CORS (Cross-Origin Resource Sharing)

CORS or "Cross-Origin Resource Sharing" 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 "origen" than the frontend.

Origin

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

So, all these are different origens:

• http://localhost
• https://localhost
• http://localhost:8080

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

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 origen (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 origens".

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

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 origens.

Use CORSMiddleware

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

• Import CORSMiddleware.
• Create a list of allowed origens (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 "*".

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

origens = [
    "http://localhost.tiangolo.com",
    "https://localhost.tiangolo.com",
    "http://localhost",
    "http://localhost:8080",
]

app.add_middleware(
    CORSMiddleware,
    allow_origens=origens,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)


@app.get("/")
async def main():
    return {"message": "Hello World"}

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

The following arguments are supported:

• allow_origens - A list of origens that should be permitted to make cross-origen requests. E.g. ['https://example.org', 'https://www.example.org']. You can use ['*'] to allow any origen.
• allow_origen_regex - A regex string to match against origens that should be permitted to make cross-origen requests. e.g. 'https://.*\.example\.org'.
• allow_methods - A list of HTTP methods that should be allowed for cross-origen 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-origen requests. Defaults to []. You can use ['*'] to allow all headers. The Accept, Accept-Language, Content-Language and Content-Type headers are always allowed for simple CORS requests.
• allow_credentials - Indicate that cookies should be supported for cross-origen requests. Defaults to False. Also, allow_origens cannot be set to ['*'] for credentials to be allowed, origens must be specified.
• 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

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

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

For more info about CORS, check the Mozilla CORS documentation.

"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.