# CVE-2026-12631: Broken access-control denial in k_thread_join/k_thread_abort syscall validation in Zephyr kernel

> Unprivileged users can crash the kernel or bypass access controls due to incorrect syscall validation logic in Zephyr RTOS.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/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:** 18 Aug 2026
- **Updated:** 19 Aug 2026
- **Also known as:** GHSA-crfw-75jw-hjm3

## What is CVE-2026-12631?

CVE-2026-12631 is a medium-severity missing authorization vulnerability in zephyrproject zephyr, published on 18 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

The Zephyr kernel validates the k_thread_join() and k_thread_abort() system calls (declared __syscall in include/zephyr/kernel.h) through thread_obj_validate() in kernel/thread.c. Its default switch branch is the access-denied path, taken when k_object_validate() returns -EPERM (the calling user thread was never granted access to the target thread object) or -EBADF (the supplied pointer is not a registered kernel object of the right type). That branch invoked K_OOPS(K_SYSCALL_VERIFY_MSG(ret, "access denied")), but K_SYSCALL_VERIFY_MSG treats a true expression as success; the non-zero error code ret therefore read as "verified OK", the kernel oops was never raised, and control fell through to CODE_UNREACHABLE.

Because k_thread_join() and k_thread_abort() are system calls, an unprivileged user-mode thread (under CONFIG_USERSPACE) can reach this denial path directly by calling either syscall on a thread object it does not own. Instead of the offending thread being cleanly terminated, execution reaches __builtin_unreachable() while running in supervisor mode inside the syscall handler.

On Clang builds CODE_UNREACHABLE emits an illegal-instruction trap, so a user thread can deterministically crash the kernel — a locally triggerable denial of service that escapes the userspace sandbox. On GCC builds the path is undefined behavior: the compiler may drop the return-value handling for thread_obj_validate(), so it can return an undefined bool; if that is false, the caller proceeds into the real k_thread_join()/k_thread_abort() implementation for a thread the user was never authorized to access, an access-control bypass.

The fix changes the verification expression to ret == 0, so a denied (non-zero) result now correctly raises K_OOPS and terminates the offending caller.

Weakness: CWE-862 (Missing Authorization)

## Which versions are affected by CVE-2026-12631?


## How do I fix CVE-2026-12631?

No fix has been published for CVE-2026-12631 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-12631 being exploited?

There is no known exploitation of CVE-2026-12631 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-12631 and fix it if it is affected.

What it is: CVE-2026-12631 is a medium-severity missing authorization vulnerability in zephyrproject zephyr, published on 18 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-2026-12631 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-2026-12631/ (Markdown: https://insights.sbomify.com/CVE-2026-12631/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-12631
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-12631
- OSV: https://osv.dev/vulnerability/CVE-2026-12631

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-12631/
Summaries are generated and may contain mistakes; the official description above is the record.