In 2008, Mr. Kaminsky was widely hailed as a digital Paul Revere after he found a serious flaw in the internet’s basic plumbing that could allow skilled coders to take over websites, siphon off bank credentials or even shut down the internet. Mr. Kaminsky alerted the Department of Homeland Security, executives at Microsoft and Cisco, and other internet security experts to the problem and helped spearhead a patch.

He was a respected practitioner of “penetration testing,” the business of compromising the security of computer systems at the behest of owners who want to harden their systems from attack. It was a profession that his mother, Trudy Maurer, said he had first developed a knack for as a 4-year-old in San Francisco, after his father gave him a computer from Radio Shack. By age 5 he had taught himself to code.

His childhood paralleled the 1983 movie “War Games,” in which a teenager, played by Matthew Broderick, unwittingly accesses a U.S. military supercomputer. When Daniel was 11, his mother said, she received an angry phone call from someone who identified himself as a network administrator for the Western United States. The administrator said someone at her residence was “monkeying around in territories where he shouldn’t be monkeying around.”  Without her knowledge, Daniel had been examining military websites. The administrator vowed to “punish” him by cutting off the family’s internet access. Mrs. Maurer warned the administrator that if he made good on his threat, she would take out an advertisement in The San Francisco Chronicle denouncing the Pentagon’s security.

“I will take out an ad that says, ‘Your security is so crappy, even an 11-year-old can break it,’” she recalled telling the administrator, in an interview on Monday.  They settled on a compromise punishment: three days without internet.

Nearly two decades after he lost his access to the internet, Mr. Kaminsky wound up saving it. What Mr. Kaminsky discovered in 2008 was a problem with the internet’s basic address system, known as the Domain Name System, or DNS, a dynamic phone book that converts human-friendly web addresses like NYTimes.com and Google.com into their machine-friendly numeric counterparts. He found a way that thieves or spies could covertly manipulate DNS traffic so that a person typing the website for a bank would instead be redirected to an impostor site that could steal the user’s account number and password.

Mr. Kaminsky’s first call was to Paul Vixie, a longtime steward of the internet’s DNS system. The usually unflappable Mr. Vixie recalled that his panic grew as he listened to Mr. Kaminsky’s explanation.  “I realized we were looking down the gun barrel of history,” Mr. Vixie recalled. “It meant everything in the digital universe was going to have to get patched.”

Over several days they cobbled together a solution in stealth, a fix that Mr. Vixie compared to dog excrement. But given the threat of internet apocalypse, he recalled it as being the best dog excrement “we could have ever come up with.”

By the time Mr. Kaminsky took the stage at Black Hat annual hacking conference in Las Vegas that August, the web had been spared. Mr. Kaminsky, who typically donned a T-shirt, shorts and flip flops, appeared onstage in a suit that his mother had bought for him. She had also requested that he wear closed-toed shoes. He complied, sort of — twirling onto the stage in roller skates.

When his talk was complete, Mr. Kaminsky was approached by a stranger in the crowd. It was the administrator who had kicked Mr. Kaminsky off the internet years earlier. Now he wanted to thank Mr. Kaminsky and to ask for an introduction to “the meanest mother he ever met.”