An Early Warning System Against Computer Invaders

An electronic sneak attack could happen almost any day, according to computer-security experts. Virtually any computer with a telephone link is highly vulnerable. Even determined teenagers, armed with a personal computer and a modem, regularly get past the usual safeguards.

And while it's hard enough to shield a single computer against intrusion, it's all but impossible to protect computer networks. That's because intruders can bounce from computer to computer on the network, never making enough unsuccessful stabs on any one machine to raise an alarm. For example, the Internet network, which links 300,000 government, university, and commercial computers in 35 countries, has been the target of a "continuous stream" of attacks, says Ira W. Morrison, a researcher at Lawrence Livermore National Laboratory.