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

> A use-after-free in Zephyr RTOS allows unprivileged users to escape the sandbox and escalate privileges by freeing an armed timer without cancellation.

- **Severity:** High 8.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Aug 2026
- **Updated:** 14 Aug 2026
- **Also known as:** GHSA-x96g-542c-gccq

## What is CVE-2026-12366?

CVE-2026-12366 is a high-severity use after free vulnerability in zephyrproject zephyr, published on 14 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-12366?


## How do I fix CVE-2026-12366?

No fix has been published for CVE-2026-12366 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-12366 being exploited?

There is no known exploitation of CVE-2026-12366 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-12366 and fix it if it is affected.

What it is: CVE-2026-12366 is a high-severity use after free vulnerability in zephyrproject zephyr, published on 14 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-12366 in the wild and no public exploit code.

Affected product: zephyrproject zephyr. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-12366/ (Markdown: https://insights.sbomify.com/CVE-2026-12366/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-12366
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-12366
- OSV: https://osv.dev/vulnerability/CVE-2026-12366

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-12366/
Summaries are generated and may contain mistakes; the official description above is the record.