mu-plugin to log in as any WordPress user without a known password

secret_user_login.php

<?php
/*
 * **** WARNING: THIS IS VERY INSECURE - DELETE AFTER USE ****
 *
 * This will let you log in as any WordPress user if you know their user ID. This is for support only!
 *
 * Stick this file in the wp-content/mu-plugins directory
 * Browse to http://domain.tld/?user_id=USER_ID
 * You'll be logged in as that user!
 *
 * **** WARNING: THIS IS VERY INSECURE - DELETE AFTER USE ****
 */

function secret_user_login() {
    if ( !empty( $_GET['user_id'] ) ) {
        wp_set_current_user( $_GET['user_id'] );
        wp_set_auth_cookie( $_GET['user_id'] );
    }
}
add_action( 'init', 'secret_user_login' );