About CORS

Cross-Origin Resource Sharing (CORS) is a security feature that is implemented by web browsers and that controls how web pages in one domain can request and interact with resources that are hosted on a different domain. By default, web browsers only allow requests to resources that are hosted on the same domain as the web page that served the original request. Access to web pages or resources that are hosted on a different domain is restricted to prevent potential security vulnerabilities, such as cross-site request forgery (CRSF).

When CORS is enabled in a web browser and a request for a different domain comes in, the web browser checks whether this request is allowed or not. To do that, it typically sends a preflight request (HTTP OPTIONS method) to the server or service that serves the requested resource. The service returns the methods that are permitted to send the actual cross-origin request, such as GET, POST, etc. If the request to the different domain is allowed, the response includes CORS-specific headers that instruct the web browser how to make the cross-origin request. For example, the CORS headers typically include the origin that is allowed to access the resource, and the credentials or headers that must be included in the cross-origin request.

Note that the preflight request is optional. Web browsers can also be configured to send the cross-origin directly. However, access to the request resource is granted only if CORS headers were returned in the response. If no headers are returned during the preflight request, the web browser denies access to the resource in the other domain.

CORS policies are typically implemented to limit access to server resources for JavaScripts that are embedded in a web page, such as:

  • A JavaScript on a web page at example.com tries to access a different domain, such as api.com.
  • A JavaScript on a web page at example.com tries to access a different subdomain, such as api.example.com.
  • A JavaScript on a web page at example.com tries to access a different port, such as example.com:3001.
  • A JavaScript on a web page at https://example.com tries to access the resources by using a different protocol, such as http://example.com.

For more information, see the CORS API.

Configuration options

You can configure the CORS policy at two levels:

  • VirtualHostOption: Configure a CORS policy in the VirtualHostOption that is applied to a gateway proxy. The policy is automatically attached to all the listeners that are defined for one or multiple hosts on the gateway.
  • RouteOption: Configure a CORS policy in the RouteOption. Then, you can apply the policy to one or multiple routes by using the targetRefs field on the RouteOption or the ExtensionRef filter on the HTTPRoute. For more information, see Policy attachment.

By default, the configuration of the RouteOption takes precedence over the VirtualHostOption. However, you can change this behavior for the exposeHeaders CORS option by using the corsPolicyMergeSettings field in the VirtualHostOption. Currently, only exposeHeaders is configurable. You cannot merge other CORS options such as allowHeaders or allowOrigin.

For example, you might want to expose the CORS origin header for traffic that reaches any of the gateway listeners with the VirtualHostOption. Additionally, each team or product might have their own headers on a per route basis that you want to expose also, such as product-a. By setting the corsPolicyMergeSettings to UNION, the exposed headers are merged together. This way, both the VirtualHostOption and RouteOption exposeHeaders CORS policies are applied. When the routes are called, the exposed headers from the VirtualHostOption and RouteOption are both returned, such as origin and product-a.

For more information about the supported merge strategies, see the API docs.

Before you begin

  1. Follow the Get started guide to install Gloo Gateway, set up a gateway resource, and deploy the httpbin sample app.

  2. Get the external address of the gateway and save it in an environment variable.

      export INGRESS_GW_ADDRESS=$(kubectl get svc -n gloo-system gloo-proxy-http -o jsonpath="{.status.loadBalancer.ingress[0]['hostname','ip']}")
echo $INGRESS_GW_ADDRESS  
  
      kubectl port-forward deployment/gloo-proxy-http -n gloo-system 8080:8080

Set up CORS policies

  1. Deploy the Petstore app.

      kubectl apply -f https://raw.githubusercontent.com/solo-io/gloo/v1.16.x/example/petstore/petstore.yaml

    Example output:

      deployment.apps/petstore created
service/petstore created

  2. Verify that the Petstore app is up and running.

      kubectl get pods

    Example output:

      NAME                        READY   STATUS    RESTARTS   AGE
petstore-66cddd5bb4-x7vdd   1/1     Running   0          26s

  3. Create the CORS policy at either the virtual host level or the route level. Policies at the virtual host level apply to all routes that are attached to the gateway that the virtual host targets. If you have a CORS policy at both virtual host and route levels, the route policy takes precedence by default.

    • Create a VirtualHostOption resource to define your CORS rules. The following example allows requests from the example.com/ and *.example.com origins. The example also sets the corsPolicyMergeSettings to configure how conflicting CORS policies at the virtual host and route levels are handled. In the example, you configure a UNION merge strategy for the exposeHeaders field. Now, the CORS policy applies to requests so that all the exposeHeaders values from both the virtual host and route are included. Currently, only exposeHeaders is configurable. You cannot merge other CORS options such as allowHeaders or allowOrigin.
        kubectl apply -n gloo-system -f- <<EOF
apiVersion: gateway.solo.io/v1
kind: VirtualHostOption
metadata:
  name: cors
  namespace: gloo-system
spec:
  options:
    corsPolicyMergeSettings:
      exposeHeaders: UNION
    cors:
      allowCredentials: true
      allowHeaders:
      - origin
      allowMethods:
      - GET
      - POST
      - OPTIONS
      allowOrigin:
      - https://example.com/
      allowOriginRegex:
      - https://\[a-zA-Z0-9\]\*.example
      exposeHeaders:
      - origin
      - vh-header
      maxAge: 1d
  targetRefs:
  - group: gateway.networking.k8s.io
    kind: Gateway
    name: http
    namespace: gloo-system
EOF

    1. Create a RouteOption resource to define your CORS rules. The following example allows requests from the example.com/ and *.example.com origins.

        kubectl apply -f- <<EOF
apiVersion: gateway.solo.io/v1
kind: RouteOption
metadata:
  name: cors
  namespace: default
spec:
  options:
    cors:
      allowCredentials: true
      allowHeaders:
      - origin
      allowMethods:
      - GET
      - POST
      - OPTIONS
      allowOrigin:
      - https://example.com/
      allowOriginRegex:
      - https://\[a-zA-Z0-9\]\*.example
      exposeHeaders:
      - origin
      - route-header
      maxAge: 1d
EOF

    2. Create an HTTPRoute resource for the Petstore app that applies the RouteOption resources that you created.

        kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1beta1
kind: HTTPRoute
metadata:
  name: petstore-cors
  namespace: default
spec:
  parentRefs:
  - name: http
    namespace: gloo-system
  hostnames:
    - cors.example
  rules:
    - filters:
        - type: ExtensionRef
          extensionRef:
            group: gateway.solo.io
            kind: RouteOption
            name: cors
      backendRefs:
        - name: petstore
          port: 8080
EOF

  4. Send a request to the Petstore app on the cors.example domain and use https://example.com/ as the origin. Verify that your request succeeds and that you get back CORS headers, such as access-control-allow-origin, access-control-allow-credentials, and access-control-expose-headers.

      curl -v -X GET http://$INGRESS_GW_ADDRESS:8080/api/pets -H "host: cors.example:8080" \
 -H "Origin: https://example.com/" -H "Access-Control-Request-Method: GET"
  
      curl -v -X GET localhost:8080/api/pets -H "host: cors.example:8080" \
 -H "Origin: https://example.com/" -H "Access-Control-Request-Method: GET"

    Example output: Notice that the access-control-expose-headers value changes depending on the resources that you created.

    • If you created only a VirtualHostOption, you see the origin and vh-header headers.
    • If you created only a RouteOption or both a RouteOption and VirtualHostOption, you see the origin and route-header headers, because the RouteOption takes precedence.
    • If you created both options and also configured the corsPolicyMergeSettings on the VirtualHostOptions with a UNION merge strategy, then you see all the origin, vh-header, and route-header headers.
         * Mark bundle as not supporting multiuse
   < HTTP/1.1 200 OK
   < content-type: text/xml
   < date: Mon, 03 Jun 2024 17:05:31 GMT
   < content-length: 86
   < x-envoy-upstream-service-time: 7
   < access-control-allow-origin: https://example.com/
   < access-control-allow-credentials: true
   < access-control-expose-headers: origin, vh-header, route-header
   < server: envoy
   < 
   [{"id":1,"name":"Dog","status":"available"},{"id":2,"name":"Cat","status":"pending"}]

  5. Send another request to the Petstore app. This time, you use notallowed.com as your origin. Although the request succeeds, you do not get back any CORS headers, because notallowed.com is not configured as a supported origin.

      curl -v -X GET http://$INGRESS_GW_ADDRESS:8080/api/pets -H "host: cors.example:8080" \
 -H "Origin: https://notallowed.com/" -H "Access-Control-Request-Method: GET"
  
      curl -v -X GET localhost:8080/api/pets -H "host: cors.example:8080" \
 -H "Origin: https://notallowed.com/" -H "Access-Control-Request-Method: GET"

    Example output:

      * Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< content-type: text/xml
< date: Mon, 03 Jun 2024 17:20:10 GMT
< content-length: 86
< x-envoy-upstream-service-time: 3
< server: envoy
< 
[{"id":1,"name":"Dog","status":"available"},{"id":2,"name":"Cat","status":"pending"}]

Cleanup

Remove the resources that you created in this guide.

  kubectl delete -f https://raw.githubusercontent.com/solo-io/gloo/v1.16.x/example/petstore/petstore.yaml
kubectl delete virtualhostoption cors -n gloo-system
kubectl delete routeoption cors -n default
kubectl delete httproute petstore-cors -n default