---
id: CVE-2026-87038
title: >-
  Time-of-check/time-of-use double fetch in the Zephyr i3c_transfer() syscall
  handler allows kernel-mode control-flow hijack from user mode
summary: >-
  The user-mode verification handler for the i3c_transfer() system call,
  z_vrfy_i3c_transfer() in drivers/i3c/i3c_handlers.c, validated the
  caller-supplied struct i3c_device_desc in place instead of snapshotting it. It
  confirmed only that …
severity: high
cvss: 7
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
cwe:
  - CWE-367
vendor: zephyrproject
product: zephyr
affected:
  - zephyr >= 3.2.0 < 4.3.2
  - zephyr >= 4.4.0 < 4.5.0
published: '2026-11-04'
updated: '2026-11-04'
sourceUpdated: '2026-11-04T06:09:03.711Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-87038'
references:
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/commit/550437bc9efd7b733f13b9be73e77fd98858c8a2
    label: Fix commit
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-86xj-2q6x-h82w
    label: GHSA-86xj-2q6x-h82w
tags:
  - cve.org
epss: 0.00087
epssPercentile: 0.00326
ingestedAt: '2026-11-05T20:33:06.717Z'
---

## Overview

The user-mode verification handler for the i3c_transfer() system call, z_vrfy_i3c_transfer() in drivers/i3c/i3c_handlers.c, validated the caller-supplied struct i3c_device_desc in place instead of snapshotting it. It confirmed only that the descriptor was readable by the calling thread (K_SYSCALL_MEMORY_READ()), read target->bus once to check it with K_SYSCALL_OBJ(), and then passed the caller's live pointer down. The implementation z_impl_i3c_transfer() re-reads target->bus from that user memory and dispatches through it as DEVICE_API_GET(i3c, target->bus)->i3c_xfers(...), so the bus that was validated is not necessarily the bus that is used. DEVICE_API_GET() performs no runtime type check outside CONFIG_DEVICE_API_ASSERT builds.

On a build with CONFIG_USERSPACE and an I3C controller driver, a user thread that has been granted an I3C device object can exploit this with a second thread in the same memory domain: one thread calls i3c_transfer() while the other rewrites the bus field of the descriptor. The window between the check and the use spans the num_msgs validation, the message-array access check, a stack VLA allocation and a memcpy() of up to 32 struct i3c_msg, so the race is repeatable and can simply be retried until it lands.

When the substitution succeeds, the kernel dereferences an attacker-chosen pointer as a struct device, loads a function pointer from the attacker-controlled api table and calls it in supervisor mode with attacker-influenced arguments. This yields kernel-mode code execution from an unprivileged user thread — a complete escape of the userspace memory-protection sandbox; lesser outcomes are an arbitrary two-level kernel read or a kernel fault. Builds without CONFIG_USERSPACE, or without an I3C driver, are unaffected.

The fix copies the descriptor into a kernel-stack local with k_usermode_from_copy(), validates the bus in that copy, and dispatches on the copy, so the validated value is provably the value used.

## Affected

- `zephyr >= 3.2.0 < 4.3.2`
- `zephyr >= 4.4.0 < 4.5.0`

## Remediation

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