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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.

How does CORS work?

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 to get back the methods that are allowed to use when sending 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 details, see this article.

How to configure a VirtualService for CORS

In order to allow your VirtualService to work with CORS, you need to add a new set of configuration options in the VirtualHost part of your VirtualService

apiVersion: gateway.solo.io/v1
kind: VirtualService
metadata:
  name: corsexample
  namespace: gloo-system
spec:
  displayName: corsexample
  virtualHost:
    options:
      cors:
        (...)
    domains:
    - '*'

Some apps, such as httpbin, have built-in CORS policies that allow all origins. These policies take precedence over CORS policies that you might configure in Gloo Gateway.

Available Fields

The following fields are available when specifying CORS on your VirtualService:

Field	Type	Description	Default
`allowOrigin`	`[]string`	Specifies the origins that will be allowed to make CORS requests. An origin is allowed if either allow_origin or allow_origin_regex match.
`allowOriginRegex`	`[]string`	Specifies regex patterns that match origins that will be allowed to make CORS requests. An origin is allowed if either `allow_origin` or `allow_origin_regex` match. Note that Gloo Gateway uses ECMAScript regex grammar. For example, to match all subdomains `https://example.com`, do not use `https://.example.com`, but instead use `https://[a-zA-Z0-9].example.com`.
`allowMethods`	`[]string`	Specifies the content for the access-control-allow-methods header.
`allowHeaders`	`[]string`	Specifies the content for the access-control-allow-headers header.
`exposeHeaders`	`[]string`	Specifies the content for the access-control-expose-headers header.
`maxAge`	`string`	Specifies the content for the access-control-max-age header.
`allowCredentials`	`bool`	Specifies whether the resource allows credentials.
`disableForRoute`	`bool`	If set, the CORS Policy (specified on the virtual host) is disabled for this route.	false

For more information, see the API docs.

Try out an example

Follow the steps to deploy the Petstore Hello World app in your cluster.
Edit the virtual service that exposes the Petstore app to add in a CORS policy.
```
kubectl edit virtualservice default -n gloo-system
```

Add the following CORS configuration to the spec.virtualHostoptions section of your virtual service. The CORS policy in this example configures the Petstore to allow cross-origin requests for the https://example.com and https://*.gloo.dev domains. With this setup, you can host scripts or other resources on the https://*.gloo.dev or https://solo.io domains, even if your application is not being served from that location.

...
spec:
  virtualHost:
    domains:
    - '*'
    options:
      cors:
        allowCredentials: true
        allowHeaders:
        - origin
        allowMethods:
        - GET
        - POST
        - OPTIONS
        allowOrigin:
        - https://example.com
        allowOriginRegex:
        - https://[a-zA-Z0-9]*.example.com
        exposeHeaders:
        - origin
        maxAge: 1d

Send a request to the Petstore app for the origin https://example.com and verify that the CORS headers are returned. The presence of these CORS headers instruct a web browser to grant access to the remote resource.

A preflight request to the Petstore sample app cannot be simulated as part of this guide because the Petstore app does not support the OPTIONS method.

curl -vik -H "Origin: https://example.com" \
-H "Access-Control-Request-Method: GET" \
-X GET $(glooctl proxy url)/all-pets

Example output:

> GET /all-pets HTTP/1.1
> User-Agent: curl/7.77.0
> Accept: */*
> Origin: https://example.com
> Access-Control-Request-Method: GET
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< access-control-allow-origin: https://example.com
access-control-allow-origin: https://example.com
< access-control-allow-credentials: true
access-control-allow-credentials: true
< access-control-expose-headers: origin
access-control-expose-headers: origin
< server: envoy
server: envoy

< 
[{"id":1,"name":"Dog","status":"available"},{"id":2,"name":"Cat","status":"pending"}]

Send another request to the Petstore app. This time, include the origin https://notallowed.com that is not configured in your virtual service. Verify that no CORS headers are returned for the provided origin.

The request still returns a 200 HTTP response code, because a curl client is used to make the request in this example. However, CORS policies are enforced in a web browser. If this type of request is sent through a web browser and no CORS headers are returned in the response, the web browser denies access to the requested resource as the cross-origin instructions are missing.

curl -vik -H "Origin: https://notallowed.com" \
-H "Access-Control-Request-Method: GET" \
-X GET $(glooctl proxy url)/all-pets

Example output:

GET /all-pets HTTP/1.1
> Host: ab2e5d3c1c8f0466b9cee8494e87a90d-1027513527.us-east-1.elb.amazonaws.com
> User-Agent: curl/7.77.0
> Accept: */*
> Origin: https://notallowed.com
> Access-Control-Request-Method: GET
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< content-type: text/xml
content-type: text/xml   
< content-length: 86
content-length: 86
< x-envoy-upstream-service-time: 1
x-envoy-upstream-service-time: 1
< server: envoy
server: envoy

< 
[{"id":1,"name":"Dog","status":"available"},{"id":2,"name":"Cat","status":"pending"}]