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.