Middleware can be used to wrap a Quart app instance and alter the ASGI process. A very simple example would be to reject requests based on the presence of a header,

class RejectMiddleware:

    def __init__(self, app):
        self.app = app

    def __call__(self, scope):
        for header, value in scope['headers']:
             if header == 'X-Secret' and value == 'very-secret':
                 return self.app(scope)
        return self.error_response

    def error_response(self, receive, send):
        await send({
            'type': 'http.response.start',
            'status': 401,
            'headers': ['content-length', '0'],
        await send({
            'type': 'http.response.body',
            'body': b'',
            'more_body': False,

 quart_app.asgi_app = RejectMiddleware(quart_app.asgi_app)

This can then be extended and used with any ASGI Middleware and served with any ASGI server.