The only way to mitigate the damage an LLM can do because of prompt injection is to limit what that LLM can do in the first place. That’s what they mean by limiting its usefulness. If an LLM has access to an api and I want it to abuse that API in some way, I can attack its prompt and eventually get it to use the api the way I want
All apis have to authorize and authenticate if they do sensitive stuff. Otherwise youre asking for it.
Yes, but whatever the LLM has authenticated access to, an attacker can convince it to mess with on their behalf.