feat: fail closed on invalid dashboard auth (#262)

src/dashboard_auth.py

@@ -3,6 +3,7 @@
 import base64
 import hashlib
 import hmac
+import ipaddress
 import json
 import os
 import secrets
@@ -14,6 +15,9 @@ from fastapi import Request
 SESSION_COOKIE = "stackchain_session"
 CSRF_COOKIE = "stackchain_csrf"
 DEFAULT_TTL_SECONDS = 8 * 60 * 60
+MIN_SECRET_LENGTH = 24
+OPERATOR_MODE = "operator"
+INSECURE_LOCAL_MODE = "insecure-local"
 
 
 @dataclass(frozen=True)
@@ -26,17 +30,42 @@ def access_token() -> str:
     return os.getenv("STACKCHAIN_DASHBOARD_ACCESS_TOKEN", "")
 
 
+def mode() -> str:
+    return os.getenv("STACKCHAIN_DASHBOARD_AUTH_MODE", OPERATOR_MODE).strip().lower()
+
+
+def configuration_error() -> str | None:
+    configured_mode = mode()
+    if configured_mode == INSECURE_LOCAL_MODE:
+        return None
+    if configured_mode != OPERATOR_MODE:
+        return "unsupported authentication mode"
+    operator_secret = access_token()
+    signing_secret = os.getenv("STACKCHAIN_DASHBOARD_SESSION_SECRET", "")
+    if len(operator_secret) < MIN_SECRET_LENGTH or len(signing_secret) < MIN_SECRET_LENGTH:
+        return "operator mode requires two sufficiently long secrets"
+    if hmac.compare_digest(operator_secret, signing_secret):
+        return "operator and session secrets must be independent"
+    return None
+
+
 def enabled() -> bool:
-    return bool(access_token())
+    return mode() == OPERATOR_MODE and configuration_error() is None
+
+
+def is_loopback_request(request: Request) -> bool:
+    host = request.client.host if request.client else ""
+    try:
+        return ipaddress.ip_address(host).is_loopback
+    except ValueError:
+        return host == "localhost"
 
 
 def _secret() -> bytes:
     configured = os.getenv("STACKCHAIN_DASHBOARD_SESSION_SECRET", "")
-    if configured:
-        return configured.encode()
-    return hmac.new(
-        access_token().encode(), b"stackchain-dashboard-session-signing", hashlib.sha256
-    ).digest()
+    if configuration_error() is not None or mode() != OPERATOR_MODE:
+        raise RuntimeError("Dashboard authentication is not configured")
+    return configured.encode()
 
 
 def _encode(raw: bytes) -> str:

src/main.py

@@ -430,10 +430,26 @@ app.include_router(frontend_router)
 
 @app.middleware("http")
 async def require_operator_session(request: Request, call_next):
-    if not dashboard_auth.enabled():
+    path = dashboard_auth.application_path(request)
+    if path == "/healthz":
+        return await call_next(request)
+
+    if dashboard_auth.configuration_error() is not None:
+        return JSONResponse(
+            {"detail": "Dashboard authentication is not configured"},
+            status_code=503,
+            headers={"Cache-Control": "no-store"},
+        )
+
+    if dashboard_auth.mode() == dashboard_auth.INSECURE_LOCAL_MODE:
+        if not dashboard_auth.is_loopback_request(request):
+            return JSONResponse(
+                {"detail": "Insecure local mode requires a loopback client"},
+                status_code=403,
+                headers={"Cache-Control": "no-store"},
+            )
         return await call_next(request)
 
-    path = dashboard_auth.application_path(request)
     public = (
         path in {"/healthz", "/readyz", "/login", "/manifest.webmanifest"}
         or path.startswith("/static/")