Access-Control-Allow-Credentials

The Access-Control-Allow-Credentials response header tells browsers whether the server allows cross-origin HTTP requests to include credentials.

Credentials are cookies, TLS client certificates, or authentication headers containing a username and password. By default, these credentials are not sent in cross-origin requests, and doing so can make a site vulnerable to CSRF attacks.

A client can ask that credentials should be included in cross-site requests in several ways:

• Using fetch(), by setting the credentials option to "include".
• Using XMLHttpRequest, by setting the XMLHttpRequest.withCredentials property to true.
• Using EventSource(), by setting the EventSource.withCredentials property to true.

If the client has asked for credentials to be included:

• If the request is preflighted, then the preflight request does not include credentials. If the server's response to the preflight request sets the Access-Control-Allow-Credentials header to true, then the real request will include credentials: otherwise, the browser reports a network error.
• If the request is not preflighted, then the request will include credentials, and if the server's response does not set the Access-Control-Allow-Credentials header to true, the browser reports a network error.

Header type	Response header
Forbidden header name	no

Access-Control-Allow-Credentials: true

Access-Control-Allow-Credentials: true

fetch(url, {
  credentials: "include",
});

const xhr = new XMLHttpRequest();
xhr.open("GET", "http://example.com/", true);
xhr.withCredentials = true;
xhr.send(null);