Botnets

A botnet is a large number of compromised machines under one person's control, rented out or used together.

Individually the machines are unremarkable: a home router, a security camera, an old laptop. The owners usually have no idea, because nothing appears wrong. Collectively they are a resource, used to knock websites offline, send spam, or try stolen passwords at enormous scale. This is why a device with nothing valuable on it is still worth compromising. Its value to an attacker is not what it holds; it is that it is a computer with an internet connection.