CVE-2026-19741: OpenThread joiner autostart uses a publicly documented default Joining Device Credential (PSKd)
Zephyr's OpenThread integration defined CONFIGOPENTHREADJOINERPSKD with default "J01NME" in modules/openthread/Kconfig.thread (previously subsys/net/l2/openthread/Kconfig, then subsys/net/l2/openthread/Kconfig.thread). J01NME is the example Joining Device Credential used throughout the public OpenThread and Thread commissioning documentation, including Zephyr's own samples/net/sockets/echoclient/README.rst, so it is not a secret. A firmware image built with CONFIGOPENTHREADJOINERAUTOSTART=y that never overrode the PSKd therefore shipped with a publicly known joining credential compiled in, with no build-time or run-time warning.
The PSKd is the only value that authenticates Thread commissioning: joiner and commissioner run a DTLS-PSK handshake keyed on it, after which the commissioner supplies the full operational dataset. In otconfigureinstance() (modules/openthread/openthread.c), a device with no stored dataset calls otJoinerStart() with the configured PSKd, and otjoinerstarthandler() calls otThreadSetEnabled() on success. An unauthenticated attacker within 802.15.4 radio range of an uncommissioned device (factory-fresh, after a factory reset, or after a settings erase) can run any stock Thread commissioner registered with J01NME, complete the handshake, and hand the device an attacker-controlled operational dataset.
The device then joins and enables Thread on the attacker's mesh. The attacker becomes its router, seeing, dropping, injecting and spoofing all of the device's IPv6 traffic and impersonating the peers the application expects, while the device is cut off from the network it was meant to join and stays mis-provisioned until it is reset. Only application-layer security with independently provisioned credentials survives. Exposure is limited to builds that enable joiner autostart and leave the PSKd unset; the option is not enabled by default and no in-tree sample uses the vulnerable combination.
The fix removes the default from CONFIGOPENTHREADJOINERPSKD and adds a BUILDASSERT() in modules/openthread/openthread.c, active only under CONFIGOPENTHREADJOINERAUTOSTART, that fails the build unless a 6-to-32-character PSKd is configured. Products should provision a per-device PSKd from factory data and call otJoinerStart() from the application rather than relying on a credential compiled identically into every unit.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
Provision a unique per-device PSKd from factory data instead of using the publicly known default J01NME; alternatively, call otJoinerStart() from the application with the provisioned credential rather than relying on an identically compiled credential.
Zephyr OpenThread joiner autostart CONFIG_OPENTHREAD_JOINER_PSKD = A per-device PSKd of 6 to 32 characters
Event History
Frequently Asked Questions
Which devices are exposed to this issue?
Devices are exposed if they were built with CONFIG_OPENTHREAD_JOINER_AUTOSTART=y, retained the default CONFIG_OPENTHREAD_JOINER_PSKD value of "J01NME", and have no stored Thread operational dataset. This includes factory-fresh devices, devices after a factory reset, and devices after settings erasure.
What does an attacker need to exploit it?
An attacker must be within 802.15.4 radio range of an uncommissioned affected device. The publicly documented default PSKd is sufficient to authenticate the Thread commissioning DTLS-PSK handshake; no existing device credentials are required.
How can I determine whether a firmware build is affected?
Check whether CONFIG_OPENTHREAD_JOINER_AUTOSTART is enabled and whether CONFIG_OPENTHREAD_JOINER_PSKD was left as "J01NME" rather than overridden. A build matching those conditions is relevant when the device has no stored operational dataset.
What can be done if updating the firmware is not immediately possible?
Avoid deploying or resetting affected devices into an uncommissioned state while they are reachable over 802.15.4. Build firmware with a PSKd other than the documented default before enabling joiner autostart.