The attacker really does know the system when someone leaves your company

A common maxim of computer security is “the attacker knows the system”. In other words, the system should still be secure even given a hypothetical attacker who knows exactly how it works. It’s the flip-side of relying on “security by obscurity”.

In cryptography, the same concept is referred to as Kerckhoffs’s principle, but can be applied to systems in general.

Most engineers are aware of this concept, but it’s also common to find a vague attitude that you can still “get away with” relying on little details that a hypothetical attacker won’t actually know, e.g.

These are obviously bad ideas when made explicit like this, but they can sneak in under the radar due to a subconscious belief that an attacker won’t really know the system in that much detail.

You can immediately put a stop to this belief, subconscious or otherwise, with the example of a disgruntled former employee. This employee has left the company, but remembers how these obscure details work. They might even have implemented them, and might also have the source code from that time.

This hypothetical employee doesn’t have to be disgruntled either. Maybe they are unscrupulous and someone else offers them money for the source code.

Bearing this kind of attacker in mind, the security risks of the above ideas seem quite severe, and you won’t be inclined to let them into the codebase.