The wifi function and the other functions handled by the car’s on-board computer are in no way connected. Your car’s on-board computer is safe from being “hacked” by some hacker coming in through the wifi. Its like saying you have a computer in your basement, which is not connected to any other computer, will catch a virus from a computer you have upstairs on the internet. Moreover, even if they were connected, they use specialized chips to store their programming on, which are known by various acronyms, i.e. EEPROM, (Electronically Eraseable Programmable Read Only Memory) and various permutations of these kinds of chips, which CANNOT be reprogrammed remotely.
The on-board computers are really a good way for the shade-tree mechanic to diagnose complex problems. For a 100 - 300 dollar investment, you can get the hardware necessary to connect directly to your car’s onboard computer. No more check engine lights, etc. The VW computers are really light years beyond most other cars, and are invaluable for me, when I want to find out what is wrong with my car. They aren’t all that difficult to use, and the companies that make the software/cables make it easy for you to understand what everything means. If you by a VW, I highly recommend you get a connection to your computer, as you will need it to do something as basic as changing your brakes.
I know, I bought my automobile computer on Ebay for just over $50. The plug is supposed to be “universal” and works on every vehicle I own (Toyota Tacoma and Accura TL).
As far as being “impossible”, I beg to disagree. If the car is communicating to the car computer through the I2C or CAN bus. The CAN bus handles things such as timing, gas/air mixtures and when the transmission should shift. By virtue of the car computer giving this information to a central processing unit - the bus is open. There is no SECURE transaction on the CAN bus - there is no SUPERVISOR mode in a CAN bus transaction. Thus, by virtue of the CAN bus tying into the network - the network is vulnerable. One must ASSUME there is some firewall between the USER interface and the CAN bus. However, I have seen nothing in the bus protocols or interface circuits that state such a device is in existence.