[softwarefounder]

Authentication.

i.e. Have you successfully implemented a good OAuth[2] security system? Can you explain the difference between JWTs, and session auth? Why choose one over the other. Talk to me about SSL a little bit, even at a high-level. How do you secure APIs? Talk to me about how you encrypt passwords, and sensative data.

Have you had to deal with PCI DSS?

This is one of the differences between "I'm a developer who has maintained a simple CRUD app" and "I'm a senior developer who can build a secure system".

Or in other words, authentication separates the men from the boys.