{"id":"CVE-2026-19947","revision":"60ece6dbdc684ca4902c948e166d69b8","records":[{"revision":"60ece6dbdc684ca4902c948e166d69b8","observedAt":"2026-10-23T22:48:20.692+00:00","statement":{"source":"cve-program","id":"CVE-2026-19947","aliases":[],"title":"Missing kernel object permission check in devmux syscalls allows unprivileged user threads to retarget a device multiplexer","description":"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.\n\nAny 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.\n\nAn 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.\n\nThe 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.","state":0,"publishedAt":"2026-10-21T18:19:35.637+00:00","modifiedAt":"2026-10-23T21:23:27.801+00:00","affected":[{"ecosystem":"https://github.com/zephyrproject-rtos/zephyr","name":"zephyr","ranges":[],"versions":[],"rawVersionEvidence":"{\n                    \"vendor\": \"zephyrproject\",\n                    \"product\": \"zephyr\",\n                    \"collectionURL\": \"https://github.com/zephyrproject-rtos/zephyr\",\n                    \"packageName\": \"zephyr\",\n                    \"defaultStatus\": \"unaffected\",\n                    \"programFiles\": [\n                        \"drivers/misc/devmux/devmux.c\",\n                        \"include/zephyr/drivers/misc/devmux/devmux.h\"\n                    ],\n                    \"programRoutines\": [\n                        {\n                            \"name\": \"z_vrfy_devmux_select_get\"\n                        },\n                        {\n                            \"name\": \"z_vrfy_devmux_select_set\"\n                        }\n                    ],\n                    \"versions\": [\n                        {\n                            \"version\": \"3.6.0\",\n                            \"status\": \"affected\",\n                            \"lessThan\": \"4.5.0\",\n                            \"versionType\": \"semver\"\n                        }\n                    ]\n                }"}],"metrics":[{"system":"CVSS 3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L","score":5.3,"severity":"MEDIUM"}],"references":["https://github.com/zephyrproject-rtos/zephyr/commit/4b7f98345d8f894f4b8aabe52a2623d843669fac","https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-8m8g-f5jf-x28x"],"sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-19947","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-19947","identifiers":["CVE-2026-19947"]},"retained":false,"historicalAffected":[]}],"coverage":[]}