WebAssembly protects you from untrusted code getting out of its sandbox and accessing data outside its own context, and that's the whole point.

But it will not (and hasn't been designed to) magically remove crashing bugs from your code, just as (e.g.) Javascript doesn't protect you from calling an undefined function.

But those bugs can't do any harm to your machine except crashing the WebAssembly application itself.

*this doesn't mean the sandbox itself doesn't contain bugs of course, but it's the job of the sandbox to prevent stuff running inside it from accessing stuff outside