Last October at Melbourne’s grand Intercontinental Hotel scores of technophiles watched a researcher for IOActive, a Seattle-based computer-security firm, demonstrate an ingenious new way to kill someone—a method that one can imagine providing a sensational plot twist in an episode of Homeland.

The IOActive researcher, a man named Barnaby Jack, was so worried about the implications of his work that he intentionally obscured many of the details in his presentation. As a further precaution, he asked the attendees not to take any pictures—a tough request in a crowd full of smartphones and laptops.

Jack’s work concerned pacemakers and implantable cardioverter-defibrillators (I.C.D.’s). More than three million American heart patients carry around these small, computerized devices, which monitor their heartbeat and deliver jolts of electricity to stabilize it when needed. To check and adjust these devices, many doctors use wand-like wireless programmers that they wave a few inches above patients’ chests—a straightforward and seemingly safe procedure. But now, with a custom-built transmitter, Jack had discovered how to signal an I.C.D. from 30 feet away. It reacted as if the signal were in fact coming from the manufacturer’s official I.C.D. programmer. Instructed by the counterfeit signal, the I.C.D. suddenly spat out 830 volts—an instantly lethal zap. Had the device been connected to an actual human heart, the fatal episode would likely have been blamed on a malfunction.

Let’s face it: Barnaby Jack is a man who is quite literally looking for trouble. This is a guy who had demonstrated the year before how he could wirelessly direct an implantable insulin pump to deliver a lethal dose. The year before that, he hacked an ATM to make it spray out bills like a slot machine. But trouble-making is what he’s paid to do at IOActive, and in that role he has developed a particular respect for the looming power of smartphones. Terrorists have already used cell phones to kill people in the crudest possible way: detonating explosives in Iraq and Afghanistan. But smartphones bring a new elegance to the endeavor and will bring new possibilities for mayhem into the most mundane areas of life.

The day is not far off, Jack says, when the manipulation of medical devices, for which he had needed to build special equipment, will be done routinely and remotely by punching keys on a smartphone. Indeed, in just a few minutes of online searching, I was able to find a dozen ventures developing smartphone apps for medical devices: pacemakers, defibrillators, cochlear implants, insulin pumps, cardiovascular monitors, artificial pancreases, and all the other electronic marvels doctors now are inserting into human bodies.

To engineers, the advantages are clear. Smartphones can relay patients’ data to hospital computers in a continuous stream. Doctors can alter treatment regimens remotely, instead of making patients come in for a visit. If something goes wrong, medical professionals can be alerted immediately and the devices can be rapidly adjusted over the air. Unfortunately, though, the disadvantages are equally obvious to people like Barnaby Jack: doctors will not be the only people dialing in. A smartphone links patients’ bodies and doctors’ computers, which in turn are connected to the Internet, which in turn is connected to any smartphone anywhere. The new devices could put the management of an individual’s internal organs, in the hands of every hacker, online scammer, and digital vandal on Earth.

I asked Jack if he thought anyone would actually use smartphones to try to fiddle with other people’s pacemakers, or change the dosage of their medications, or compromise their eyesight, or take control of their prosthetic limbs, or raise the volume of their hearing aids to a paralyzing shriek. Will this become a tempting new way to settle a score or hurry up an inheritance? He said, “Has there ever been a box connected to the Internet that people haven’t tried to break into?” He had a point: a few years ago, anonymous vandals inserted flashing animated images into an Epilepsy Foundation online forum, triggering migraines and seizure-like reactions in some unfortunate people who came across them. (The vandals were never found.) Jack was reluctant to go into detail about what he thinks the future may hold. “I’m not comfortable trying to predict exact scenarios,” he said. But then he added, calm as a State Department spokesman, “I can say that I wouldn’t want to discover a virus in my insulin pump.”