Not sure if there is a more proper stackexchange for this... I have heard vaguely of problems with usb devices being insecure for obvious reasons, but what about headphone jacks? Can they impersonate devices like keyboards, send data such as keystrokes to the machine? I am trying to imagine a scenario like the in-line microphone housing a gsm broadcaster/receiver, which would receive power from the port. So a malicious device wouldn't necessarily have to interface with the computer/phone in an obtrusive manner, but just sit there and suck up all audio within range and transmit it across a cellular network? Is this feasible? Also, do fbi agents sneak into my room at night and move things around in a subtle way in an effort to "gaslight" me? Just kidding about the last question.
Edit to explain difference from "can headphones transmit malware": What about the other scenario I described where only power is transmitted TO the headphones and the inline microphone houses wifi or cellular transmitter/receiver?
Edit for clarification: A lot of interesting points have been brought up, but I feel like I must not have described this well, since answers range from Van-Eck phreaking scenarios, to alteration of the laptop's hardware. The laptop has nothing to do with what I am describing. Assume the laptop (hardware and software) is known to be completely clean and trusted. I am talking about a malicious set of headphones with inline microphone (the little box on one of the wires to the headphones). Inside the inline microphone box is a transmitting device (ideally it would be a cellular transmitter). The inline microphone would be always on, and transmitting all data to some malicious actor over the cellular transmitter. The inline microphone and headphones would still function as normal from the users perspective. The transmitter would receive power from the 3.5mm jack.