An issue was discovered in myStrom WiFi Switch V1 before 2.66, WiFi Switch V2 before 3.80, WiFi Switch EU before 3.80, WiFi Bulb before 2.58, WiFi LED Strip before 3.80, WiFi Button before 2.73, and WiFi Button Plus before 2.73. The process of registering a device with a cloud account was based on an activation code derived from the device MAC address. By guessing valid MAC addresses or using MAC addresses printed on devices in shops and reverse engineering the protocol, an attacker would have been able to register previously unregistered devices to their account. When the rightful owner would have connected them after purchase to their WiFi network, the devices would not have registered with their account, would subsequently not have been controllable from the owner's mobile app, and would not have been visible in the owner's account. Instead, they would have been under control of the attacker.