A computer worm is a term used to describe an unwanted program that uses system or application vulnerabilities to infect a computer without the user doing anything but connecting to an infected network A computer worm spreads without the user taking any action and usually exploits a bug (or vulnerability) in an operating system or some other program that may be running on a computer. The computer worm requires a computer software vulnerability to spread. The computer worm attacks at the network and application layers.