CORS

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

⚠️

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 AgentgatewayPolicy CORS that you might configure. For more information, see the Kubernetes Gateway API docs and CORS design docs.
AgentgatewayPolicy: For more flexibility to reuse the CORS policy across HTTPRoutes, specific routes, and Gateways, configure a CORS policy in the AgentgatewayPolicy. You can attach an AgentgatewayPolicy to a Gateway or the routes in an HTTPRoute resource.

Before you begin

Set up an agentgateway proxy.
Install the httpbin sample app.

Set up CORS policies

Create a CORS policy for the httpbin app in an HTTPRoute or AgentgatewayPolicy. The following example sets up custom HTTP methods and max age for requests for the https://example.com/ origin.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: httpbin
  namespace: httpbin
spec:
  parentRefs:
    - name: agentgateway-proxy
      namespace: agentgateway-system
  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

kubectl apply -f- <<EOF
apiVersion: agentgateway.dev/v1alpha1
kind: AgentgatewayPolicy
metadata:
  name: httpbin-cors
  namespace: agentgateway-system
spec: 
  targetRefs:
    - group: gateway.networking.k8s.io
      kind: Gateway
      name: agentgateway-proxy   
  traffic:
    cors:
      allowCredentials: true
      allowHeaders:
        - "Origin"
        - "Authorization"
        - "Content-Type"             
      allowMethods:
        - "GET"
        - "POST"
        - "OPTIONS"               
      allowOrigins:
        - "https://example.com"
      exposeHeaders:
      - "Origin"
      - "X-TrafficPolicy-Header"
      maxAge: 86400
EOF

Send a request to the httpbin app and use https://example.com as the origin. Verify that your request succeeds and that you get back the configured CORS headers.
curl -I -X OPTIONS http://$INGRESS_GW_ADDRESS:80/get -H "host: www.example.com" \ -H "Origin: https://example.com"
curl -I -X OPTIONS localhost:8080/headers -H "host: www.example.com" \ -H "Origin: https://example.com"
Example output: Notice that the access-control-* values reflect your CORS policy and change 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.
Example output:
```
HTTP/1.1 200 OK
access-control-allow-origin: https://example.com
access-control-allow-methods: GET,POST,OPTIONS
access-control-allow-headers: origin
access-control-max-age: 86400
content-length: 0
```

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 your configured CORS settings such as max age, allowed orgin, or allowed methods, because notallowed.com is not configured as a supported origin.

curl -I -X OPTIONS http://$INGRESS_GW_ADDRESS:80/get -H "host: www.example.com" \
 -H "Origin: https://notallowed.com"

curl -I -X OPTIONS localhost:8080/headers -H "host: www.example.com" \
 -H "Origin: https://notallowed.com"

Example output:

HTTP/1.1 200 OK
access-control-allow-credentials: true
access-control-allow-methods: GET, POST, HEAD, PUT, DELETE, PATCH, OPTIONS
access-control-allow-origin: https://notallowed.com
access-control-max-age: 3600
content-length: 0

Cleanup

You can remove the resources that you created in this guide.

Restore the HTTPRoute for the httpbin app.

kubectl apply -f- <<EOF
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: httpbin
  namespace: httpbin
spec:
  parentRefs:
    - name: agentgateway-proxy
      namespace: agentgateway-system
  hostnames:
    - "www.example.com"
  rules:
    - backendRefs:
        - name: httpbin
          port: 8000
EOF

kubectl delete AgentgatewayPolicy httpbin-cors -n agentgateway-system

CSRF