The flaw has to do with the protocol used in a train system known as the End-of-Train and Head-of-Train. A Flashing Rear End Device (FRED), also known as an End-of-Train (EOT) device, is attached to the back of a train and sends data via radio signals to a corresponding device in the locomotive called the Head-of-Train (HOT). Commands can also be sent to the FRED to apply the brakes at the rear of the train.

These devices were first installed in the 1980s as a replacement for caboose cars, and unfortunately, they lack encryption and authentication protocols. Instead, the current system uses data packets sent between the front and back of a train that include a simple BCH checksum to detect errors or interference. But now, the CISA is warning that someone using a software-defined radio could potentially send fake data packets and interfere with train operations...

…the saboteurs appear to have sent simple so-called “radio-stop” commands via radio frequency to the trains they targeted. Because the trains use a radio system that lacks encryption or authentication for those commands, Olejnik says, anyone with as little as $30 of off-the-shelf radio equipment can broadcast the command to a Polish train­—sending a series of three acoustic tones at a 150.100 megahertz frequency­—and trigger their emergency stop function...