A Web Security Checklist

Before shipping anything that accepts user input: use parameterized queries, never string-built SQL. Escape (or let your framework's templates escape) any user content before it's rendered as HTML. Rely on your framework's built-in CSRF protection on every form that changes data. Mark session cookies HttpOnly and Secure, and serve the whole site over HTTPS. Validate and sanitize on the server, every time, even if the browser already checked it first.

None of this is exotic. Almost every major real-world breach traces back to one of these basics being skipped somewhere, not to some exotic, never-before-seen technique.