They've already been through rounds of attacks and fixes. They've paid people and publically discussed the vulunerabilities that were discovered. They have separate computing systems for the ui (which must have endless attack surfaces since it's an old webkit browser, please find a lot of problems so they will update it) and the drivetrain. If you don't have a tesla it's very interesting how it works. The ui system can be rebooted while the car is on and driving, it's completely separate.