CORS (Cross-Origin Resource Sharing)

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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 “origin” than the frontend.

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

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

To achieve this, the backend must have a list of “allowed origins”.

In this case, it would have to include http://localhost:8080 for the 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 origins.

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 if 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()
origins = [
    "http://localhost.tiangolo.com",
    "https://localhost.tiangolo.com",
    "http://localhost",
    "http://localhost:8080",
]
app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    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 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. eg. '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 CORS requests.
• allow_credentials - Indicate that cookies should be supported for cross-origin requests. Defaults to False.
• 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.