Access-Control-Allow-Origin
The HTTP Access-Control-Allow-Origin
response header indicates whether the response can be shared with requesting code from the given origin.
Header type | Response header |
---|---|
Forbidden header name | No |
Syntax
Access-Control-Allow-Origin: *
Access-Control-Allow-Origin: <origin>
Access-Control-Allow-Origin: null
Directives
*
(wildcard)-
The requesting code from any origin is allowed to access the resource. For requests without credentials, the literal value
*
can be specified as a wildcard. Attempting to use the wildcard with credentials results in an error. <origin>
-
Specifies a single origin. If the server supports clients from multiple origins, it must return the origin for the specific client making the request.
null
-
Specifies the origin "null".
Note: The value
null
should not be used. It may seem safe to returnAccess-Control-Allow-Origin: "null"
; however, the origin of resources that use a non-hierarchical scheme (such asdata:
orfile:
) and sandboxed documents is serialized asnull
. Many browsers will grant such documents access to a response with anAccess-Control-Allow-Origin: null
header, and any origin can create a hostile document with anull
origin. Therefore, thenull
value for theAccess-Control-Allow-Origin
header should be avoided.
Examples
A response that tells the browser to allow code from any origin to access a resource will include the following:
Access-Control-Allow-Origin: *
A response that tells the browser to allow requesting code from the origin https://developer.mozilla.org
to access a resource will include the following:
Access-Control-Allow-Origin: https://developer.mozilla.org
Limiting the possible Access-Control-Allow-Origin
values to a set of allowed origins requires code on the server side to check the value of the Origin
request header, compare that to a list of allowed origins, and then if the Origin
value is in the list, set the Access-Control-Allow-Origin
value to the same value as the Origin
value.
CORS and caching
Suppose the server sends a response with an Access-Control-Allow-Origin
value with an explicit origin (rather than the *
wildcard). In that case, the response should also include a Vary
response header with the value Origin
— to indicate to browsers that server responses can differ based on the value of the Origin
request header.
Access-Control-Allow-Origin: https://developer.mozilla.org
Vary: Origin
Specifications
Specification |
---|
Fetch Standard # http-access-control-allow-origin |
Browser compatibility
BCD tables only load in the browser