Unvalidated input can always be an exploit vector.

Except in C, validation of user input can in itself be an exploit vector.

Turtles all the way down.

That’s true in other languages as well. Any programmatic task can end up being an exploit vector.

No? That's the whole point of formal verification?

You can even kind of retrofit this to C. The classic example is "sel4". You just need a set of proofs that the code doesn't trigger UB. This ends up being much larger and more complicated than the C itself.

You can fail to verify something which you actually wanted to verify (i.e you made a proof of something else instead of the thing that mattered). See WPA2 KRACK as an example.

Yeah, but only in C* can those errors end up as more UB.

* terms and limits may apply.

Many other languages also have UB. But almost none is as trigger happy as C and C++ are.