asgi-csrf

ASGI middleware for protecting against CSRF attacks

Installation

pip install asgi-csrf

Background

See the OWASP guide to Cross Site Request Forgery (CSRF) and their Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet.

This middleware implements the Double Submit Cookie pattern, where a cookie is set that is then compared to a csrftoken hidden form field or a x-csrftoken HTTP header.

Usage

Decorate your ASGI application like this:

from asgi_csrf import asgi_csrf
from .my_asgi_app import app


app = asgi_csrf(app, signing_secret="secret-goes-here")

The middleware will set a csrftoken cookie, if one is missing. The value of that token will be made available to your ASGI application through the scope["csrftoken"] function.

Your application code should include that value as a hidden form field in any POST forms:

<form action="/login" method="POST">
    ...
    <input type="hidden" name="csrftoken" value="{{ request.scope.csrftoken() }}">
</form>

Note that request.scope["csrftoken"]() is a function that returns a string. Calling that function also lets the middleware know that the cookie should be set by that page, if the user does not already have that cookie.

If the cookie needs to be set, the middleware will add a Vary: Cookie header to the response to ensure it is not incorrectly cached by any CDNs or intermediary proxies.

The middleware will return a 403 forbidden error for any POST requests that do not include the matching csrftoken - either in the POST data or in a x-csrftoken HTTP header (useful for JavaScript fetch() calls).

The signing_secret is used to sign the tokens, to protect against subdomain vulnerabilities.

If you do not pass in an explicit signing_secret parameter, the middleware will look for a ASGI_CSRF_SECRET environment variable.

If it cannot find that environment variable, it will generate a random secret which will persist for the lifetime of the server.

This means that if you do not configure a specific secret your user's csrftoken cookies will become invalid every time the server restarts! You should configure a secret.

Always setting the cookie if it is not already set

By default this middleware only sets the csrftoken cookie if the user encounters a page that needs it - due to that page calling the request.scope["csrftoken"]() function, for example to populate a hidden field in a form.

If you would like the middleware to set that cookie for any incoming request that does not already provide the cookie, you can use the always_set_cookie=True argument:

app = asgi_csrf(app, signing_secret="secret-goes-here", always_set_cookie=True)

Configuring the cookie

The middleware can be configured with several options to control how the CSRF cookie is set:

app = asgi_csrf(
    app,
    signing_secret="secret-goes-here",
    cookie_name="csrftoken",
    cookie_path="/",
    cookie_domain=None,
    cookie_secure=False,
    cookie_samesite="Lax"
)

• cookie_name: The name of the cookie to set. Defaults to "csrftoken".
• cookie_path: The path for which the cookie is valid. Defaults to "/", meaning the cookie is valid for the entire domain.
• cookie_domain: The domain for which the cookie is valid. Defaults to None, which means the cookie will only be valid for the current domain.
• cookie_secure: If set to True, the cookie will only be sent over HTTPS connections. Defaults to False.
• cookie_samesite: Controls how the cookie is sent with cross-site requests. Can be set to "Strict", "Lax", or "None". Defaults to "Lax".

Other cases that skip CSRF protection

If the request includes an Authorization: Bearer ... header, commonly used by OAuth and JWT authentication, the request will not be required to include a CSRF token. This is because browsers cannot send those headers in a context that can be abused.

If the request has no cookies at all it will be allowed through, since CSRF protection is only necessary for requests from authenticated users.

always_protect

If you have paths that should always be protected even without cookies - your login form for example (to avoid login CSRF attacks) you can protect those paths by passing them as the always_protect parameter:

app = asgi_csrf(
    app,
    signing_secret="secret-goes-here",
    always_protect={"/login"}
)

skip_if_scope

There may be situations in which you want to opt-out of CSRF protection even for authenticated POST requests - this is often the case for web APIs for example.

The skip_if_scope= parameter can be used to provide a callback function which is passed an ASGI scope and returns True if CSRF protection should be skipped for that request.

This example skips CSRF protection for any incoming request where the request path starts with /api/:

def skip_api_paths(scope)
    return scope["path"].startswith("/api/")

app = asgi_csrf(
    app,
    signing_secret="secret-goes-here",
    skip_if_scope=skip_api_paths
)

Custom errors with send_csrf_failed

By default, when a CSRF token is missing or invalid, the middleware will return a 403 Forbidden response page with a short error message.

You can customize this behavior by passing a send_csrf_failed function to the middleware. This function should accept the ASGI scope and send functions, and the message_id of the error that occurred.

The message_id will be an integer representing an item from the asgi_csrf.Errors enum.

This example shows how you could customize the error message based on that message_id:

async def custom_csrf_failed(scope, send, message_id):
    assert scope["type"] == "http"
    await send(
        {
            "type": "http.response.start",
            "status": 403,
            "headers": [[b"content-type", b"text/html; charset=utf-8"]],
        }
    )
    await send(
        {
            "type": "http.response.body",
            "body": {
                Errors.FORM_URLENCODED_MISMATCH: "custom form-urlencoded error",
                Errors.MULTIPART_MISMATCH: "custom multipart error",
                Errors.FILE_BEFORE_TOKEN: "custom file before token error",
                Errors.UNKNOWN_CONTENT_TYPE: "custom unknown content type error",
            }
            .get(message_id, "")
            .encode("utf-8"),
        }
    )


app = asgi_csrf(
    app,
    signing_secret="secret-goes-here",
    send_csrf_failed=custom_csrf_failed
)