From 3d8c6101b6d2dfb301ad851303f3060cedeaff95 Mon Sep 17 00:00:00 2001
From: ciregenz <ciregenz@berkeley.edu>
Date: Fri, 31 Jul 2026 11:44:24 -0700
Subject: [PATCH] [eric] security: make the 9router state dir owner-only, its
 db.json held live tokens at 0644

---
 backend/apps/nine_router/process.py           | 21 +++++++
 .../tests/test_router_data_dir_permissions.py | 56 +++++++++++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 backend/tests/test_router_data_dir_permissions.py

diff --git a/backend/apps/nine_router/process.py b/backend/apps/nine_router/process.py
index 79eea7ad..88e011df 100644
--- a/backend/apps/nine_router/process.py
+++ b/backend/apps/nine_router/process.py
@@ -17,6 +17,7 @@ import os
 import secrets
 import shutil
 import socket
+import stat
 import subprocess
 import tempfile
 import time
@@ -115,6 +116,25 @@ def p_nine_router_data_dir() -> str:
     return os.path.join(os.path.expanduser("~"), ".9router")
 
 
+def harden_data_dir_permissions() -> None:
+    """Make the 9Router state dir owner-only. Its db.json holds live subscription access AND refresh
+    tokens in plaintext and 9Router writes it 0644, so on a shared machine any other local account
+    can read them. We tighten the DIRECTORY rather than the file because 9Router rewrites db.json on
+    every token refresh, which would drop a chmod on the file itself within the hour."""
+    if os.name == "nt":
+        return
+    data_dir = p_nine_router_data_dir()
+    try:
+        if not os.path.isdir(data_dir):
+            return
+        current = stat.S_IMODE(os.stat(data_dir).st_mode)
+        if current & 0o077:
+            os.chmod(data_dir, 0o700)
+            logger.info("tightened 9router data dir from %s to 0700", oct(current))
+    except OSError:
+        logger.warning("could not tighten 9router data dir permissions", exc_info=True)
+
+
 p_cli_token_cache: str | None = None
 
 
@@ -353,6 +373,7 @@ async def ensure_running():
         p_start_lock = asyncio.Lock()
     async with p_start_lock:
         await p_ensure_running_impl()
+    harden_data_dir_permissions()
     # Arm both healers the moment the router becomes a live dependency; users who never route through it never spawn them.
     if is_running():
         start_watchdog()
diff --git a/backend/tests/test_router_data_dir_permissions.py b/backend/tests/test_router_data_dir_permissions.py
new file mode 100644
index 00000000..68d75271
--- /dev/null
+++ b/backend/tests/test_router_data_dir_permissions.py
@@ -0,0 +1,56 @@
+"""9Router's state dir must be owner-only.
+
+Its db.json carries live subscription access AND refresh tokens in plaintext, and 9Router writes
+that file 0644. On a shared machine every other local account could read them. We tighten the
+directory rather than the file because 9Router rewrites db.json on every token refresh, so a chmod
+on the file itself would be undone within the hour.
+
+Run:
+    cd backend && .venv/bin/python -m pytest tests/test_router_data_dir_permissions.py -v
+"""
+
+import os
+import stat
+
+import pytest
+
+import backend.apps.nine_router.process as process
+
+
+@pytest.fixture
+def p_data_dir(tmp_path, monkeypatch):
+    d = tmp_path / "9router"
+    d.mkdir()
+    monkeypatch.setenv("DATA_DIR", str(d))
+    return d
+
+
+@pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits only")
+def test_group_and_world_readable_dir_is_tightened(p_data_dir):
+    os.chmod(p_data_dir, 0o755)
+    process.harden_data_dir_permissions()
+    assert stat.S_IMODE(os.stat(p_data_dir).st_mode) == 0o700
+
+
+@pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits only")
+def test_tokens_are_unreadable_by_others_afterwards(p_data_dir):
+    """The property that actually matters, stated as the attacker sees it: no bit outside the owner."""
+    os.chmod(p_data_dir, 0o755)
+    (p_data_dir / "db.json").write_text('{"providerConnections":[]}')
+    process.harden_data_dir_permissions()
+    assert stat.S_IMODE(os.stat(p_data_dir).st_mode) & 0o077 == 0
+
+
+@pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits only")
+def test_already_tight_dir_is_left_alone(p_data_dir):
+    os.chmod(p_data_dir, 0o700)
+    before = os.stat(p_data_dir).st_mtime_ns
+    process.harden_data_dir_permissions()
+    assert stat.S_IMODE(os.stat(p_data_dir).st_mode) == 0o700
+    assert os.stat(p_data_dir).st_mtime_ns == before
+
+
+def test_missing_dir_does_not_raise(tmp_path, monkeypatch):
+    """Runs before 9Router has ever started, so the dir legitimately may not exist yet."""
+    monkeypatch.setenv("DATA_DIR", str(tmp_path / "nope"))
+    process.harden_data_dir_permissions()