CVE-2026-19947 in Zephyr
Summary
by MITRE • 10/21/2026
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.
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Analysis
by VulDB Data Team • 10/21/2026
The vulnerability resides in the Zephyr RTOS device multiplexer pseudo-driver implementation within drivers/misc/devmux/devmux.c, specifically affecting systems where CONFIG_USERSPACE and CONFIG_DEVMUX are enabled alongside CONFIG_DEVICE_MUTABLE. The core technical flaw is a complete failure of authorization checks for two system calls: devmux_select_get() and devmux_select_set(). When these syscalls are invoked from user mode, the corresponding verifier functions z_vrfy_devmux_select_get() and z_vrfy_devmux_select_set() forward requests directly to kernel implementations without performing any validation. Crucially, they omit both k_object_access_check calls and driver object verification, allowing unprivileged threads to interact with device resources for which they have no granted permissions. This authorization bypass is compounded by a configuration issue where DEVICE_DT_INST_DEFINE registers devmux instances with a NULL driver API pointer. Consequently, the build script gen_kobject_list.py excludes these devices from the kernel object list, meaning no struct k_object exists for them and thus no permission model can be applied or enforced at all.
From an operational perspective, this defect allows any user-mode thread to read and modify which downstream device a multiplexer forwards traffic to. Device pointers such as DEVICE_DT_GET(DT_NODELABEL(devmux0)) are compile-time constants accessible to application code, enabling attackers to target specific instances without needing prior access grants. While the kernel implementations do perform bounds checking on selection indices and validate pointers against static instance tables preventing memory safety issues like out-of-bounds reads or arbitrary pointer dereferences, the lack of authorization control renders these safeguards insufficient for security isolation. The vulnerability is strictly an authorization failure rather than a memory corruption issue, yet it effectively bypasses the userspace object-permission model designed to isolate applications from hardware resources.
The impact of this vulnerability is significant within the context of Zephyr's device abstraction layer. Since devmux devices are commonly used for switching console and shell-uart outputs between different downstream peripherals, an attacker can manipulate these selections to disrupt system observability or intercept sensitive data. Specifically, a malicious thread could suppress console, log, and shell output entirely by redirecting it to a closed port, effectively blinding administrators and security monitoring tools. Alternatively, the attacker could redirect this output to a downstream device that is observable by unprivileged processes, leading to information leakage of kernel logs or interactive session data. Furthermore, flipping the selection underneath another legitimate thread can cause service disruption for critical applications relying on consistent hardware access paths. The impact remains bounded to the set of devices defined in the devicetree configuration, preventing broader system compromise but severely degrading reliability and confidentiality within that scope.
This vulnerability maps directly to CWE-269 Improper Privilege Management, as it involves a failure to enforce proper authorization for privileged operations. In terms of MITRE ATT&CK tactics, this aligns with Defense Evasion techniques where an attacker bypasses security controls to maintain access or disrupt monitoring capabilities such as Log Tampering or Indicator Removal on Host. The remediation strategy addresses the root cause by properly initializing the devmux driver API structure via __subsystem struct devmux_driver_api and attaching a DEVICE_API instance to each device, thereby ensuring they are registered as tracked K_OBJ_DRIVER_DEVMUX kernel objects with valid permission models. Additionally, the fix introduces explicit checks in both verifier functions: K_OOPS(devmux_inst_get(dev) == SIZE_MAX) ensures the device pointer is valid within the static table, and K_OOPS(k_object_access_check(dev) != 0) enforces that the calling thread possesses the necessary access rights before allowing any modification or retrieval of multiplexer state. This restores the integrity of the userspace isolation model for devmux devices.