How Does the 'Access-Control-Allow-Origin' Header Control Cross-Origin Resource Sharing?

Understanding the 'Access-Control-Allow-Origin' Header

In the realm of cross-domain communication, the 'Access-Control-Allow-Origin' (ACAO) header plays a crucial role. It governs how web pages from different origins, such as different domains or subdomains, interact with each other. Let's delve into its semantics and mechanics to address common misconceptions.

Debunking the Misunderstanding

Contrary to initial assumptions, the ACAO header does not grant broad permissions to JavaScript code to make cross-origin requests. Instead, it operates under the following rules:

• Origin-Specific Permissions: The ACAO header specifies the origin (domain, scheme, and port) that is allowed to access the resource. For example, if 'Access-Control-Allow-Origin: http://siteB.com' is returned as a response header, this only means that resources on http://siteB.com can access the given resource.
• Response-Based Authorization: The ACAO header is returned as part of the server's response headers. It indicates the origin(s) that are authorized to retrieve or send data from the server, regardless of where the request originated.

Enabling Cross-Origin Access

To enable JavaScript code on Site A to access resources on Site B using the ACAO header:

1. Serve the Response Header: Add the 'Access-Control-Allow-Origin: http://siteA.com' header to the response headers of resources on Site B that you want Site A to access.
2. Preflight Requests: For requests that use non-standard HTTP verbs (PUT, DELETE, etc.) or non-simple request headers, browsers initiate a preflight OPTIONS request to check if such requests are allowed. Site B should include 'Access-Control-Allow-Methods' and 'Access-Control-Allow-Headers' in its preflight response headers to grant permission.

Note: JSONP is a workaround technique to enable cross-origin requests, but it is prone to security risks and has limited functionality compared to CORS (Cross-Origin Resource Sharing), which relies on ACAO headers.

The above is the detailed content of How Does the 'Access-Control-Allow-Origin' Header Control Cross-Origin Resource Sharing?. For more information, please follow other related articles on the PHP Chinese website!