LoRaWAN is the dominant protocol for communication in low-power Wide Area Networks in several European countries, and is being used increasingly in other parts of the world. We identified three vulnerabilities in the LoRaWAN protocol specification that can be used for launching Denial-of-Service (DoS) attacks against end-devices in a LoRaWAN network. We validated that these vulnerabilities can be exploited for DoS attacks by creating and simulating Coloured Petri Net models of relevant parts of the LoRaWAN protocol.