David D. Clark, an MIT scientist whose air of genial wisdom earned him the nickname “Albus Dumbledore,” can remember exactly when he grasped the Internet’s dark side. He was presiding over a meeting of network engineers when news broke that a dangerous computer worm — the first to spread widely — was slithering across the wires. One of the engineers, working for a leading computer company, piped up with a claim of responsibility for the security flaw that the worm was exploiting. “Damn,” he said. “I thought I had fixed that bug.” But as the attack raged in November 1988, crashing thousands of machines and causing millions of dollars in damage, it became clear that the failure went beyond a single man. The worm was using the Internet’s essential nature — fast, open and frictionless — to deliver malicious code along computer lines designed to carry harmless files or e-mails.