Perfect security is actually pretty easy to achieve, for example, if you turn off your computer, it's impossible to hack over the internet. But it's also useless - you want some people to access the machine some of the time.

Another way you have very good (and fake) security, is by reducing the scope of the problem - if you check every request against a very strong crypto signature, it's also practically unbreakable.

But then you have issues like who signs these tokens, who decides which user gets what permissions etc. etc. which mean you have to weaken practical security to make it useful.

Another example is the L4 microkernel, which 'proven' to be bug-free and secure. But it also doesn't do like 99% of the stuff a regular kernel like Linux does, and sort of pushes the issue to the user.

So even defining 'perfect' security in a way that the system remains practically usable is very difficult and potentially impossible.

We come down to to the permissions problem - each person should have as much permissions to do their work as necessary and nothing more - which is essentially a reformulation of Gödel's incompleteness theorem.