Today I read about a researcher who bought a digital TV recorder and connected it to the internet - probably using port forwarding to enable attackers to connect to the recorder. After a while someone was able to connect with Telnet, and upload software to mine bitcoins.
If this is possible, other malware could be installed that tries to infect other computers in the network. Because these devices are not monitored they are ideal for hiding out.
So your average Windows computer is infected with malware, it finds a tv recorder in your network, infects that. The Windows PC is cleaned of its malware, but the recorder stays undetected, and after a while it infects another computer, etc...
How can I detect infected devices on my local network? What methods are there to prevent this from happening?