---
id: CVE-2026-9728
title: >-
  The userspace syscall verifier z_vrfy_mbox_send() in
  drivers/mbox/mbox_handlers.c validated the nested msg->data/msg->size fields
  by reading them directly out of live userspace memory, and then forwarded the
  original, still-mutable users…
summary: >-
  The userspace syscall verifier z_vrfy_mbox_send() in
  drivers/mbox/mbox_handlers.c validated the nested msg->data/msg->size fields
  by reading them directly out of live userspace memory, and then forwarded the
  original, still-mutable users…
severity: medium
cvss: 6.4
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L'
cwe:
  - CWE-367
published: '2026-08-24'
updated: '2026-10-12'
sourceUpdated: '2026-10-12T15:10:00.207'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-9728'
references:
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/commit/ab35eaccec5976f05c196f176d0c32885754496f
    label: vulnerabilities@zephyrproject.org
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-47q2-w832-7w67
    label: vulnerabilities@zephyrproject.org
tags:
  - nvd
epss: 0.0011
epssPercentile: 0.01191
ingestedAt: '2026-10-12T15:55:08.121Z'
---

## Overview

The userspace syscall verifier z_vrfy_mbox_send() in drivers/mbox/mbox_handlers.c validated the nested msg->data/msg->size fields by reading them directly out of live userspace memory, and then forwarded the original, still-mutable userspace struct mbox_msg * pointer to z_impl_mbox_send() and the underlying driver. Between the access check and the driver's use of msg->data, the validated pointer could be replaced, leaving a time-of-check/time-of-use window.

On a system built with CONFIG_USERSPACE, any unprivileged userspace thread may invoke the mbox_send() system call. A second thread sharing the caller's address space can race to overwrite msg->data with a supervisor (kernel) address after the verifier's bounds check has passed but before the driver dereferences it. The driver then reads from the attacker-chosen address in supervisor context (for example memcpy(&data32, msg->data, msg->size) in the NXP mailbox driver, whose bytes are subsequently emitted to the peer mailbox endpoint).

The impact is a userspace-to-supervisor access-control bypass: disclosure of kernel memory contents (high confidentiality impact), or, for an invalid/unmapped target address, a faulting kernel read causing denial of service. The fix snapshots the entire struct mbox_msg into a kernel-stack copy with k_usermode_from_copy() and validates and forwards that immutable copy, closing the race.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.