CORS Tester

Cross-Origin Resource Sharing (CORS) is a browser security mechanism that controls which origins can access resources on a server. It uses HTTP headers to tell browsers whether a cross-origin request is allowed.

A preflight is an OPTIONS request the browser sends before the actual request when using custom headers, non-simple methods (PUT, DELETE), or credentials. The server must respond with appropriate Access-Control-Allow-* headers.

Common causes: missing Access-Control-Allow-Origin header, origin not in the allowed list, method not permitted, custom headers not allowed, or using credentials with a wildcard (*) origin.

Yes. Requests are sent from our server, not your browser. No cookies, tokens, or credentials from your browser session are included in the test requests.