Most iOS jailbreaks have required a USB connection, so it seems the USB port was/is an attack surface. I haven't looked at the individual exploits used in each jailbreak, though.
It's been a while since I looked at it, but they were often used to get the phone into DFU mode and upload data that way. Seeing as DFU mode requires the phone itself to be rebooted, you'd likely notice what was going on unless you left it unattended.
And this is a more "embedded" example: http://www.tomshardware.com/news/Apple-Lightning-Cable-Hacke...