[pravus]

There's no reason to have a URL (or any data) encoded in the state parameter. The purpose of the parameter is to provide an opaque lookup key which you can utilize to provide correct, validated responses. This is usually done in some sort of database or Redis-like cache. My workflows have always used a random UUID for the state key and I just encode the necessary (validated) data items needed for the next step as a JSON blob. It's essentially a very short-lived web session.

If for some reason you really do need to transmit this data in-band (ultra rare use case) you should at least be using something like HMAC to verify that all carriers have transported the data unmodified. It is your responsibility to ensure the integrity of the data end-to-end.