# CORS (Cross-Origin Resource Sharing) { #cors-cross-origin-resource-sharing }

<a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS" class="external-link" target="_blank">CORS or "Cross-Origin Resource Sharing"</a> refers to the situations when a frontend running in a browser has JavaScript code that communicates with a backend, and the backend is in a different "origin" than the frontend.

## Origin { #origin }

An origin is the combination of protocol (`http`, `https`), domain (`myapp.com`, `localhost`, `localhost.tiangolo.com`), and port (`80`, `443`, `8080`).

So, all these are different origins:

* `http://localhost`
* `https://localhost`
* `http://localhost:8080`

Even if they are all in `localhost`, they use different protocols or ports, so, they are different "origins".

## Steps { #steps }

So, let's say you have a frontend running in your browser at `http://localhost:8080`, and its JavaScript is trying to communicate with a backend running at `http://localhost` (because we don't specify a port, the browser will assume the default port `80`).

Then, the browser will send an HTTP `OPTIONS` request to the `:80`-backend, and if the backend sends the appropriate headers authorizing the communication from this different origin (`http://localhost:8080`) then the `:8080`-browser will let the JavaScript in the frontend send its request to the `:80`-backend.

To achieve this, the `:80`-backend must have a list of "allowed origins".

In this case, the list would have to include `http://localhost:8080` for the `:8080`-frontend to work correctly.

## Wildcards { #wildcards }

It's also possible to declare the list as `"*"` (a "wildcard") to say that all are allowed.

But that will only allow certain types of communication, excluding everything that involves credentials: Cookies, Authorization headers like those used with Bearer Tokens, etc.

So, for everything to work correctly, it's better to specify explicitly the allowed origins.

## Use `CORSMiddleware` { #use-corsmiddleware }

You can configure it in your **FastAPI** application using the `CORSMiddleware`.

* Import `CORSMiddleware`.
* Create a list of allowed origins (as strings).
* Add it as a "middleware" to your **FastAPI** application.

You can also specify whether your backend allows:

* Credentials (Authorization headers, Cookies, etc).
* Specific HTTP methods (`POST`, `PUT`) or all of them with the wildcard `"*"`.
* Specific HTTP headers or all of them with the wildcard `"*"`.

{* ../../docs_src/cors/tutorial001_py39.py hl[2,6:11,13:19] *}


The default parameters used by the `CORSMiddleware` implementation are restrictive by default, so you'll need to explicitly enable particular origins, methods, or headers, in order for browsers to be permitted to use them in a Cross-Domain context.

The following arguments are supported:

* `allow_origins` - A list of origins that should be permitted to make cross-origin requests. E.g. `['https://example.org', 'https://www.example.org']`. You can use `['*']` to allow any origin.
* `allow_origin_regex` - A regex string to match against origins that should be permitted to make cross-origin requests. e.g. `'https://.*\.example\.org'`.
* `allow_methods` - A list of HTTP methods that should be allowed for cross-origin requests. Defaults to `['GET']`. You can use `['*']` to allow all standard methods.
* `allow_headers` - A list of HTTP request headers that should be supported for cross-origin requests. Defaults to `[]`. You can use `['*']` to allow all headers. The `Accept`, `Accept-Language`, `Content-Language` and `Content-Type` headers are always allowed for <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple_requests" class="external-link" rel="noopener" target="_blank">simple CORS requests</a>.
* `allow_credentials` - Indicate that cookies should be supported for cross-origin requests. Defaults to `False`.

    None of `allow_origins`, `allow_methods` and `allow_headers` can be set to `['*']` if `allow_credentials` is set to `True`. All of them must be <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#credentialed_requests_and_wildcards" class="external-link" rel="noopener" target="_blank">explicitly specified</a>.

* `expose_headers` - Indicate any response headers that should be made accessible to the browser. Defaults to `[]`.
* `max_age` - Sets a maximum time in seconds for browsers to cache CORS responses. Defaults to `600`.

The middleware responds to two particular types of HTTP request...

### CORS preflight requests { #cors-preflight-requests }

These are any `OPTIONS` request with `Origin` and `Access-Control-Request-Method` headers.

In this case the middleware will intercept the incoming request and respond with appropriate CORS headers, and either a `200` or `400` response for informational purposes.

### Simple requests { #simple-requests }

Any request with an `Origin` header. In this case the middleware will pass the request through as normal, but will include appropriate CORS headers on the response.

## More info { #more-info }

For more info about <abbr title="Cross-Origin Resource Sharing">CORS</abbr>, check the <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS" class="external-link" target="_blank">Mozilla CORS documentation</a>.

/// note | Technical Details

You could also use `from starlette.middleware.cors import CORSMiddleware`.

**FastAPI** provides several middlewares in `fastapi.middleware` just as a convenience for you, the developer. But most of the available middlewares come directly from Starlette.

///
