[CVE-2026-63943][MODERATE REGULAR] Input: xpad - fix out-of-bounds access for Share button

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

CVE: CVE-2026-63943
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: Input: xpad - fix out-of-bounds access for Share button
Commit: bcfb4833cd4078a1a356ef451838b75cd233099e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bcfb4833cd4078a1a356ef451838b75cd233099e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63943
Analysis date: Sun, 19 Jul 2026 22:22:34 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like with manual review
Manual review required: YES

Summary:
A malformed USB Xbox controller can send a short input packet that makes the xpad driver read outside the received packet while processing the Share button, potentially causing a kernel crash or very limited information exposure.

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

The original Linux kernel CVE announcement for CVE-2026-63943 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63943

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: 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-2026-63943	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:L/I:N/A:H';CWE-125;CWE-129;*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 'An out-of-bounds read in the xpad Xbox One input path can occur because xpadone_process_packet uses the device supplied packet length to read the Share button byte at an offset derived from len. A malformed or malicious USB controller can send a short GIP_CMD_INPUT packet so the driver reads outside the actual packet data while reporting KEY_RECORD. For the CVSS the PR:N is used because no privileges are required once the attacker can present the malicious physical USB device to the system. The issue is not network reachable and requires physical device access or an equivalent USB device injection path. Impact is primarily denial of service from a possible kernel fault, with only limited confidentiality concern in the paranoid score because the primitive is an out-of-bounds read and not an out-of-bounds write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like with manual review (with actual score 1)  SKIP CVE-2026-63943 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB SIMPLEFIX HARDWARE LINUS PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like with manual review**

## 2. Signal breakdown

Conservative signals:

* Firmware-mediated external influence: +1. A malicious or broken USB Xbox controller can control `urb->actual_length` and packet contents.
* Availability impact realistic: +1. The out-of-bounds read may fault or trigger a kernel crash depending on buffer placement and hardening.
* Physical-only or rare hardware-only condition: -2. The attack requires physical USB device access or an equivalent USB-device-injection path.

Paranoid additional signals:

* Confidentiality impact plausible: +1. The primitive is a limited OOB read, but only one bit is used for an input event, so disclosure is weak and highly constrained.
* Broad/default/common subsystem exposure: +1. USB input and xpad-style controller support are common on general-purpose Linux systems, although the exact Share-button path is device-specific.

Not counted:

* No remote/network reachability.
* No local unprivileged software-only trigger.
* No OOB write, UAF, double free, type confusion, refcount abuse, callback control, or reclaim primitive.
* No plausible privilege escalation from the patch context.

## 3. Reachability analysis

The bug is triggered by a malformed or malicious Xbox One compatible USB controller sending a short `GIP_CMD_INPUT` packet with `actual_length < 18` or `< 26`. No OS privileges are required once the attacker can present the device, so PR is effectively not applicable from the device side, but the access vector is physical rather than local software or network. Containers and namespaces do not materially lower the requirement because the vulnerable parsing occurs in the host kernel USB input driver. The path may be available on systems with the `xpad` driver loaded or auto-loadable for matching hardware.

Call-site confidence: high. The vulnerable index calculation and its use in `input_report_key()` are visible in the patch.

## 4. Severity interpretation

This behaves more like an ordinary Moderate or Low-like physical-device memory-safety issue than an Important kernel vulnerability. The theoretical issue is a kernel out-of-bounds read, but the demonstrated primitive is read-only, narrow, and used only to derive a key state bit. Realistic exploitation is mostly DoS-oriented and requires malicious physical hardware or USB injection, with no evidence of a write primitive or privilege escalation path.

## 5. One-sentence report phrase

A malformed USB Xbox controller can send a short input packet that makes the xpad driver read outside the received packet while processing the Share button, potentially causing a kernel crash or very limited information exposure.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a kernel OOB read controlled by an external device, but it should be reviewed as low-priority physical-access exposure rather than as an Important-class candidate.

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

Patch: Input: xpad - fix out-of-bounds access for Share button
Commit: bcfb4833cd4078a1a356ef451838b75cd233099e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bcfb4833cd4078a1a356ef451838b75cd233099e

Commit description:

xpadone_process_packet() receives len directly from urb->actual_length
and uses it to index the share-button byte at data[len - 18] or
data[len - 26]. Since both len and data[0] are under the device's
control, a broken controller can send a GIP_CMD_INPUT packet with
actual_length < 18 (e.g. 5 bytes) and reach this code path, causing
accesses beyond the actual array.

Fix this by calculating the offset and checking bounds against the
packet length.

Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 4ef4636 ("Input: xpad - fix Share button on Xbox One controllers")
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/input/joystick/xpad.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=bcfb4833cd4078a1a356ef451838b75cd233099e

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

The original Linux kernel CVE announcement for CVE-2026-63943 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63943

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: 0
  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).