fix: cap and type-check the unauthenticated consent POST body (SEC-01)

src/aegis_gitea_mcp/oauth/callback.py

@@ -16,14 +16,21 @@ import html
 from typing import Any
 from urllib.parse import urlsplit
 
+from mcp.server.transport_security import RequestBodyLimitMiddleware
+from starlette.datastructures import Headers
 from starlette.requests import Request
 from starlette.responses import HTMLResponse, RedirectResponse, Response
-from starlette.routing import Route
+from starlette.routing import Route, request_response
+from starlette.types import ASGIApp, Receive, Scope, Send
 
 from aegis_gitea_mcp.oauth.provider import CONSENT_PATH, GiteaOAuthProvider
 from aegis_gitea_mcp.oauth.upstream import CALLBACK_PATH
 
 COOKIE_TTL_SECONDS = 600
+#: Largest request body the consent form may send (three short fields).
+MAX_CONSENT_BODY_BYTES = 16 * 1024
+_MAX_FORM_FIELDS = 8
+_MAX_FORM_PART_BYTES = 4096
 _INVALID_MESSAGE = (
     "This sign-in request is invalid or has expired. Start the sign-in again from your MCP client."
 )
@@ -100,6 +107,28 @@ def _text(value: Any, limit: int = 200) -> str:
     return value[:limit] if isinstance(value, str) else ""
 
 
+class _UrlencodedFormOnly:
+    """ASGI guard for the consent POST, applied before any body is read.
+
+    The route is reachable without authentication, so it must not parse
+    arbitrary bodies: anything but ``application/x-www-form-urlencoded`` gets
+    415 without reading a byte, and the body is capped at
+    :data:`MAX_CONSENT_BODY_BYTES` (413 beyond it).
+    """
+
+    def __init__(self, app: ASGIApp) -> None:
+        self._limited = RequestBodyLimitMiddleware(app, MAX_CONSENT_BODY_BYTES)
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] == "http":
+            content_type = Headers(scope=scope).get("content-type", "")
+            if content_type.split(";")[0].strip().lower() != "application/x-www-form-urlencoded":
+                response = Response("Unsupported media type", status_code=415)
+                await response(scope, receive, send)
+                return
+        await self._limited(scope, receive, send)
+
+
 def build_browser_routes(provider: GiteaOAuthProvider) -> list[Route]:
     """Return the consent and callback routes for ``provider``."""
     cookies = _Cookies(provider.base_url)
@@ -138,7 +167,12 @@ def build_browser_routes(provider: GiteaOAuthProvider) -> list[Route]:
         origin = request.headers.get("origin")
         if origin is not None and origin != expected_origin:
             return _invalid()
-        form = await request.form()
+        try:
+            form = await request.form(
+                max_files=0, max_fields=_MAX_FORM_FIELDS, max_part_size=_MAX_FORM_PART_BYTES
+            )
+        except Exception:
+            return _invalid()
         decision = provider.finish_consent(
             _text(form.get("request")),
             _text(form.get("csrf")),
@@ -168,6 +202,12 @@ def build_browser_routes(provider: GiteaOAuthProvider) -> list[Route]:
 
     return [
         Route(CONSENT_PATH, consent_page, methods=["GET"]),
-        Route(CONSENT_PATH, consent_submit, methods=["POST"]),
-        Route(CALLBACK_PATH, callback, methods=["GET"]),
+        Route(
+            CONSENT_PATH, _UrlencodedFormOnly(request_response(consent_submit)), methods=["POST"]
+        ),
+        Route(
+            CALLBACK_PATH,
+            RequestBodyLimitMiddleware(request_response(callback), MAX_CONSENT_BODY_BYTES),
+            methods=["GET"],
+        ),
     ]