Security researcher Jonathan Brossard created a proof-of-concept hardware backdoor called Rakshasa that replaces a computer’s BIOS (Basic Input Output System) and can compromise the operating system at boot time without leaving traces on the hard drive. The only way to get rid of the malware is to shut down the computer and manually reflash every peripheral, a method that is impractical for most users because it requires specialized equipment and advanced knowledge. Backdoors can be installed by anyone with physical access to your hardware either at manufacturing time, or in the office with a USB stick.
Source: https://thehackernews.com/2012/08/researcher-demonstrate-hardware-based.html