You are correct that like all modern ICs, there are protection diodes for ESD and some amount of over/under voltage protection, but you need to be careful.
The STM32F102 has 5V tolerant pins and and non-5V tolerant pins–the analog inputs are not 5V tolerant. There is a table on page 37 of that spec that shows that for the 5V tolerant pins, the max and min allowable input pin voltages are Vdd+4.0 and Vss-0.3, where Vdd is 3.3V for the core and Vss is 0V, that means -0.3 to 7.3V on those pin, but that just means that the chip will survive, not function at those levels. For the non-5V tolerant pins, the upper limit is 4V and the lower limit is the same -0.3V. The larger the over or under voltage you put on the inputs, the more current is flowing “backwards” through the device, until finally the diodes fail.
So if you drive your inputs too far above or below the range, the STM32F102 will be damaged and unfortunately it has already happened to one or two users here on the forum, so be careful.
If you want to give details of what you are trying to do, we can help you more.