# CVE-2026-12366: Use-after-free freeing an armed dynamically-allocated k_timer in Zephyr userspace object disposal

Status: published. Published: 2026-08-14. Updated: 2026-11-04.

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.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: zephyr) — https://github.com/zephyrproject-rtos/zephyr/commit/1e68351a2572f9ae480be71da4aca9aa90db3fb2

## Affected products and versions

- zephyrproject / zephyr (zephyr): ≥ 1.12.0; < 4.5.0 (affected, semver). Source: CNA: zephyr

## Severity and prioritization

- CVSS 3.1: 8.8 (high); CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H; source: CNA: zephyr

- CISA KEV: No listing in collected sources

- EPSS: 0.17% as of 2026-11-03; percentile 5.75%

- SSVC: exploitation none; automatable no; technical impact total; source ADP: CISA-ADP

- Weaknesses: CWE-416

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/zephyrproject-rtos/zephyr/commit/1e68351a2572f9ae480be71da4aca9aa90db3fb2 (CNA: zephyr; patch)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-x96g-542c-gccq (CNA: zephyr)

## Provenance

- CNA: zephyr: https://www.cve.org/CVERecord?id=CVE-2026-12366; retrieved 2026-11-04; source updated 2026-11-04

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-12366; retrieved 2026-11-04; source updated Not reported

Revision: b770f3c668e393f34f079eefba8161ec1a583f6e0b70a06de893ae00a8d82eb3

Canonical: https://y2.dev/cve/CVE-2026-12366

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