How to make authorization system post authentication phase?

[Tsimopak]

I have a system in which the user might be logged in and he can be related to different groups.
I also have a pages and each page can define which people have access.

So essentially, i want to make a middleware or laverage next auth api somehow to check on backend if the user has access to current page and if not, to redirect him somewhere, how can i do that? because i don't have that info when making for example a custom CredientialsProvider, but that's depends on current user page