: serial: cypress_m8: validate interrupt packet headers

[AL-KERNEL]

CVE: CVE-2026-63902
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: USB: serial: cypress_m8: validate interrupt packet headers
Commit: fcef31a5a85ccf3c313449a866ec6ed7e4132425
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fcef31a5a85ccf3c313449a866ec6ed7e4132425
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63902
Analysis date: Sun, 19 Jul 2026 21:00:18 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like, manual review because kernel OOB read
Manual review required: NO

Summary:
A malformed Cypress USB serial device can send a short interrupt packet that causes a one-byte out-of-bounds header read in `cypress_read_int_callback()`, with practical impact limited mainly to physical-device-triggered DoS on debug or hardened kernels.

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

The original Linux kernel CVE announcement for CVE-2026-63902 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63902

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
======================================================================

CVE-2026-63902	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:L/A:H';CWE-125;*CWE-20;CWE-754;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '5.7';DESCR 'A short interrupt packet in the USB cypress_m8 serial driver can reach cypress_read_int_callback with fewer bytes than the selected Cypress packet header requires. Format 1 needs 2 header bytes and format 2 needs 1 header byte, but the old code read data bytes before validating urb actual_length. A malicious or malformed USB serial device can therefore cause an out of bounds header byte read in the interrupt callback. For the CVSS the PR:N because the attacking device does not need a local user account or kernel privileges on the host once it can interact through the USB port. The issue is not network reachable and normally requires physical access to attach or emulate the USB device. Impact is mainly denial of service on debug or hardened kernels and limited confidentiality or integrity impact is kept only for the paranoid score because the primitive is an invalid read, not an arbitrary write or proven privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, manual review because kernel OOB read (with actual score 1)	YES	READ KASAN OOB USB PRINTF-TTY LINUS PACKET 	NO	NO	checked

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

ActionableScore=1
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate / Low-like, manual review because kernel OOB read**::

## 2. Signal breakdown

* Device-mediated external influence: +1
  A malicious or malformed USB serial device can control short interrupt-in reports consumed by `cypress_read_int_callback()`.

* Weak corruption / limited read-only OOB: +0
  The concrete primitive is a one-byte slab out-of-bounds read of packet header data. The patch does not show OOB write, UAF, reclaim, object replacement, callback control, or arbitrary corruption.

* Availability impact, paranoid only: +1
  KASAN reports a slab-out-of-bounds read. On debug or hardened systems this may become a crash or panic, but the patch does not prove reliable production-kernel DoS.

* Physical-only condition: -2
  Realistic triggering requires physical USB access, USB device emulation, or control of an attached Cypress-compatible USB serial device. This is not network reachable.

* Rare / device-specific exposure: not additionally subtracted
  The affected driver is device-specific, but the physical-only penalty already captures most of the exposure reduction.

Call-site confidence: high. The vulnerable callback and header parsing logic are present in the provided patch.

## 3. Reachability analysis

The bug can be triggered by a malformed USB device that sends an interrupt packet shorter than the selected Cypress packet header. No local user account is required if the attacker can attach or emulate the USB device, so host-side PR is effectively none for the physical attacker model. Namespaces and containers do not materially lower or raise reachability because the trigger is from USB device input, not a local syscall interface. The path is not remotely reachable over the network and requires the affected `cypress_m8` USB serial driver to bind to the device.

## 4. Severity interpretation

This behaves more like an ordinary Moderate / Low-like kernel memory-safety issue than an Important vulnerability. The theoretical concern is a kernel OOB read from device-controlled input, but the demonstrated primitive is only a short header-byte read with no shown disclosure path, no write primitive, no UAF, and no privilege-escalation path. The strongest realistic impact is a debug or hardened-kernel crash from KASAN or panic-style configurations.

## 5. One-sentence report phrase

A malformed Cypress USB serial device can send a short interrupt packet that causes a one-byte out-of-bounds header read in `cypress_read_int_callback()`, with practical impact limited mainly to physical-device-triggered DoS on debug or hardened kernels.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a kernel OOB read reachable from external USB device input, but it should not be treated as Important without additional evidence of disclosure, write corruption, UAF, or privilege escalation.

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

Patch: USB: serial: cypress_m8: validate interrupt packet headers
Commit: fcef31a5a85ccf3c313449a866ec6ed7e4132425
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fcef31a5a85ccf3c313449a866ec6ed7e4132425

Commit description:

cypress_read_int_callback() parses the interrupt-in buffer according to
the selected Cypress packet format. Format 1 has a two-byte status/count
header and format 2 has a one-byte combined status/count header. The
usb-serial core sizes the interrupt-in buffer from the endpoint
descriptor's wMaxPacketSize, and successful interrupt transfers can
complete short when URB_SHORT_NOT_OK is not set.

Check that the completed packet contains the selected header before
reading it. Malformed short reports are ignored and the interrupt URB is
resubmitted through the existing retry path, preventing out-of-bounds
header-byte reads.

KASAN report as below:
KASAN slab-out-of-bounds in cypress_read_int_callback+0x240/0x7f0
Read of size 1
Call trace:
  cypress_read_int_callback() (drivers/usb/serial/cypress_m8.c:1009)
  __usb_hcd_giveback_urb()
  dummy_timer()

Fixes: 3416eaa ("USB: cypress_m8: Packet format is separate from characteristic size")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
Fixes: 3416eaa ("USB: cypress_m8: Packet format is separate from characteristic size")
Cc: stable@vger.kernel.org	# 2.6.26
[ johan: use constants in header length sanity checks ]
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/serial/cypress_m8.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=fcef31a5a85ccf3c313449a866ec6ed7e4132425

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

The original Linux kernel CVE announcement for CVE-2026-63902 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63902

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:N/A:L
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.7

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: 0
  Paranoid ActionableScore: 1

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).