403 Forbidden

<?php
/**
Plugin Name: SSO
Author: Garth Mortensen, Mike Hansen
Version: 0.3
License: GPLv2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
*/

function sso_check() {
	if ( ! isset( $_GET['salt'] ) || ! isset( $_GET['nonce'] ) ) { sso_req_login(); }
	if ( sso_check_blocked() ) { sso_req_login(); }

	$nonce = esc_attr( $_GET['nonce'] );
	$salt = esc_attr( $_GET['salt'] );
	$user;

	if ( ! empty( $_GET['user'] ) ) {
		$user = esc_attr( $_GET['user'] );
	} else {
		$user = get_users( array( 'role' => 'administrator', 'number' => 1 ) );
		if ( is_array( $user ) && is_a( $user[0], 'WP_User' ) ) {
			$user = $user[0];
			$user = $user->ID;
		} else {
			$user = 0;
		}
	}

	$bounce = $_GET['bounce'];
	$hash = base64_encode( hash( 'sha256', $nonce . $salt, false ) );
	$hash = substr( $hash, 0, 64 );

	if ( get_transient( 'sso_token' ) == $hash ) {
		if ( is_email( $user ) ) {
			$user = get_user_by( 'email', $user );
		} else {
			$user = get_user_by( 'id', (int) $user );
		}
		if ( is_a( $user, 'WP_User' ) ) {
			wp_set_current_user( $user->ID, $user->user_login );
			wp_set_auth_cookie( $user->ID );
			do_action( 'wp_login', $user->user_login, $user );
			delete_transient( 'sso_token' );
			wp_safe_redirect( admin_url( $bounce ) );
		} else {
			sso_req_login();
		}
	} else {
		sso_add_failed_attempt();
		sso_req_login();
	}
	die();
}
add_action( 'wp_ajax_nopriv_sso-check', 'sso_check' );
add_action( 'wp_ajax_sso-check', 'sso_check' );

function sso_req_login() {
	wp_safe_redirect( wp_login_url() );
}

function sso_get_attempt_id() {
	return 'sso' . esc_url( $_SERVER['REMOTE_ADDR'] );
}

function sso_add_failed_attempt() {
	$attempts = get_transient( sso_get_attempt_id(), 0 );
	$attempts++;
	set_transient( sso_get_attempt_id(), $attempts, 300 );
}

function sso_check_blocked() {
	$attempts = get_transient( sso_get_attempt_id(), 0 );
	if ( $attempts > 4 ) {
		return true;
	}
	return false;
}