Johnson & Johnson recently disclosed that one if its insulin pumps might be subject to hacking. This follows assertions about pacemakers and implanted defibrillators might also be subject to attack. No doubt some wireless medical devices will have security vulnerabilities with at least software if not hardware attack vectors.
The motives for attack are perhaps equally important in any case. Hacking a fleet of cars can have widespread visibility and will be associated with a different set of motives than a personal attack via a medical device. However, murder or assassination are potential uses for these types of flaws.
“No instances of medical-device hacking have been disclosed.” according to the related WSJ article. Of course, when a diabetic dies of an insulin excess or deficit, murder by hacking might not be on the post mortum evaluation list. The abuses here are (hopefully) rare, but the lack of disclosure does not imply the lack of a successful attack.