Denying Service

A more serious threat, however, comes from skilled hackers who launch a denial-of-service attack, in which a Web server is flooded with so many requests that it stops responding altogether.

Previously one of the most common attacks, DoS attacks are now much harder to accomplish. Large Internet companies counter them by buying larger Internet pipes, which are harder to fill with the junk data hackers throw at them. The more bandwidth a company has, the more service the hacker needs to interrupt in order to produce a noticeable effect.

Hackers quickly learned that a single computer couldn't send enough phony requests to deny service, so they came up with a clever approach that employs dozens of hacked computers, working in synch to execute a distributed denial-of-service attack.

A DDoS attack uses as many computers as the hacker can control (called "zombies") to send bogus data requests to a targeted server. To unleash the attack, the hacker sends just one command, which propagates to all of the zombies and causes a near-instantaneous death-by-data on the Web server.

A hacker can also use an army of compromised computers to steal data--such as credit card numbers and proprietary corporate files--without leaving a clear trail. The hacker hops from machine to machine and then launches an attack that passes through all of them, creating a maze of connections for authorities to sift through.

University systems are prime targets for such activity, since administrators often leave student accounts active after students have graduated. A hacker can take over the account and use it as a base to attack another system.

In December 2000 hackers broke into a U.S. Air Force system in Virginia and downloaded code for controlling communication and spy satellites to a computer in Sweden. The Swedish company that owned the system housing the data had no idea hackers were using its computer, and cooperated with authorities.

From Sweden the activity was traced to a university machine in Germany, which authorities also believe was being used by a distant hacker.