1) Create new user "rocco"
2) Create new private site called "test"
3) Login as user "rocco"
4) Try to access the URL /share/page/site/test/dashboard -> server error
5) Try to access the URL /share/page/site/test/site-members -> access is granted
If you have a custom page on the private site "test", any user who is aware of the URL can access it.