Access-Control-Allow-Origin response header indicates whether the response can be shared with requesting code from the given origin.
For requests without credentials, the literal value "
*" can be specified as a wildcard; the value tells browsers to allow requesting code from any origin to access the resource. Attempting to use the wildcard with credentials results in an error.
Specifies an origin. Only a single origin can be specified. If the server supports clients from multiple origins, it must return the origin for the specific client making the request.
Specifies the origin "null".
nullshould not be used: "It may seem safe to return
Access-Control-Allow-Origin: "null", but the serialization of the Origin of any resource that uses a non-hierarchical scheme (such as
file:) and sandboxed documents is defined to be "null". Many User Agents will grant such documents access to a response with an
Access-Control-Allow-Origin: "null"header, and any origin can create a hostile document with a "null" Origin. The "null" value for the ACAO header should therefore be avoided."
A response that tells the browser to allow code from any origin to access a resource will include the following:
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:
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
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.
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