US08-01: Make the Trust Boundary Configurable and Authenticated (#96)

photo_pipeline/api/routes/session.py

@@ -3,21 +3,52 @@
 It sets the ``HttpOnly``/``SameSite=Strict`` session cookie and returns the CSRF
 token in the body. A foreign page can call this — it just cannot read the answer,
 because the app sends no CORS headers — and the cookie it received is never attached
-to a request that foreign page initiates.
+to a request that foreign page initiated.
+
+When an access secret is configured (mandatory as soon as the app is reachable from
+another machine, US08-01) this is also the authentication gate: the secret buys the
+cookie, and every route behind it keeps asking for exactly the session and CSRF token
+it asked for before. Wrong secrets are counted, and a burst of them stops being
+answered — otherwise a proxy-exposed deployment could be guessed at indefinitely.
 """
 
 from __future__ import annotations
 
+import logging
+import secrets
+
 from fastapi import APIRouter, Request
 from fastapi.responses import JSONResponse
 
-from photo_pipeline.api.security import SESSION_COOKIE
+from photo_pipeline.api.security import ACCESS_SECRET_HEADER, SESSION_COOKIE
 
 router = APIRouter(tags=["session"])
 
+log = logging.getLogger(__name__)
+
+
+def _refuse(status: int, code: str, message: str) -> JSONResponse:
+    return JSONResponse(status_code=status, content={"error": {"code": code, "message": message}})
+
 
 @router.get("/session")
 def start_session(request: Request) -> JSONResponse:
+    config = request.app.state.config
+    secret = config.access_secret
+    if secret is not None:
+        limiter = request.app.state.access_limiter
+        if limiter.blocked():
+            return _refuse(429, "too_many_attempts", "too many failed attempts; retry later")
+        offered = request.headers.get(ACCESS_SECRET_HEADER, "")
+        if not secrets.compare_digest(offered, secret.get_secret_value()):
+            limiter.record_failure()
+            # The client address is the whole record: the offered secret, the issued
+            # session, and the request body all stay out of the log.
+            log.warning(
+                "access secret rejected", extra={"client": _client(request), "path": "/session"}
+            )
+            return _refuse(401, "access_denied", "a valid access secret is required")
+
     session = request.app.state.session
     response = JSONResponse({"csrf_token": session.csrf_token})
     response.set_cookie(
@@ -25,6 +56,13 @@ def start_session(request: Request) -> JSONResponse:
         session.id,
         httponly=True,
         samesite="strict",
+        # HTTPS outside means the cookie must never travel over a plain hop, even one
+        # this process cannot see. Loopback http keeps working unchanged.
+        secure=request.scope.get("state", {}).get("external_scheme") == "https",
         path="/",
     )
     return response
+
+
+def _client(request: Request) -> str:
+    return request.client.host if request.client else "unknown"