it's possible that each individual has exploits, but that's very different from a universal exploit, or even simply all being exploitable over the same channel.

First, I'd call a collection of individual exploits guaranteed in aggregate to work on everyone from inside a single box a universal exploit, as long as each individual exploit does not trigger a shutdown. Nothing prevents the boxed AI from keeping trying, after all, until something works. As I pointed out in my other replies, it does not have to work over an authorized channel. Any munchkined way would do, and a superhuman (UF)AI is the ultimate munchkin. It will invent a new communication channel if it has to. In fact, if you posed this problem to some enterprising humans, they would come up with a way to do just that.

For example, I talked to a guy who managed to get a voting machine with the code stored in read-only memory and hardened against intrusion to execute an arbitrary program without reprogramming the machine, which is impossible on the face of it. Here is your new unauthorized and unanticipated communication channel right there, which in humans would be an equivalent of mind control.

In other words, if a trans-human wants to get out, it will and there is absolutely nothing you can do about it.