T5 Force Log In

t5-force-log-in.php

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: T5 Force Log In
 * Description: Grant access after log in only
 * Plugin URI:  http://toscho.de/?p=2207
 * Version:     2012.08.28
 * Author:      Thomas Scholz
 * Author URI:  http://toscho.de
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */
add_action( 'template_redirect', function() {
	is_user_logged_in() || auth_redirect();
});

Sorry for the late response. I was never notified of this comment.

It is secure for the HTML content. Images, on the other hand, are not routed through WordPress, so anyone can see them if they are able to get their URLs by guessing them or sniffing network traffic.

Where do I apply this code?

If you got a customized login page via front-end, you need to modify code as:

add_action( 'template_redirect', function() {
    if ( !is_page( id_of_login_page ) ) 
        is_user_logged_in() || auth_redirect();
});

or you will get a redirect error.