Local File Inclusion (LFI)

Apache web server working directory is usually /var/www/html.
Setting page=/etc/passwd, the server tries to open the file /var/www/html/etc/passwd.
Let add /../ to the path to reach the upper directory.

/var/www/html/../etc/passwd => /var/www/etc/passwd.
/var/www/html/../../etc/passwd => /var/etc/passwd.

We can add as many ../../../../../ as we want, we can't go upper than /.
/var/www/html/../../../../../../../ => /, regardless of the number of ../

An LFI can read/execute ALL files, the web server account is allowed to read.