Access-Control-Allow-Headers header
Baseline Widely available *
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
* Some parts of this feature may have varying levels of support.
The HTTP Access-Control-Allow-Headers response header is used in response to a preflight request to indicate the HTTP headers that can be used during the actual request. This header is required if the preflight request contains Access-Control-Request-Headers.
Note: The CORS-safelisted request headers are always allowed and usually aren't listed in Access-Control-Allow-Headers unless there is a need to circumvent the additional safelist restrictions.
| Header type | Response header |
|---|---|
| Forbidden request header | No |
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. TheAuthorizationheader 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 CORS-safelisted request headers.
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 CORS-safelisted request headers 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 preflight request involving Access-Control-Allow-Headers.
Request
First, the preflight request is an OPTIONS request that includes some combination of the three preflight request headers: Access-Control-Request-Method, Access-Control-Request-Headers, and Origin.
The preflight request below tells the server that we want to send a CORS GET request with the headers listed in Access-Control-Request-Headers (Content-Type and X-Requested-With).
OPTIONS /resource/foo Access-Control-Request-Method: GET Access-Control-Request-Headers: content-type,x-requested-with Origin: https://foo.bar.org 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://foo.bar.org 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
| Specification |
|---|
| Fetch # http-access-control-allow-headers |
Browser compatibility
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