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from __future__ import annotations
import functools
import re
import typing
from starlette.datastructures import Headers, MutableHeaders
from starlette.responses import PlainTextResponse, Response
from starlette.types import ASGIApp, Message, Receive, Scope, Send
ALL_METHODS = ("DELETE", "GET", "HEAD", "OPTIONS", "PATCH", "POST", "PUT")
SAFELISTED_HEADERS = {"Accept", "Accept-Language", "Content-Language", "Content-Type"}
class CORSMiddleware:
def __init__(
self,
app: ASGIApp,
allow_origins: typing.Sequence[str] = (),
allow_methods: typing.Sequence[str] = ("GET",),
allow_headers: typing.Sequence[str] = (),
allow_credentials: bool = False,
allow_origin_regex: str | None = None,
expose_headers: typing.Sequence[str] = (),
max_age: int = 600,
) -> None:
if "*" in allow_methods:
allow_methods = ALL_METHODS
compiled_allow_origin_regex = None
if allow_origin_regex is not None:
compiled_allow_origin_regex = re.compile(allow_origin_regex)
allow_all_origins = "*" in allow_origins
allow_all_headers = "*" in allow_headers
preflight_explicit_allow_origin = not allow_all_origins or allow_credentials
simple_headers = {}
if allow_all_origins:
simple_headers["Access-Control-Allow-Origin"] = "*"
if allow_credentials:
simple_headers["Access-Control-Allow-Credentials"] = "true"
if expose_headers:
simple_headers["Access-Control-Expose-Headers"] = ", ".join(expose_headers)
preflight_headers = {}
if preflight_explicit_allow_origin:
# The origin value will be set in preflight_response() if it is allowed.
preflight_headers["Vary"] = "Origin"
else:
preflight_headers["Access-Control-Allow-Origin"] = "*"
preflight_headers.update(
{
"Access-Control-Allow-Methods": ", ".join(allow_methods),
"Access-Control-Max-Age": str(max_age),
}
)
allow_headers = sorted(SAFELISTED_HEADERS | set(allow_headers))
if allow_headers and not allow_all_headers:
preflight_headers["Access-Control-Allow-Headers"] = ", ".join(allow_headers)
if allow_credentials:
preflight_headers["Access-Control-Allow-Credentials"] = "true"
self.app = app
self.allow_origins = allow_origins
self.allow_methods = allow_methods
self.allow_headers = [h.lower() for h in allow_headers]
self.allow_all_origins = allow_all_origins
self.allow_all_headers = allow_all_headers
self.preflight_explicit_allow_origin = preflight_explicit_allow_origin
self.allow_origin_regex = compiled_allow_origin_regex
self.simple_headers = simple_headers
self.preflight_headers = preflight_headers
async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
if scope["type"] != "http": # pragma: no cover
await self.app(scope, receive, send)
return
method = scope["method"]
headers = Headers(scope=scope)
origin = headers.get("origin")
if origin is None:
await self.app(scope, receive, send)
return
if method == "OPTIONS" and "access-control-request-method" in headers:
response = self.preflight_response(request_headers=headers)
await response(scope, receive, send)
return
await self.simple_response(scope, receive, send, request_headers=headers)
def is_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
def preflight_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")
headers = dict(self.preflight_headers)
failures = []
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
# 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 def simple_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 def send(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"]
has_cookie = "cookie" in request_headers
# If request includes any cookie headers, then we must respond
# with the specific origin instead of '*'.
if self.allow_all_origins and has_cookie:
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
def allow_explicit_origin(headers: MutableHeaders, origin: str) -> None:
headers["Access-Control-Allow-Origin"] = origin
headers.add_vary_header("Origin")
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