Last Friday around midnight, the 1.6 million people living in Dallas woke up to the screeching sounds of sirens that was triggered as a result of a supposed computer hack outside of the emergency notification network. The emergency system was used to warn its residence of tornadoes and other dangerous weather conditions. The alarms were blaring for 95 minutes until the administrators shut down the system manually. Initially the attack was thought to be caused by a network hack, but it wasn’t entirely accurate.
Dallas City Manager later clarified that the “hack” used a radio signal that spoofed the system used to control the siren network. He did not disclosed any additional details, but noted that it was not a software issue but rather a radio issue. Experts speculates that older Emergency Alert Systems are usually controlled by tone combinations that are broadcast over the National Weather Service’s weather radio. The sirens receive their commands from a Dual-Tone Multi-Frequency (DTMF) or Audio Frequency Shift Keying (AFSK) that sends encoded commands from the command center. If these frequency were not monitored, then an attacker can send endless combinations until they get the right one. Then all they had to do is repeat the signal.
Article: https://arstechnica.com/information-technology/2017/04/dallas-siren-hack-used-radio-signals-to-spoof-alarm-says-city-manager/
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