It seems like the WASM is simply a fallback if no other decoder is available. If the data source is untrusted, simply don’t run the WASM decoders.

“Some code is untrusted” does not mean code should never be executed. There are more use cases with trusted sources than untrusted.

So I define the data type to be "asdklfjaslkdfjiolsadfjoiusadfoiasfoikasjfdoisadf" and give you a decoder for it.