Nov 7, 2018

Cross-Origin Resource Sharing (CORS)

A resource makes a cross-origin HTTP request when it requests a resource from a different domain than the one which served itself. For example, an HTML page served from http://i88.ca makes an <img> src request for http://dbbig.com/image.jpg. Many pages on the web today load resources like CSS stylesheets, images and scripts from separate domains.
For security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts.  For example,XMLHttpRequest follows the same-origin policy. So, a web application using XMLHttpRequest could only make HTTP requests to its own domain. To improve web applications, developers asked browser vendors to allow XMLHttpRequest to make cross-domain requests.
The W3C Web Applications Working Group recommends the new Cross-Origin Resource Sharing (CORS) mechanism. CORS gives web servers cross-domain access controls, which enable secure cross-domain data transfers. Modern browsers use CORS in an API container - such as XMLHttpRequest - to mitigate risks of cross-origin HTTP requests.
This cross-origin sharing standard is used to enable cross-site HTTP requests for:

EDIT

The Cross-Origin Resource Sharing standard works by adding new HTTP headers that allow servers to describe the set of origins that are permitted to read that information using a web browser.  Additionally, for HTTP request methods that can cause side-effects on user data (in particular, for HTTP methods other than GET, or for POST usage with certain MIME types), the specification mandates that browsers "preflight" the request, soliciting supported methods from the server with an HTTP OPTIONS request method, and then, upon "approval" from the server, sending the actual request with the actual HTTP request method.  Servers can also notify clients whether "credentials" (including Cookies and HTTP Authentication data) should be sent with requests.