TOCTOU race in mbox_send syscall verifier allows userspace to leak kernel memory

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-9728
StatePUBLISHED
Assignerzephyr
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-24 15:16:49 UTC
Updated2026-08-26 16:59:23 UTC
DescriptionThe 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.

Risk And Classification

Primary CVSS: v3.1 6.4 MEDIUM from [email protected]

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L

EPSS: 0.000830000 probability, percentile 0.002740000 (date 2026-08-26)

Problem Types: CWE-367 | CWE-367 race

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.4MEDIUMCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L
3.1CNACVSS6.4MEDIUMCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
High
Privileges Required
Low
User Interaction
None
Scope
Changed
Confidentiality
High
Integrity
None
Availability
Low

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:L

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Zephyrproject Zephyr affected 3.0.0 4.4.2 semver Not specified

References

ReferenceSourceLinkTags
github.com/zephyrproject-rtos/zephyr/commit/ab35eaccec5976f05c196f176d0c... [email protected] github.com
github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-47q2-w832-... [email protected] github.com
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