George Ledin teaches students how to write viruses, and it makes computer-security software firms sick.

In a windowless underground computer lab in California, young men are busy cooking up viruses, spam and other plagues of the computer age. Grant Joy runs a program that surreptitiously records every keystroke on his machine, including user names, passwords, and credit-card numbers. And Thomas Fynan floods a bulletin board with huge messages from fake users. Yet Joy and Fynan aren't hackers—they're students in a computer-security class at Sonoma State University. And their professor, George Ledin, has showed them how to penetrate even the best antivirus software.

The companies that make their living fighting viruses aren't happy about what's going on in Ledin's classroom. He has been likened to A.Q. Khan, the Pakistani scientist who sold nuclear technology to North Korea....

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