The New Zealand-born Jack, 35, was found dead on Thursday evening by "a loved one" at an apartment in San Francisco's Nob Hill neighbourhood, according to a police spokesman. He would not say what caused Jack's death but said police had ruled out foul play.
The San Francisco Medical Examiner's Office said it was conducting an autopsy, although it could be a month before the cause of death is determined.
Jack was one of the world's most prominent "white hat" hackers - those who use their technical skills to find security holes before criminals can exploit them.
His genius was finding bugs in the tiny computers embedded in equipment, such as medical devices and cash machines. He often received standing ovations at conferences for his creativity and showmanship while his research forced equipment makers to fix bugs in their software.
Jack had planned to demonstrate his techniques to hack into pacemakers and implanted defibrillators at the Black Hat hackers convention in Las Vegas next Thursday. He told Reuters last week that he could kill a man from 30 feet (nine meters) away by attacking an implanted heart device.
"He was passionate about finding security bugs before the bad guys," said longtime security industry executive Stuart McClure, who gave Jack one of his first jobs and also had worked with him at Intel Corp's McAfee, a computer security company.
"He was one of those people who was put on this earth to find vulnerabilities that can be exploited in a malicious way to hurt people," McClure said.
Jack became one of the world's most famous hackers after a 2010 demonstration of "Jackpotting" - getting ATMs to spew out bills. A clip of his presentation has been viewed more than 2.6 million times on YouTube.
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