---
id: CVE-2026-12366
title: >-
  Use-after-free freeing an armed dynamically-allocated k_timer in Zephyr
  userspace object disposal
summary: >-
  Zephyr's dynamic kernel-object disposal path unref_check() in
  kernel/userspace/userspace.c frees an object's storage (k_free(dyn->data))
  once its reference count reaches zero, after running a per-object-type
  cleanup. The cleanup switch h…
severity: high
cvss: 8.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H'
cvssSource: cna
cwe:
  - CWE-416
vendor: zephyrproject
product: zephyr
affected:
  - zephyr >= 1.12.0 < 4.5.0
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-08-14T19:21:20.823611Z'
published: '2026-08-14'
updated: '2026-11-04'
sourceUpdated: '2026-11-04T04:59:26.531Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-12366'
references:
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/commit/1e68351a2572f9ae480be71da4aca9aa90db3fb2
    label: Fix commit
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-x96g-542c-gccq
    label: GHSA-x96g-542c-gccq
tags:
  - cve.org
epss: 0.00165
epssPercentile: 0.05768
ingestedAt: '2026-11-05T20:33:06.701Z'
---

## Overview

Zephyr's dynamic kernel-object disposal path unref_check() in kernel/userspace/userspace.c frees an object's storage (k_free(dyn->data)) once its reference count reaches zero, after running a per-object-type cleanup. The cleanup switch handled only K_OBJ_MSGQ and K_OBJ_STACK; there was no K_OBJ_TIMER case. A dynamically-allocated, initialized, and armed k_timer keeps its embedded struct _timeout dnode linked in the global timeout queue (_timeout_q), so freeing the timer storage without cancelling the timeout leaves a dangling node in that queue.

When the timer next expires, the timeout machinery walks _timeout_q and invokes z_timer_expiration_handler() on the freed node, dereferencing and writing freed (and reusable) kernel heap in kernel/ISR context. This is a deterministic use-after-free that does not depend on SMP: the queued node is simply never unlinked at free time.

The disposal is reachable from an unprivileged user thread under CONFIG_USERSPACE + CONFIG_DYNAMIC_OBJECTS: a thread that holds the last permission on such a timer drops it via the k_object_release() syscall (or by exiting, through k_thread_perms_all_clear()), and can arm the timer itself via the k_timer_start() syscall. The free and the expiration handler run at kernel privilege while the actor is a user thread, so the bug is a sandbox-escape memory-corruption primitive usable for privilege escalation. The fix adds k_timer_cleanup() (cancel the timeout and wait for any in-flight handler) and calls it for K_OBJ_TIMER before freeing.

## Affected

- `zephyr >= 1.12.0 < 4.5.0`

## Remediation

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