Zephyr Project up to 4.4.x Device Multiplexer Pseudo-Driver devmux.c z_vrfy_devmux_select_get/z_vrfy_devmux_select_set improper authorization

Summaryinfo

A vulnerability described as very critical has been identified in Zephyr Project Zephyr up to 4.4.x. Impacted is the function z_vrfy_devmux_select_get/z_vrfy_devmux_select_set of the file drivers/misc/devmux/devmux.c of the component Device Multiplexer Pseudo-Driver. The manipulation results in improper authorization. This vulnerability was named CVE-2026-19947. The attack needs to be approached locally. There is no available exploit. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability was found in Zephyr Project Zephyr up to 4.4.x. It has been classified as very critical. Affected is the function z_vrfy_devmux_select_get/z_vrfy_devmux_select_set of the file drivers/misc/devmux/devmux.c of the component Device Multiplexer Pseudo-Driver. The manipulation with an unknown input leads to a improper authorization vulnerability. CWE is classifying the issue as CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

The device multiplexer pseudo-driver in drivers/misc/devmux/devmux.c exposes two system calls, devmux_select_get() and devmux_select_set(). When CONFIG_USERSPACE is enabled, their user-mode verifiers z_vrfy_devmux_select_get() and z_vrfy_devmux_select_set() forwarded directly to the kernel implementations without performing any kernel-object validation — no k_object_access_check() and no driver object check. Compounding this, DEVICE_DT_INST_DEFINE() registered devmux instances with a NULL driver API pointer, which causes scripts/build/gen_kobject_list.py to skip them entirely, so a devmux device had no struct k_object and no permission on it could be granted or withheld in the first place. Any thread running in user mode can reach the defect: device pointers such as DEVICE_DT_GET(DT_NODELABEL(devmux0)) are compile-time constants available to application code, and the syscall accepted them from a thread that had never been granted access to the device. The kernel implementations do validate the pointer against the static devmux instance table and bounds-check the selection index, so this is not a memory-safety issue; the failure is purely one of authorization. Exposure requires CONFIG_USERSPACE=y together with the experimental CONFIG_DEVMUX=y (which depends on CONFIG_DEVICE_MUTABLE) and a zephyr,devmux devicetree node. An unprivileged user-mode thread can read and change which downstream device a multiplexer forwards to, bypassing the userspace object-permission model. Because a devmux forwards the data, configuration and API of the selected device — its documented use being zephyr,console and zephyr,shell-uart switching — an attacker can suppress console, log and shell output, redirect that output to a downstream device it is able to observe, or flip the selection underneath another thread that is legitimately using the multiplexer. The impact is bounded to the devicetree-fixed set of downstream devices; no out-of-bounds access or arbitrary pointer dereference is reachable. The fix declares __subsystem struct devmux_driver_api and attaches a DEVICE_API(devmux, devmux_api) instance to each devmux device so that it becomes a tracked K_OBJ_DRIVER_DEVMUX kernel object, and adds K_OOPS(devmux_inst_get(dev) == SIZE_MAX) plus K_OOPS(k_object_access_check(dev) != 0) to both verifiers.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-19947 since 08/15/2026. The exploitability is told to be easy. The attack needs to be approached locally. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 10/21/2026). The MITRE ATT&CK project declares the attack technique as T1548.002.

Upgrading to version 4.5.0 eliminates this vulnerability.

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Productinfo

Type

• Project Management Software

Vendor

• Zephyr Project

Name

• Zephyr

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.4

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CPE 2.2info

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