And that's exactly why it's so dangerous to use pickle, because it doesn't prevent you from executing untrusted code. It doesn't matter if you only pickle strings and bytes, as long as you unpickle any data, an attacker can replace that data with something which results in arbitrary code execution.

If you only want to store bytes, the safe way to do so is to use the dbm module directly, which doesn't pickle/unpickle the data you provide.