Middleware¶

defis_allowed_origin(self, origin: str) -> bool:
 if self.allow_all_origins:
 return True

 if self.allow_origin_regex is not None and self.allow_origin_regex.fullmatch(origin):
 return True

 return origin in self.allow_origins

defpreflight_response(self, request_headers: Headers) -> Response:
 requested_origin = request_headers["origin"]
 requested_method = request_headers["access-control-request-method"]
 requested_headers = request_headers.get("access-control-request-headers")
 requested_private_network = request_headers.get("access-control-request-private-network")

 headers = dict(self.preflight_headers)
 failures: list[str] = []

 if self.is_allowed_origin(origin=requested_origin):
 if self.preflight_explicit_allow_origin:
 # The "else" case is already accounted for in self.preflight_headers
 # and the value would be "*".
 headers["Access-Control-Allow-Origin"] = requested_origin
 else:
 failures.append("origin")

 if requested_method not in self.allow_methods:
 failures.append("method")

 # If we allow all headers, then we have to mirror back any requested
 # headers in the response.
 if self.allow_all_headers and requested_headers is not None:
 headers["Access-Control-Allow-Headers"] = requested_headers
 elif requested_headers is not None:
 for header in [h.lower() for h in requested_headers.split(",")]:
 if header.strip() not in self.allow_headers:
 failures.append("headers")
 break

 if requested_private_network is not None:
 if self.allow_private_network:
 headers["Access-Control-Allow-Private-Network"] = "true"
 else:
 failures.append("private-network")

 # We don't strictly need to use 400 responses here, since its up to
 # the browser to enforce the CORS policy, but its more informative
 # if we do.
 if failures:
 failure_text = "Disallowed CORS " + ", ".join(failures)
 return PlainTextResponse(failure_text, status_code=400, headers=headers)

 return PlainTextResponse("OK", status_code=200, headers=headers)

async defsimple_response(self, scope: Scope, receive: Receive, send: Send, request_headers: Headers) -> None:
 send = functools.partial(self.send, send=send, request_headers=request_headers)
 await self.app(scope, receive, send)

async defsend(self, message: Message, send: Send, request_headers: Headers) -> None:
 if message["type"] != "http.response.start":
 await send(message)
 return

 message.setdefault("headers", [])
 headers = MutableHeaders(scope=message)
 headers.update(self.simple_headers)
 origin = request_headers["Origin"]

 # If credentials are allowed, then we must respond with the specific origin instead of '*'.
 if self.allow_all_origins and self.allow_credentials:
 self.allow_explicit_origin(headers, origin)

 # If we only allow specific origins, then we have to mirror back the Origin header in the response.
 elif not self.allow_all_origins and self.is_allowed_origin(origin=origin):
 self.allow_explicit_origin(headers, origin)

 await send(message)

@staticmethod
defallow_explicit_origin(headers: MutableHeaders, origin: str) -> None:
 headers["Access-Control-Allow-Origin"] = origin
 headers.add_vary_header("Origin")