access-control-allow-origin header javascript code example

Example 1: access-control-allow-origin

Access-Control-Allow-Origin: *

Example 2: cross origin even with allow header

header('Access-Control-Allow-Origin: http://localhost:8100');
header ("Access-Control-Expose-Headers: Content-Length, X-JSON");
header ("Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE, OPTIONS");
header ("Access-Control-Allow-Headers: Content-Type, Authorization, Accept, Accept-Language, X-Authorization");
header('Access-Control-Max-Age: 86400');

if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') {
    // The request is using the POST method
    header("HTTP/1.1 200 OK");
    return;

}

Example 3: Access to XMLHttpRequest at 'http://localhost/MySQL_pracs/InsertUser.php' from origin 'http://localhost:4200' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

//Access to XMLHttpRequest at 'http://localhost/[api path].php' from origin 'http://localhost:4200' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

//The error is simply saying that "Content-Type" is missing from "Access-Control-Allow-Headers".

//Therefore we need to add "Content-Type" to "Access-Control-Allow-Headers".

<?php 
header('Access-Control-Allow-Headers: Content-Type');
-----
?>

Example 4: what is cors

“CORS” stands for Cross-Origin Resource Sharing. 
It allows you to make requests from one website to another website 
in the browser, which is normally prohibited by another browser policy 
called the Same-Origin Policy (SOP).

Example 5: header access-control particular domain

Access-Control-Allow-Origin: http://siteA.com

Example 6: what is CORS

Use CORS to allow cross-origin access. 
CORS is a part of HTTP that lets servers specify any other hosts 
from which a browser should permit loading of content.

How to block cross-origin access
To prevent cross-origin writes, 
	check an unguessable token in the request — known as a Cross-Site Request Forgery (CSRF) token. 
    prevent cross-origin reads of pages that require this token.
To prevent cross-origin reads of a resource, 
	ensure that it is not embeddable.
    prevent embedding because embedding a resource always leaks some information about it.
To prevent cross-origin embeds, 
	ensure that your resource cannot be interpreted
    Browsers may not respect the Content-Type header. 
For example, if you point a <script> tag at an HTML document, the browser will try to parse the HTML as JavaScript. When your resource is not an entry point to your site, you can also use a CSRF token to prevent embedding.