I recall there was a hack that you could use to corrupt the DC's flash memory or BIOS, thus leading to a system that wouldn't boot anymore, but I don't know if that was really true.
The DC has nowhere to store malicious code, because you can't modify the contents of the game disc the system is playing, as everything in RAM gets cleared when you cut the power. As such, it isn't really possible for your DC to get infected with a virus or anything. People who write viruses want to infect as many people as possible, and you'd never reach that goal by targetting the DC with your malware. There's no reason to try to infect all 50 people worldwide who still use their DCs online.