boneIO black_debian_images ships with a Mosquitto MQTT broker that uses the same default credentials on all devices. These credentials are publicly documented on github, and there is no indication in the instructions to change them. A remote attacker can use them to gain full read and write access to the broker, allowing control of connected relays, blinds, and gates, and injection of fake entities into a linked Home Assistant installation.
This issue was fixed in version 1.6.0 of the image: MQTT passwords are generated per device at first boot. Devices upgraded from earlier versions keep their existing broker credentials; from boneio 1.6.0 the web panel reports credentials that are still the published defaults, and the owner has to change them.