In June 2018, Tesla starting shipping the Model S with a FOB that has improved cryptography to guard against some newer security hacks. The Model X FOB is already resistant to these hacks.
Tesla is making a new set of FOBs available to owners of the older FOB for $190. Contact service if you are interested.
This is a more serious issue for Europeans, as there have been a number of thefts of cars with FOBs (all makes, not just Tesla), that this new FOB should help resist. You can also use the new PIN to drive feature, which also protects the car from any kind of FOB attack, including outright theft of the FOB. So far, we’ve yet to hear of FOB security attack in the USA leading to the theft of a Tesla.