[CVE-2026-63956][IMPORTANT] USB: serial: cypress_m8: fix memory corruption with small endpoint

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

CVE: CVE-2026-63956
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: USB: serial: cypress_m8: fix memory corruption with small endpoint
Commit: 4fcb22218f0a7229b7ce3b3952fb644def293fa5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4fcb22218f0a7229b7ce3b3952fb644def293fa5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63956
Analysis date: Sun, 19 Jul 2026 14:53:55 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-63956 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63956

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-63956	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;**CWE-476;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 memory corruption issue in the USB serial cypress_m8 driver occurs because a malicious USB device can report an interrupt-out endpoint max packet size smaller than the packet header and data layout expected by the driver. This can cause attacker-controlled slab corruption or a NULL pointer dereference when the driver later uses the undersized interrupt-out buffer. For the CVSS the PR:N is used because the malicious device does not need privileges on the target system once it can be attached or emulated as a USB device. The issue is not network reachable in the normal USB model and requires physical or equivalent USB device attachment. Impact is at least denial of service via kernel crash and may include confidentiality and integrity impact because the patch context explicitly identifies user-controlled slab corruption.';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

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

ActionableScore=7
ActionableScoreLower=5

1. **ActionableScore**
* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

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 an interrupt-out endpoint smaller than expected.
* Privilege escalation plausible: +1 conservative, +2 paranoid. Slab corruption is a plausible LPE concern, but the patch does not show a concrete reclaim, overwrite target, or exploit chain.
* Reliable kernel crash / strong DoS: +1. The commit also mentions NULL-pointer dereference as an expected failure mode.
* Privileged kernel/device-management memory corruption path: +1. The bug is in USB serial device probing and trusts device-provided endpoint metadata.
* Confidentiality impact plausible: +1 paranoid. Plausible only due to slab corruption class, not demonstrated.
* Integrity impact plausible: +1 paranoid. Plausible only due to slab corruption class, not demonstrated.
* Availability impact realistic: +1 paranoid. A malicious device can plausibly crash the kernel during probe/use.
* Physical-only or rare hardware-only condition: -2. Triggering requires physical or equivalent USB device attachment, not network access.

3. **Reachability analysis**
A malicious USB device, or a USB emulation path such as gadget, VM USB passthrough, or compromised peripheral, can trigger the bug by presenting an interrupt-out endpoint with max packet size below 8. No privileges are needed on the victim once the attacker can attach or emulate the device. Namespaces and containers generally do not matter unless they control USB device passthrough. The affected driver is device-specific, but USB driver autoload/probing makes the path realistic where such devices are accepted.

4. **Severity interpretation**
This is not an ordinary validation-only issue because the commit explicitly identifies user-controlled slab corruption. Realistic exploitation is constrained by physical USB reachability and by lack of a demonstrated overwrite/reclaim primitive, so the conservative result is Actionable Moderate. The paranoid interpretation is Strong Important candidate because attacker-controlled slab corruption in kernel USB probe paths has plausible confidentiality, integrity, and availability consequences.

5. **One-sentence report phrase**
A malicious USB device can report an undersized cypress_m8 interrupt-out endpoint, causing user-controlled slab corruption or NULL-pointer dereference in the USB serial driver, with physical or equivalent USB access required and possible impact ranging from kernel crash to plausible privilege escalation.

6. **Manual review recommendation**
MANUAL CHECK REQUIRED. The bug has explicit attacker-controlled slab corruption, even though the trigger is physical or USB-emulated rather than network reachable.

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

Patch: USB: serial: cypress_m8: fix memory corruption with small endpoint
Commit: 4fcb22218f0a7229b7ce3b3952fb644def293fa5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4fcb22218f0a7229b7ce3b3952fb644def293fa5

Commit description:

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

Fixes: 3416eaa ("USB: cypress_m8: Packet format is separate from characteristic size")
Cc: stable@vger.kernel.org	# 2.6.26
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
[ johan: adjust context for 6.18 ]
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.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=4fcb22218f0a7229b7ce3b3952fb644def293fa5

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

The original Linux kernel CVE announcement for CVE-2026-63956 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63956

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: 5
  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).