Suitcase-sized devices have been developed to do just that. Various possible operations with obvious effects include knocking out telephone switches, crashing stock markets, attacking electronic routers for rail systems, attacking bank accounts, disrupting air traffic control and denying service with, for instance, a ping attack. Within a network, a computer can send systematic queries to all addresses and analyze the associated return time, very similar to sonar. Net groups with similar times of return can be associated into a hierarchical structure. A Trojan horse is different from a virus in that a virus can be duplicated thousands of times and function according to a previous set of instructions, while a Trojan horse is designed to facilitate access and interaction between its creator and the system it infiltrates.
Worms can take up more and more memory and disk space until they stop the computer cold.
The famous Internet worm of November 1988 replicated itself on more than 6,000 networks around the world. As a result, customized messages can be generated for each target sector of society. An attempt to send a falsified message to someone. For example, I could dial up a university phone registration system, pretending to be someone I have a grudge against, and drop their classes. Since these systems are automated, all I would need to know in most cases is a person's Social Security number and birthdate.