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CORS

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APPLIES TO: All API Management tiers

The cors policy adds cross-origin resource sharing (CORS) support to an operation or an API to allow cross-domain calls from browser-based clients.

Note

Set the policy's elements and child elements in the order provided in the policy statement. To help you configure this policy, the portal provides a guided, form-based editor. Learn more about how to set or edit API Management policies.

Policy statement

<cors allow-credentials="false | true" terminate-unmatched-request="true | false">
    <allowed-origins>
        <origin>origin uri</origin>
    </allowed-origins>
    <allowed-methods preflight-result-max-age="number of seconds">
        <method>HTTP verb</method>
    </allowed-methods>
    <allowed-headers>
        <header>header name</header>
    </allowed-headers>
    <expose-headers>
        <header>header name</header>
    </expose-headers>
</cors>

Attributes

Name Description Required Default
allow-credentials The Access-Control-Allow-Credentials header in the preflight response will be set to the value of this attribute and affect the client's ability to submit credentials in cross-domain requests. Policy expressions are allowed. No false
terminate-unmatched-request Controls the processing of cross-origin requests that don't match the policy settings. Policy expressions are allowed.

When OPTIONS request is processed as a preflight request and Origin header doesn't match policy settings:
- If the attribute is set to true, immediately terminate the request with an empty 200 OK response
- If the attribute is set to false, check inbound for other in-scope cors policies that are direct children of the inbound element and apply them. If no cors policies are found, terminate the request with an empty 200 OK response.

When GET or HEAD request includes the Origin header (and therefore is processed as a simple cross-origin request), and doesn't match policy settings:
- If the attribute is set to true, immediately terminate the request with an empty 200 OK response.
- If the attribute is set to false, allow the request to proceed normally and don't add CORS headers to the response.		 No true

Elements

Name Description Required Default
allowed-origins Contains origin elements that describe the allowed origins for cross-domain requests. allowed-origins can contain either a single origin element that specifies * to allow any origin, or one or more origin elements that contain a URI. Yes N/A
allowed-methods This element is required if methods other than GET or POST are allowed. Contains method elements that specify the supported HTTP verbs. The value * indicates all methods. No If this section isn't present, GET and POST are supported.
allowed-headers This element contains header elements specifying names of the headers that can be included in the request. Yes N/A
expose-headers This element contains header elements specifying names of the headers that will be accessible by the client. No N/A

Caution

Use the * wildcard with care in policy settings. This configuration may be overly permissive and may make an API more vulnerable to certain API security threats.

allowed-origins elements

Name Description Required Default
origin The value can be either * to allow all origins, or a URI that specifies a single origin. The URI must include a scheme, host, and port. Do not include quotation marks. Yes If the port is omitted in a URI, port 80 is used for HTTP and port 443 is used for HTTPS.

allowed-methods attributes

Name Description Required Default
preflight-result-max-age The Access-Control-Max-Age header in the preflight response will be set to the value of this attribute and affect the user agent's ability to cache the preflight response. Policy expressions are allowed. No 0

allowed-methods elements

Name Description Required Default
method Specifies an HTTP verb. Policy expressions are allowed. At least one method element is required if the allowed-methods section is present. N/A

allowed-headers elements

Name Description Required Default
header Specifies a header name. At least one header element is required in allowed-headers if that section is present. N/A

expose-headers elements

Name Description Required Default
header Specifies a header name. At least one header element is required in expose-headers if that section is present. N/A

Usage

Usage notes

  • You may configure the cors policy at more than one scope (for example, at the product scope and the global scope). Ensure that the base element is configured at the operation, API, and product scopes to inherit needed policies at the parent scopes.
  • Only the cors policy is evaluated on the OPTIONS request during preflight. Remaining configured policies are evaluated on the approved request.
  • This policy can only be used once in a policy section.

About CORS

CORS is an HTTP header-based standard that allows a browser and a server to interact and determine whether or not to allow specific cross-origin requests (XMLHttpRequest calls made from JavaScript on a web page to other domains). This allows for more flexibility than only allowing same-origin requests, but is more secure than allowing all cross-origin requests.

CORS specifies two types of cross-origin requests:

  • Preflighted (or "preflight") requests - The browser first sends an HTTP request using the OPTIONS method to the server, to determine if the actual request is permitted to send. If the server response includes the Access-Control-Allow-Origin header that allows access, the browser follows with the actual request.

  • Simple requests - These requests include one or more extra Origin headers but don't trigger a CORS preflight. Only requests using the GET and HEAD methods and a limited set of request headers are allowed.

cors policy scenarios

Configure the cors policy in API Management for the following scenarios:

  • Enable the interactive test console in the developer portal. Refer to the developer portal documentation for details.

    Note

    When you enable CORS for the interactive console, by default API Management configures the cors policy at the global scope.

  • Enable API Management to reply to preflight requests or to pass through simple CORS requests when the backends don't provide their own CORS support.

    Note

    If a request matches an operation with an OPTIONS method defined in the API, preflight request processing logic associated with the cors policy will not be executed. Therefore, such operations can be used to implement custom preflight processing logic - for example, to apply the cors policy only under certain conditions.

Common configuration issues

  • Subscription key in header - If you configure the cors policy at the product scope, and your API uses subscription key authentication, the policy won't work when the subscription key is passed in a header. As a workaround, modify requests to include a subscription key as a query parameter.
  • API with header versioning - If you configure the cors policy at the API scope, and your API uses a header-versioning scheme, the policy won't work because the version is passed in a header. You may need to configure an alternative versioning method such as a path or query parameter.
  • Policy order - You may experience unexpected behavior if the cors policy is not the first policy in the inbound section. Select Calculate effective policy in the policy editor to check the policy evaluation order at each scope. Generally, only the first cors policy is applied.
  • Empty 200 OK response - In some policy configurations, certain cross-origin requests complete with an empty 200 OK response. This response is expected when terminate-unmatched-request is set to its default value of true and an incoming request has an Origin header that doesn’t match an allowed origin configured in the cors policy.

Example

This example demonstrates how to support preflight requests, such as those with custom headers or methods other than GET and POST. To support custom headers and other HTTP verbs, use the allowed-methods and allowed-headers sections as shown in the following example.

<cors allow-credentials="true">
    <allowed-origins>
        <!-- Localhost useful for development -->
        <origin>http://localhost:8080/</origin>
        <origin>http://example.com/</origin>
    </allowed-origins>
    <allowed-methods preflight-result-max-age="300">
        <method>GET</method>
        <method>POST</method>
        <method>PATCH</method>
        <method>DELETE</method>
    </allowed-methods>
    <allowed-headers>
        <!-- Examples below show Azure Mobile Services headers -->
        <header>x-zumo-installation-id</header>
        <header>x-zumo-application</header>
        <header>x-zumo-version</header>
        <header>x-zumo-auth</header>
        <header>content-type</header>
        <header>accept</header>
    </allowed-headers>
    <expose-headers>
        <!-- Examples below show Azure Mobile Services headers -->
        <header>x-zumo-installation-id</header>
        <header>x-zumo-application</header>
    </expose-headers>
</cors>

Related content

For more information about working with policies, see: