# CORS
CORS is a technique to prevent websites from doing bad things with your personal data. Most browsers + JavaScript toolkits not only support CORS but enforce it, which has implications for your API server which supports Swagger.
You can read about CORS here: http://www.w3.org/TR/cors.
There are two cases where no action is needed for CORS support:
1. Swagger UI is hosted on the same server as the application itself (same host *and* port).
2. The application is located behind a proxy that enables the required CORS headers. This may already be covered within your organization.
Otherwise, CORS support needs to be enabled for:
1. Your Swagger docs. For Swagger 2.0 it's the `swagger.json`/`swagger.yaml` and any externally `$ref`ed docs.
2. For the `Try it now` button to work, CORS needs to be enabled on your API endpoints as well.
### Testing CORS Support
You can verify CORS support with one of three techniques:
- Curl your API and inspect the headers. For instance:
```bash
$ curl -I "https://petstore.swagger.io/v2/swagger.json"
HTTP/1.1 200 OK
Date: Sat, 31 Jan 2015 23:05:44 GMT
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, DELETE, PUT, PATCH, OPTIONS
Access-Control-Allow-Headers: Content-Type, api_key, Authorization
Content-Type: application/json
Content-Length: 0
```
This tells us that the petstore resource listing supports OPTIONS, and the following headers: `Content-Type`, `api_key`, `Authorization`.
- Try Swagger UI from your file system and look at the debug console. If CORS is not enabled, you'll see something like this:
```
XMLHttpRequest cannot load http://sad.server.com/v2/api-docs. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access.
```
Swagger UI cannot easily show this error state.
- Use the https://www.test-cors.org website to verify CORS support. Keep in mind this will show a successful result even if `Access-Control-Allow-Headers` is not available, which is still required for Swagger UI to function properly.
### Enabling CORS
The method of enabling CORS depends on the server and/or framework you use to host your application. https://enable-cors.org provides information on how to enable CORS in some common web servers.
Other servers/frameworks may provide you information on how to enable it specifically in their use case.
### CORS and Header Parameters
Swagger UI lets you easily send headers as parameters to requests. The name of these headers *MUST* be supported in your CORS configuration as well. From our example above:
```
Access-Control-Allow-Headers: Content-Type, api_key, Authorization
```
Only headers with these names will be allowed to be sent by Swagger UI.