# CVE-2026-9728: TOCTOU race in mbox_send syscall verifier allows userspace to leak kernel memory

> A race condition in the Zephyr RTOS mbox_send syscall allows unprivileged users to leak kernel memory or cause denial of service.

- **Severity:** Medium 6.4 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L`)
- **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:** 24 Aug 2026
- **Updated:** 25 Aug 2026
- **Also known as:** GHSA-47q2-w832-7w67

## What is CVE-2026-9728?

CVE-2026-9728 is a medium-severity vulnerability in zephyrproject zephyr, published on 24 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

The userspace syscall verifier z_vrfy_mbox_send() in drivers/mbox/mbox_handlers.c validated the nested msg->data/msg->size fields by reading them directly out of live userspace memory, and then forwarded the original, still-mutable userspace struct mbox_msg * pointer to z_impl_mbox_send() and the underlying driver. Between the access check and the driver's use of msg->data, the validated pointer could be replaced, leaving a time-of-check/time-of-use window.

On a system built with CONFIG_USERSPACE, any unprivileged userspace thread may invoke the mbox_send() system call. A second thread sharing the caller's address space can race to overwrite msg->data with a supervisor (kernel) address after the verifier's bounds check has passed but before the driver dereferences it. The driver then reads from the attacker-chosen address in supervisor context (for example memcpy(&data32, msg->data, msg->size) in the NXP mailbox driver, whose bytes are subsequently emitted to the peer mailbox endpoint).

The impact is a userspace-to-supervisor access-control bypass: disclosure of kernel memory contents (high confidentiality impact), or, for an invalid/unmapped target address, a faulting kernel read causing denial of service. The fix snapshots the entire struct mbox_msg into a kernel-stack copy with k_usermode_from_copy() and validates and forwards that immutable copy, closing the race.

Weakness: CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition)

## Which versions are affected by CVE-2026-9728?


## How do I fix CVE-2026-9728?

No fix has been published for CVE-2026-9728 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-9728 being exploited?

There is no known exploitation of CVE-2026-9728 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-9728 and fix it if it is affected.

What it is: CVE-2026-9728 is a medium-severity vulnerability in zephyrproject zephyr, published on 24 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-2026-9728 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-2026-9728/ (Markdown: https://insights.sbomify.com/CVE-2026-9728/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9728
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9728
- OSV: https://osv.dev/vulnerability/CVE-2026-9728

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-9728/
Summaries are generated and may contain mistakes; the official description above is the record.