Access-Control-Allow-Origin:*与省略该请求头的差异解析求助

Great question—let’s break down the critical differences between these two scenarios, since they lead to very different browser behaviors even if you’re avoiding the wildcard in production.

What happens when you omit the Access-Control-Allow-Origin header entirely? #

When your server doesn’t send this header in its response, browsers will block all cross-origin requests outright. This is the default "deny all" behavior for CORS:

• For simple cross-origin requests (like GET/POST with standard headers), the browser sends the request, but discards the response and throws a CORS error to your frontend code.
• For preflight requests (OPTIONS requests sent before non-simple requests like PUT/DELETE or those with custom headers), the browser will immediately reject the request because there’s no explicit permission from the server.

In short: No header = no cross-origin access allowed, full stop.

What happens when you set Access-Control-Allow-Origin: *? #

This header explicitly tells browsers that any external origin is allowed to make cross-origin requests to your server. However, there’s a key security caveat:

• This wildcard does not work with credentialed requests (requests that include cookies, HTTP authentication, or client-side SSL certificates). If your frontend sends credentials along with the request, the browser will reject the response even if * is set—this is a security measure to prevent exposing sensitive data to untrusted origins.
• Unlike allowing specific origins, you can’t combine * with other origin values (you can’t do https://app.example.com, *; you have to dynamically return the origin from the request’s Origin header if you need to support multiple specific origins).

Core Comparison #

• Omitting the header: Blocks all cross-origin requests, regardless of the origin or request type.
• Setting *: Allows all non-credentialed cross-origin requests, but blocks credentialed ones.

内容的提问来源于stack exchange，提问作者Tim