Removing username from the ‘wordpress_logged_in’ cookie

I am working with a client with some strict security measures. After undergoing security review, we were notified that the user name stored in the logged in cookie, e.g.

wordpress_logged_in[username]|[hash]

is something that has to be removed. Since this is an integral part of the login system, I’m not sure how to remove it and still maintain the session.

Short introduction

After quick look inside WP source code, I think I’ve found solution…

WordPress uses two functions to set and parse auth cookies:

• wp_generate_auth_cookie
• wp_parse_auth_cookie

There is a filter in wp_generate_auth_cookie called auth_cookie which you probably could use to change the contents of cookie, but there is no filter inside wp_parse_auth_cookie, but…

Both of these functions are defined in pluggable.php, which means, that you can write your own implementations for them and overwrite default ones.

Solution

1. Write your own plugin (let’s call it Better Auth Cookie)
2. Implement your own wp_generate_auth_cookie and wp_parse_auth_cookie functions inside this plugin.
3. Activate your plugin.

You can find my sample implementation (based strongly on original versions) of these functions below:

if ( !function_exists('wp_generate_auth_cookie') ) :
/**
 * Generate authentication cookie contents.
 *
 * @since 2.5.0
 *
 * @param int $user_id User ID
 * @param int $expiration Cookie expiration in seconds
 * @param string $scheme Optional. The cookie scheme to use: auth, secure_auth, or logged_in
 * @param string $token User's session token to use for this cookie
 * @return string Authentication cookie contents. Empty string if user does not exist.
 */
function wp_generate_auth_cookie( $user_id, $expiration, $scheme="auth", $token = '' ) {
    $user = get_userdata($user_id);
    if ( ! $user ) {
        return '';
    }

    if ( ! $token ) {
        $manager = WP_Session_Tokens::get_instance( $user_id );
        $token = $manager->create( $expiration );
    }

    $pass_frag = substr($user->user_pass, 8, 4);

    $key = wp_hash( $user->user_login . '|' . $pass_frag . '|' . $expiration . '|' . $token, $scheme );

    // If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
    $algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
    $hash = hash_hmac( $algo, $user->user_login . '|' . $expiration . '|' . $token, $key );

    $cookie = $user_id . '|' . $expiration . '|' . $token . '|' . $hash;

    /**
     * Filter the authentication cookie.
     *
     * @since 2.5.0
     *
     * @param string $cookie     Authentication cookie.
     * @param int    $user_id    User ID.
     * @param int    $expiration Authentication cookie expiration in seconds.
     * @param string $scheme     Cookie scheme used. Accepts 'auth', 'secure_auth', or 'logged_in'.
     * @param string $token      User's session token used.
     */
    return apply_filters( 'auth_cookie', $cookie, $user_id, $expiration, $scheme, $token );
}
endif;


if ( !function_exists('wp_parse_auth_cookie') ) :
/**
 * Parse a cookie into its components
 *
 * @since 2.7.0
 *
 * @param string $cookie
 * @param string $scheme Optional. The cookie scheme to use: auth, secure_auth, or logged_in
 * @return array Authentication cookie components
 */
function wp_parse_auth_cookie($cookie="", $scheme="") {
    if ( empty($cookie) ) {
        switch ($scheme){
            case 'auth':
                $cookie_name = AUTH_COOKIE;
                break;
            case 'secure_auth':
                $cookie_name = SECURE_AUTH_COOKIE;
                break;
            case "logged_in":
                $cookie_name = LOGGED_IN_COOKIE;
                break;
            default:
                if ( is_ssl() ) {
                    $cookie_name = SECURE_AUTH_COOKIE;
                    $scheme="secure_auth";
                } else {
                    $cookie_name = AUTH_COOKIE;
                    $scheme="auth";
                }
        }

        if ( empty($_COOKIE[$cookie_name]) )
            return false;
        $cookie = $_COOKIE[$cookie_name];
    }

    $cookie_elements = explode('|', $cookie);
    if ( count( $cookie_elements ) !== 4 ) {
        return false;
    }

    list( $user_id, $expiration, $token, $hmac ) = $cookie_elements;

    $user = get_userdata($user_id);
    $username = ( ! $user ) ? '' : $user->user_login;

    return compact( 'username', 'expiration', 'token', 'hmac', 'scheme' );
}
endif;

My version of these functions replaces user_login with user_id. But it should be a good start for changing it to something even more complex (i.e. user specific hash, or something like this).