92 lines
3.6 KiB
Python
92 lines
3.6 KiB
Python
 
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"""Refuse state-changing API requests that another web site makes a browser send.
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The double-submit CSRF token (app/identity/deps.py ``require_csrf``) guards
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everything a signed-in user can change. The requests made before anyone is
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signed in — sign-in itself, sign-up, a password reset — have no token yet, so a
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hostile page could make a visitor's browser sign in to an account of the
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attacker's choosing (login CSRF): whatever the victim then writes, files or
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uploads lands in the attacker's account.
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Browsers say where a request comes from: ``Sec-Fetch-Site`` in every current
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browser, ``Origin`` on any cross-origin POST in older ones. A request another
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site started is refused here, before any route runs. Mail apps, calendar apps
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and servers send neither header and are not affected (DAV lives outside /api
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anyway).
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"""
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from __future__ import annotations
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import json
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from urllib.parse import urlsplit
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from starlette.datastructures import Headers
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from starlette.types import ASGIApp, Receive, Scope, Send
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from app.config import get_settings
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_SAFE_METHODS = frozenset({"GET", "HEAD", "OPTIONS"})
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_BODY = json.dumps(
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{"error": {"code": "cross_site", "message": "request refused: sent from another site"}}
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).encode()
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def _hostname(value: str) -> str:
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"""``host[:port]`` -> ``host``, lower-cased (IPv6 brackets kept)."""
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value = value.strip().lower()
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if value.startswith("["):
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return value.split("]", 1)[0] + "]"
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return value.rsplit(":", 1)[0] if value.count(":") == 1 else value
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def is_cross_site(headers: Headers) -> bool:
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"""Whether a browser reports this request as started by another site."""
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fetch_site = headers.get("sec-fetch-site")
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if fetch_site is not None:
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# "same-origin", "same-site" (our own subdomains), "none" (typed or
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# bookmarked): all fine. Only another site is refused.
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return fetch_site.strip().lower() == "cross-site"
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origin = headers.get("origin")
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if origin is None:
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return False # not a browser, or a same-origin request from an old one
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if origin.strip().lower() == "null":
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return True # sandboxed frame, data: URL, file: page
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origin_host = (urlsplit(origin.strip()).hostname or "").lower()
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# Our own domain, whatever a reverse proxy did to the Host header.
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domain = get_settings().app_domain.lower()
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if origin_host == domain or origin_host.endswith("." + domain):
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return False
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# Otherwise the name the browser used: Host, or the one a reverse proxy saw.
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ours = {_hostname(headers.get("host", ""))}
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forwarded = headers.get("x-forwarded-host")
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if forwarded:
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ours.add(_hostname(forwarded.split(",")[0]))
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return origin_host not in ours
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class CrossSiteGuard:
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"""ASGI middleware: 403 on cross-site POST/PUT/PATCH/DELETE under /api/."""
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def __init__(self, app: ASGIApp) -> None:
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self.app = app
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async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
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if (
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scope["type"] == "http"
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and scope["method"] not in _SAFE_METHODS
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and scope["path"].startswith("/api/")
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and is_cross_site(Headers(scope=scope))
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):
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await send(
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{
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"type": "http.response.start",
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"status": 403,
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"headers": [
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(b"content-type", b"application/json"),
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(b"content-length", str(len(_BODY)).encode()),
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],
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}
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)
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await send({"type": "http.response.body", "body": _BODY})
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return
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await self.app(scope, receive, send)
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