CVE-2026-9728

The userspace syscall verifier z_vrfy_mbox_send() in drivers/mbox/mbox_handlers.c validated the nested msg-&gt;data/msg-&gt;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&amp;#39;s use of msg-&gt;data, the validated pointer could be replaced, leaving a time-of-check/time-of-use window.<br /> <br /> On a system built with CONFIG_USERSPACE, any unprivileged userspace thread may invoke the mbox_send() system call. A second thread sharing the caller&amp;#39;s address space can race to overwrite msg-&gt;data with a supervisor (kernel) address after the verifier&amp;#39;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(&amp;data32, msg-&gt;data, msg-&gt;size) in the NXP mailbox driver, whose bytes are subsequently emitted to the peer mailbox endpoint).<br /> <br /> 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.