Access-Control-Request-Headers is a crucial element of the HTTP headers that constitute the Cross-Origin Resource Sharing (CORS) mechanism. This header is used when a browser is making a preflight request to inform the server of the HTTP headers that will be used when the actual request is made.
What is Access-Control-Request-Headers?
Access-Control-Request-Headers is an HTTP request header. It is used by a client (a Web API in a browser) during a preflight request to let the server know which HTTP headers will be used when the actual request is made. It’s the server’s responsibility to indicate whether these headers are acceptable.
The header follows the format:
Access-Control-Request-Headers: <field-name>[, <field-name>]*
<field-name> is the name of the header that the client plans to use in the actual request.
For instance, consider a preflight request with this header:
Here, the client indicates that it will use the
X-My-Custom-Header in the actual request.
Scenario 1: Requesting a Single Custom Header
Suppose your client needs to make a request to your server with a custom header
X-My-Custom-Header. The preflight request from the client would include:
The server should then respond with an
Access-Control-Allow-Headers header containing
X-My-Custom-Header to indicate that this header is allowed:
Scenario 2: Requesting Multiple Custom Headers
In many situations, you might need to make a request with multiple custom headers. To do this, include each header name in the
Access-Control-Request-Headers header, separated by commas:
Access-Control-Request-Headers: X-My-Custom-Header, X-Another-Custom-Header
The server would then respond with:
Access-Control-Allow-Headers: X-My-Custom-Header, X-Another-Custom-Header
Considerations and Caveats
When working with
Access-Control-Request-Headers, there are a few key points to bear in mind:
- Header Sensitivity: Ensure that any custom headers you use don’t unintentionally leak sensitive information.
- Server Responsibility: It’s up to the server to decide whether to allow or disallow the headers specified in
Access-Control-Request-Headers. If the server does not respond with an
Access-Control-Allow-Headersheader that includes the requested headers, the actual request will not be allowed.
- Preflight Requests:
Access-Control-Request-Headersis used in preflight requests, which are sent automatically by the browser under certain conditions (i.e., for “non-simple” requests) to check the server’s CORS policy.
Access-Control-Request-Headers header plays a vital role in the CORS mechanism, enabling clients to inform the server about the headers they intend to use in the actual request. A proper understanding and implementation of this header can ensure smoother and more secure cross-origin interactions in your web applications.