When you know you're looking for an exploit, it's easily verifiable and you can iterate fairly rapidly and get fast feedback, you can just have a test that you don't get a 403 back or something. LLMs, in general, seem good at problems like this.

When you're developing, there's a ton of degrees of freedom, and security encompasses only some of them. To be sure, if you set up a quickly verifiable test for every security flaw, you could probably get LLMs to satisfy them somehow, but there's a lot of those, and it's not necessarily practical.