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.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
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
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.
Sources
Also known as GHSA-47q2-w832-7w67.