From The New Yorker:

According to James Baker, the former F.B.I. general counsel who runs the national-security program at the R Street Institute, “There’s a concern that those devices that are connected to the 5G network are not going to be very secure from a cyber perspective. That presents a huge vulnerability for the system, because those devices can be turned into bots, for example, and you can have a massive botnet that can be used to attack different parts of the network.”