Documentation
Security
CORS

CORS

Enforce client-site access controls with cross-origin resource sharing (CORS).

⚠️
This feature is available in kgateway version 2.1.x or later. This feature is experimental in the upstream Kubernetes Gateway API and subject to change.

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.

Configuration options

You can configure the CORS policy at two levels:

  • HTTPRoute: For the native way in Kubernetes Gateway API, configure a CORS policy in the HTTPRoute. You can choose to apply the CORS policy to all the routes that are defined in the HTTPRoute, or to a selection of backendRefs. This route-level policy takes precedence over any TrafficPolicy CORS that you might configure. For more information, see the Kubernetes Gateway API docs and CORS design docs.
  • TrafficPolicy: For more flexibility to reuse the CORS policy across HTTPRoutes, specific routes and Gateways, configure a CORS policy in the TrafficPolicy. You can attach a TrafficPolicy to a Gateway, all HTTPRoutes via targetRefs, or an individual route via extensionRef. To attach to a backendRef, use a CORS policy in the HTTPRoute instead. For more information about attachment and merging rules, see the TrafficPolicy concept docs.

Known limitations

The CORS filter supports only exact matches, not wildcard matchers. This limitation applies to both the HTTPRoute and TrafficPolicy. For example, you cannot set the allowOrigins field to https://*.example.com/ or allowHeaders to X-Custom-*.

Before you begin

  1. Follow the Get started guide to install kgateway.

  2. Follow the Sample app guide to create a gateway proxy with an HTTP listener and deploy the httpbin sample app.

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

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

  4. Install the experimental channel of the Kubernetes Gateway API to use this feature.

    kubectl apply -f https://github.com/kubernetes-sigs/gateway-api/releases/download/v1.3.0/experimental-install.yaml

Set up CORS policies

Create a CORS policy for the httpbin app in an HTTPRoute or TrafficPolicy.

Create an HTTPRoute resource for the httpbin app that applies a CORS filter. The following example allows requests from the https://example.com/ origin.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: httpbin-cors
  namespace: httpbin
spec:
  parentRefs:
    - name: http
      namespace: kgateway-system
  hostnames:
    - cors.example
  rules:
    - filters:
        - type: CORS
          cors:
            allowCredentials: true
            allowHeaders:
              - Origin               
            allowMethods:
              - GET
              - POST
              - OPTIONS               
            allowOrigins:
              - "https://example.com/"
            exposeHeaders:
            - Origin
            - X-HTTPRoute-Header
            maxAge: 86400
      backendRefs:
        - name: httpbin
          port: 8000
EOF

  1. Create a TrafficPolicy resource for the httpbin app that applies a CORS filter. The following example allows requests from the https://example.com/ origin.

    kubectl apply -f- <<EOF
apiVersion: gateway.kgateway.dev/v1alpha1
kind: TrafficPolicy
metadata:
  name: httpbin-cors
  namespace: httpbin
spec:
  cors:
    allowCredentials: true
    allowHeaders:
      - "Origin"
      - "Authorization"
      - "Content-Type"             
    allowMethods:
      - "GET"
      - "POST"
      - "OPTIONS"               
    allowOrigins:
      - "https://example.com/"
    exposeHeaders:
    - "Origin"
    - "X-TrafficPolicy-Header"
    maxAge: 86400
EOF

  2. Attach the TrafficPolicy to a route or Gateway. The following example creates an HTTPRoute for the httpbin app that has the TrafficPolicy attached via the extensionRef filter. For more information about attachment and merging rules, see the TrafficPolicy concept docs.

    kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: httpbin-cors
  namespace: httpbin
spec:
  parentRefs:
    - name: http
      namespace: kgateway-system
  hostnames:
    - cors.example
  rules:
    - filters:
        - type: ExtensionRef
          extensionRef:
            group: gateway.kgateway.dev
            kind: TrafficPolicy
            name: httpbin-cors
      backendRefs:
        - name: httpbin
          port: 8000
EOF

Test CORS policies

Now that you have CORS policies applied via an HTTPRoute or TrafficPolicy, you can test the policies.

  1. Send a request to the httpbin 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/headers -H "host: cors.example:8080" \
 -H "Origin: https://example.com/" -H "Access-Control-Request-Method: GET"
    curl -v -X GET localhost:8080/headers -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 an HTTPRoute with a CORS filter, you see the Origin and X-HTTPRoute-Header headers.
    • If you created a TrafficPolicy with a CORS filter, you see the Origin and X-TrafficPolicy-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, X-HTTPRoute-Header
< server: envoy
...

    * 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, X-TrafficPolicy-Header
< server: envoy
<
...

  2. Send another request to the httpbin 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/headers -H "host: cors.example:8080" \
 -H "Origin: https://notallowed.com/" -H "Access-Control-Request-Method: GET"
    curl -v -X GET localhost:8080/headers -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
< 
...

Cleanup

You can remove the resources that you created in this guide. 
kubectl delete httproute httpbin-cors -n httpbin
kubectl delete httproute httpbin-cors -n httpbin
kubectl delete trafficpolicy httpbin-cors -n httpbin
Access loggingCSRF