Null pointer dereference

RIOT 2019.07 contains a NULL pointer dereference in the MQTT-SN implementation (asymcute), potentially allowing an attacker to crash a network node running RIOT. This requires spoofing an MQTT server response. To do so, the attacker needs to know the MQTT MsgID of a pending MQTT protocol message and the ephemeral port used by RIOT’s MQTT implementation. Additionally, the server IP address is required for spoofing the packet.