USB connection with overvoltage protection
Overvoltage isn't so much the issue as over-current. The host supplies the voltage and you can reasonably assume that it's not suddenly going to supply more than 5V.

The two most important failure modes to protect against are:


if your device is connected to anything else with a power supply of its own, that supply must not feed back into the USB port.
if there is a short circuit or overcurrent in the device, it must not draw more power than is reasonable for a USB device. (normally 500mA, although technically you have to negotiate the use of more than 100mA, and some standalone chargers may give you up to 2A)


A standard means of protecting against overcurrent damage is a polyfuse (resettable fuse). Current limiting in normal use can be done by one of many power controller ICs, which are also useful for supplying other voltages, charging batteries, identifying when a high-current charger is plugged in, etc.