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These are excellent suggestions Bucephalus. One note though, they mainly deal with the infrastructure/network/os level security.

A simple SQL injection in the code would still be exploitable regardless of the countermeasures above.

So on top of what you have suggested, what are your ideas to get the security right on the code level of the applications?

Well said, there are tons of issues the one can encounter even with perfectly secured infrastructure.

Somebody made good effort identifying some: https://www.hacksplaining.com/

Learn to think like somebody who will try to exploit your system and embed guards into your code as you build it.