LLMs are inherently insecure, primarily because they are inherently /gullible/. They need to be gullible for them to be useful - but this means any application that exposes them to text from untrusted sources (e.g. summarize this web page) could be subverted by a malicious attacker.
We've been talking about prompt injection for 14 months now and we don't yet have anything that feels close to a reliable fix.
I really hope someone figures this out soon, or a lot of the stuff we want to build with LLMs won't be feasible to build in a secure way.