how to bypass Access-Control-Allow-Origin?

Question

Put this on top of retrieve.php:

header('Access-Control-Allow-Origin: *');

Note that this effectively disables CORS protection, and leaves your users exposed to attack. If you're not completely certain that you need to allow all origins, you should lock this down to a more specific origin:

header('Access-Control-Allow-Origin: https://www.example.com');

Please refer to following stack answer for better understanding of Access-Control-Allow-Origin

https://stackoverflow.com/a/10636765/413670

Okay, but you all know that the * is a wildcard and allows cross site scripting from every domain?

You would like to send multiple Access-Control-Allow-Origin headers for every site that's allowed to - but unfortunately its officially not supported to send multiple Access-Control-Allow-Origin headers, or to put in multiple origins.

You can solve this by checking the origin, and sending back that one in the header, if it is allowed:

$origin = $_SERVER['HTTP_ORIGIN'];
$allowed_domains = [
    'http://mysite1.com',
    'https://www.mysite2.com',
    'http://www.mysite2.com',
];

if (in_array($origin, $allowed_domains)) {
    header('Access-Control-Allow-Origin: ' . $origin);
}

Thats much safer. You might want to edit the matching and change it to a manual function with some regex, or something like that. At least this will only send back 1 header, and you will be sure its the one that the request came from. Please do note that all HTTP headers can be spoofed, but this header is for the client's protection. Don't protect your own data with those values. If you want to know more, read up a bit on CORS and CSRF.

Why is it safer?

Allowing access from other locations then your own trusted site allows for session highjacking. I'm going to go with a little example - image Facebook allows a wildcard origin - this means that you can make your own website somewhere, and make it fire AJAX calls (or open iframes) to facebook. This means you can grab the logged in info of the facebook of a visitor of your website. Even worse - you can script POST requests and post data on someone's facebook - just while they are browsing your website.

Be very cautious when using the ACAO headers!

Warning, Chrome (and other browsers) will complain that multiple ACAO headers are set if you follow some of the other answers.

The error will be something like XMLHttpRequest cannot load ____. The 'Access-Control-Allow-Origin' header contains multiple values '____, ____, ____', but only one is allowed. Origin '____' is therefore not allowed access.

Try this:

$http_origin = $_SERVER['HTTP_ORIGIN'];

$allowed_domains = array(
  'http://domain1.com',
  'http://domain2.com',
);

if (in_array($http_origin, $allowed_domains))
{  
    header("Access-Control-Allow-Origin: $http_origin");
}

I have fixed this problem when calling a MVC3 Controller. I added:

Response.AddHeader("Access-Control-Allow-Origin", "*"); 

before my

return Json(model, JsonRequestBehavior.AllowGet);

And also my $.ajax was complaining that it does not accept Content-type header in my ajax call, so I commented it out as I know its JSON being passed to the Action.

Hope that helps.

It's a really bad idea to use *, which leaves you wide open to cross site scripting. You basically want your own domain all of the time, scoped to your current SSL settings, and optionally additional domains. You also want them all to be sent as one header. The following will always authorize your own domain in the same SSL scope as the current page, and can optionally also include any number of additional domains. It will send them all as one header, and overwrite the previous one(s) if something else already sent them to avoid any chance of the browser grumbling about multiple access control headers being sent.

class CorsAccessControl
{
    private $allowed = array();

    /**
     * Always adds your own domain with the current ssl settings.
     */
    public function __construct()
    {
        // Add your own domain, with respect to the current SSL settings.
        $this->allowed[] = 'http'
            . ( ( array_key_exists( 'HTTPS', $_SERVER )
                && $_SERVER['HTTPS'] 
                && strtolower( $_SERVER['HTTPS'] ) !== 'off' ) 
                    ? 's' 
                    : null )
            . '://' . $_SERVER['HTTP_HOST'];
    }

    /**
     * Optionally add additional domains. Each is only added one time.
     */
    public function add($domain)
    {
        if ( !in_array( $domain, $this->allowed )
        {
            $this->allowed[] = $domain;
        }
    /**
     * Send 'em all as one header so no browsers grumble about it.
     */
    public function send()
    {
        $domains = implode( ', ', $this->allowed );
        header( 'Access-Control-Allow-Origin: ' . $domains, true ); // We want to send them all as one shot, so replace should be true here.
    }
}

Usage:

$cors = new CorsAccessControl();

// If you are only authorizing your own domain:
$cors->send();

// If you are authorizing multiple domains:
foreach ($domains as $domain)
{
    $cors->add($domain);
}
$cors->send();

You get the idea.