[perlgeek]

There is no one-size-fits-all solution, and probably no "most secure", but lots of people have thought about stuff like that. If you google for "software architecture for security" or such terms, you'll find some helpful material.

https://www.computer.org/cms/CYBSI/docs/Top-10-Flaws.pdf (PDF warning) might be an interesting start.

The principles below it aren't too hard though: encrypt data in transit and in rest, think hard about key management. Keep attack surface minimal. Validate at all layers where it makes sense (for example in a multi-tenant application, you should authorize access to a row by tenant both in the application layer, and use row-level security in the DB as second layer of defense). Make your assumptions explicit; write them down. If possible, write automated tests that check if they still hold up.

While the principles are not hard to understand, designing an application with them in mind can be quite a different beast, and keeping stuff secure as features and integrations creep in requires quite some dedication.