Security experts love dreaming up nightmare scenarios. Drones that empty our 401(k)s. Email attachments that blow up the refrigerator. Software that hacks into films and replaces all footage of Margot Robbie with computer-generated representations of Dr. Phil.

Nightmare scenarios like these are all the rage at the annual gathering of computer-security experts in Las Vegas, a metropolis that is sort of a nightmare itself. At confabs like Black Hat, experts have already been talking about ransomware that will lock you in your...