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Access-Control-Allow-Origin header

files/en-us/web/http/reference/headers/access-control-allow-origin/index.md

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The HTTP Access-Control-Allow-Origin {{Glossary("response header")}} indicates whether the response can be shared with requesting code from the given {{Glossary("origin")}}.

<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td>{{Glossary("Response header")}}</td> </tr> </tbody> </table>

Syntax

http
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 return Access-Control-Allow-Origin: "null"; however, the origin of resources that use a non-hierarchical scheme (such as data: or file:) and sandboxed documents is serialized as null. Many browsers 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. Therefore, the null value for the Access-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:

http
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:

http
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 {{HTTPHeader("Origin")}} request header, compare that to a list of allowed origins, and then if the {{HTTPHeader("Origin")}} value is in the list, set the Access-Control-Allow-Origin value to the same value as the {{HTTPHeader("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 {{HTTPHeader("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.

http
Access-Control-Allow-Origin: https://developer.mozilla.org
Vary: Origin

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also