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CORSMiddleware on /register and /token forwards any non-preflight OPTIONS request straight to the body-reading handler

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新手友好度
84/100
Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
活跃
技术栈
python
领域
api, backend, security

调研方向

从 src/mcp/server/auth/routes.py 开始：阅读 _cors()（约 57-63 行）以及 /token 和 /register 的路由定义（约 118-136 行），然后确认 starlette 的 CORSMiddleware 如何判定一个请求是 preflight。在 dispatch 之前加一个守卫，使非 preflight 的 OPTIONS（没有 origin，或没有 access-control-request-method 头）返回 405 和 Allow 头，而不是到达 RegistrationHandler/TokenHandler。先用 issue 中独立的 httpx/ASGITransport 脚本复现；完成的标志是两种 OPTIONS 情况都不再返回 201/500，且 tests/ 下现有的认证测试仍然通过。

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描述

bug v1 v2

Affected versions: mcp 1.30.0 (confirmed); still present in the latest release, mcp 2.3.0, same code at src/mcp/server/auth/routes.py:119-120 (/token) and :132-136 (/register).

Where

  • routes.py:57-63: _cors() wraps the handler in starlette.middleware.cors.CORSMiddleware with allow_methods=["POST","OPTIONS"].
  • routes.py:118-123 and :130-136: both routes list OPTIONS and are wrapped by _cors.
  • starlette/middleware/cors.py:86 (starlette 1.7.0): CORSMiddleware.__call__ only treats the request as a preflight when origin is not None AND method == "OPTIONS" AND "access-control-request-method" in headers; any other OPTIONS (including one with no CORS headers at all) falls through to simple_response, which calls the wrapped handler.

Repro (standalone, mcp + starlette + httpx only)

import anyio
from pydantic import AnyHttpUrl
from starlette.applications import Starlette
from mcp.server.auth.routes import create_auth_routes
from mcp.server.auth.settings import ClientRegistrationOptions
from httpx import ASGITransport, AsyncClient

class DummyProvider:
    def __init__(self): self.clients = {}
    async def register_client(self, client_info): self.clients[client_info.client_id] = client_info
    async def get_client(self, client_id): return self.clients.get(client_id)
    async def authorize(self, *a, **k): ...
    async def load_authorization_code(self, *a, **k): ...
    async def exchange_authorization_code(self, *a, **k): ...
    async def load_refresh_token(self, *a, **k): ...
    async def exchange_refresh_token(self, *a, **k): ...
    async def revoke_token(self, *a, **k): ...

async def main():
    provider = DummyProvider()
    routes = create_auth_routes(provider, issuer_url=AnyHttpUrl("https://example.test"), client_registration_options=ClientRegistrationOptions(enabled=True))
    app = Starlette(routes=routes)
    transport = ASGITransport(app=app, raise_app_exceptions=False)
    async with AsyncClient(transport=transport, base_url="https://example.test") as client:
        r1 = await client.options("/register")
        print("OPTIONS /register, no body, no CORS headers ->", r1.status_code)
        r2 = await client.request("OPTIONS", "/register", content=b'{"redirect_uris": ["https://client.example/cb"], "client_name": "demo"}', headers={"Content-Type": "application/json"})
        print("OPTIONS /register, JSON body, no CORS headers ->", r2.status_code, r2.text[:200])
        print("Clients stored:", list(provider.clients.keys()))

anyio.run(main)

Expected vs actual

  • Expected: a non-preflight OPTIONS is rejected (405) or answered empty, never reaching the handler.
  • Actual: the empty-body OPTIONS reaches RegistrationHandler.handle, which calls request.json() on an empty body and raises an uncaught JSONDecodeError (unhandled 500), and the JSON-body OPTIONS is parsed, assigned a client_id/client_secret, stored via provider.register_client, and answered 201 — a registration created over what looked like a CORS preflight, bypassing rate limiting.

Suggested fix

Check origin is not None and "access-control-request-method" in headers before dispatch, in create_auth_routes/cors_middleware, answering a non-preflight OPTIONS with 405 + Allow header; no Starlette change required.

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