The HTTP Access-Control-Allow-Headers is used in response to a to indicate the HTTP headers that can be used during the actual request. This header is required if the preflight request contains .

The are always allowed and usually aren’t listed in Access-Control-Allow-Headers unless there is a need to circumvent the additional safelist restrictions.

Syntax

Access-Control-Allow-Headers: <header-name>
Access-Control-Allow-Headers: <header-name>, <header-name>
Access-Control-Allow-Headers: *

Directives

• <header-name>

  • : The name of a supported request header. The header may list any number of headers, separated by commas.

  * (wildcard)

  • : Any header. The value * only counts as a special wildcard value for requests without credentials (requests without HTTP cookies or HTTP authentication information). In requests with credentials, it is treated as the literal header name * without special semantics. The header doesn’t accept wildcard and always needs to be listed explicitly.

Examples

Implementing a custom header

Below is an example of an Access-Control-Allow-Headers header. It indicates that a custom header named X-Custom-Header is supported by CORS requests to the server, in addition to the .

Access-Control-Allow-Headers: X-Custom-Header

Supporting multiple headers

This example shows Access-Control-Allow-Headers when it specifies support for multiple headers.

Access-Control-Allow-Headers: X-Custom-Header, Upgrade-Insecure-Requests

Bypassing additional restrictions on CORS-safelisted headers

Although are always allowed and don’t usually need to be listed in Access-Control-Allow-Headers, listing them anyway will circumvent the additional restrictions that apply.

Access-Control-Allow-Headers: Accept

Handling preflight requests

Let’s look at an example of a involving Access-Control-Allow-Headers.

Request

First, the preflight request is an request that includes some combination of the three preflight request headers: , , and .

The preflight request below tells the server that we want to send a CORS GET request with the headers listed in ( and X-Requested-With).

OPTIONS /resource/foo
Access-Control-Request-Method: GET
Access-Control-Request-Headers: content-type,x-requested-with
Origin: https://www.example.com

Response

If the CORS request indicated by the preflight request is authorized, the server will respond to the preflight request with a message that indicates the allowed origin, methods, and headers. Below, we see that Access-Control-Allow-Headers includes the headers that were requested.

HTTP/1.1 200 OK
Content-Length: 0
Connection: keep-alive
Access-Control-Allow-Origin: https://www.example.com
Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE
Access-Control-Allow-Headers: Content-Type, x-requested-with
Access-Control-Max-Age: 86400

If the requested method isn’t supported, the server will respond with an error.

Specifications

Browser compatibility

See also