They are used in everyday life, not threatening to the majority of the population, and so ignored as just there. Yet it's complex nature is not understood or else the potential for abuse would make people see it less as a common appliance and more as an invisible potential threat. What we are talking about is the necessary tool on all Starships and other spacecrafts or stations... The Teleportation device.

They breakdown the matter of an object, send the molecules and information to another place, and then reassembling them. A lower level version of this is used for energy/matter replicators to turn items like energy or waste material into food and useful items. By rewriting the structure of the item. What no one who uses one asks... What if you hack the data and uses it to remake the object in the Teleporter?

It has been done before, using old data patterns of a previous transport to change someone. This case someone suffering from a viral premature aging to reduce them physically to their unaffected body while keeping their current mind. Why can it not be used to alter someone into a young form, a healthier form, a better form, or any mixed of the above? Restore damaged body parts and more.

Or to be used on their mind? Adding skills and memories, removing them... or modifying them. Like modifying thoughts, feelings, beliefs, memories and so on. Remaking a person mentally, just as you could physically?

Now you have the threat which faces the universe at large, without the majority knowing it. That every time they use the transporter, they hold the potential of being remade by one or more people clever enough to hack the programming of these machines for their own ends. Good, evil, neither or perverted. Someone like...