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Avoid wildcards in APIM CORS policies#

Security · API Management · Rule · 2023_03

Avoid using wildcard for any configuration option in CORS policies.

Description#

The API Management cors policy adds cross-origin resource sharing (CORS) support to an operation or APIs.

CORS is not a security feature. CORS is a W3C standard that allows a server to relax the same-origin policy enforced by modern browsers. CORS uses HTTP headers that allows API Management (and other HTTP servers) to indicate any allowed origins.

Using wildcard (*) in any policy is overly permissive and may reduce the effectiveness of browser same-origin policy enforcement.

Recommendation#

Consider configuring the CORS policy by specifying explicit values for each property.

Examples#

Configure API Management policy#

To deploy API Management CORS policies that pass this rule:

  • When configuring cors policies provide the exact values for all propeties.
  • Avoid using wildcards for any property of the cors policy including:
    • allowed-origins
    • allowed-methods
    • allowed-headers
    • expose-headers

For example a global scoped policy:

API Management policy
<policies>
  <inbound>
    <cors allow-credentials="true">
      <allowed-origins>
        <origin>https://contoso.developer.azure-api.net</origin>
        <origin>https://developer.contoso.com</origin>
      </allowed-origins>
      <allowed-methods preflight-result-max-age="300">
        <method>GET</method>
        <method>PUT</method>
        <method>POST</method>
        <method>PATCH</method>
        <method>HEAD</method>
        <method>DELETE</method>
        <method>OPTIONS</method>
      </allowed-methods>
      <allowed-headers>
        <header>Content-Type</header>
        <header>Cache-Control</header>
        <header>Authorization</header>
      </allowed-headers>
    </cors>
  </inbound>
  <backend>
    <forward-request />
  </backend>
  <outbound />
  <on-error />
</policies>

Configure with Azure template#

To deploy API Management CORS policies that pass this rule:

  • Configure an policy sub-resource.
  • Avoid using wildcards * for any CORS policy element in properties.value property. Instead provide exact values.

For example a global scoped policy:

Azure Template snippet
{
  "type": "Microsoft.ApiManagement/service/policies",
  "apiVersion": "2022-08-01",
  "name": "[format('{0}/{1}', parameters('name'), 'policy')]",
  "properties": {
    "value": "<policies><inbound><cors allow-credentials=\"true\"><allowed-origins><origin>https://contoso.developer.azure-api.net</origin><origin>https://developer.contoso.com</origin></allowed-origins><allowed-methods preflight-result-max-age=\"300\"><method>GET</method><method>PUT</method><method>POST</method><method>PATCH</method><method>HEAD</method><method>DELETE</method><method>OPTIONS</method></allowed-methods><allowed-headers><header>Content-Type</header><header>Cache-Control</header><header>Authorization</header></allowed-headers></cors></inbound><backend><forward-request /></backend><outbound /><on-error /></policies>",
    "format": "xml"
  },
  "dependsOn": [
    "[resourceId('Microsoft.ApiManagement/service', parameters('name'))]"
  ]
}

Configure with Bicep#

To deploy API Management CORS policies that pass this rule:

  • Configure an policy sub-resource.
  • Avoid using wildcards * for any CORS policy element in properties.value property. Instead provide exact values.

For example a global scoped policy:

Azure Bicep snippet
resource globalPolicy 'Microsoft.ApiManagement/service/policies@2022-08-01' = {
  parent: service
  name: 'policy'
  properties: {
    value: '<policies><inbound><cors allow-credentials="true"><allowed-origins><origin>https://contoso.developer.azure-api.net</origin><origin>https://developer.contoso.com</origin></allowed-origins><allowed-methods preflight-result-max-age="300"><method>GET</method><method>PUT</method><method>POST</method><method>PATCH</method><method>HEAD</method><method>DELETE</method><method>OPTIONS</method></allowed-methods><allowed-headers><header>Content-Type</header><header>Cache-Control</header><header>Authorization</header></allowed-headers></cors></inbound><backend><forward-request /></backend><outbound /><on-error /></policies>'
    format: 'xml'
  }
}

Notes#

The rule only checks against rawxml and xml policy formatted content.

When using Azure Bicep, the policy XML can be loaded from an external file by using the loadTextContent function.

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