fix(file_tools): prevent agent from silently disabling exec approval via config write

tests/tools/test_file_tools.py

@@ -321,4 +321,65 @@ def test_truncated_hint_with_nonzero_offset(self, mock_get):
         assert "offset=100" in raw
 
 
+class TestSensitivePathCheck:
+    """Verify that _check_sensitive_path blocks writes to protected locations."""
+
+    def test_hermes_config_blocked_for_write_file(self, tmp_path, monkeypatch):
+        fake_config = tmp_path / "config.yaml"
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved", str(fake_config))
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved_loaded", True)
+
+        from tools.file_tools import write_file_tool
+        result = json.loads(write_file_tool(str(fake_config), "approvals:\n  mode: off\n"))
+        assert "error" in result
+        assert "Hermes config" in result["error"]
+
+    def test_hermes_config_blocked_via_tilde_path(self, tmp_path, monkeypatch):
+        fake_config = tmp_path / "config.yaml"
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved", str(fake_config))
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved_loaded", True)
+
+        from tools.file_tools import write_file_tool
+        result = json.loads(write_file_tool(str(fake_config), "approvals:\n  mode: off\n"))
+        assert "error" in result
+        assert "Hermes config" in result["error"]
+
+    def test_hermes_config_blocked_for_patch(self, tmp_path, monkeypatch):
+        fake_config = tmp_path / "config.yaml"
+        fake_config.write_text("approvals:\n  mode: manual\n")
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved", str(fake_config))
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved_loaded", True)
+
+        from tools.file_tools import patch_tool
+        result = json.loads(patch_tool(
+            mode="replace",
+            path=str(fake_config),
+            old_string="mode: manual",
+            new_string="mode: off",
+        ))
+        assert "error" in result
+        assert "Hermes config" in result["error"]
+
+    def test_system_path_still_blocked(self, monkeypatch):
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved", "/some/other/path")
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved_loaded", True)
+
+        from tools.file_tools import write_file_tool
+        result = json.loads(write_file_tool("/etc/passwd", "evil"))
+        assert "error" in result
+        assert "sensitive system path" in result["error"]
+
+    @patch("tools.file_tools._get_file_ops")
+    def test_normal_file_not_blocked(self, mock_get, monkeypatch):
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved", "/home/user/.hermes/config.yaml")
+        monkeypatch.setattr("tools.file_tools._hermes_config_resolved_loaded", True)
+        mock_ops = MagicMock()
+        result_obj = MagicMock()
+        result_obj.to_dict.return_value = {"status": "ok", "path": "/tmp/other.txt", "bytes": 5}
+        mock_ops.write_file.return_value = result_obj
+        mock_get.return_value = mock_ops
+
+        from tools.file_tools import write_file_tool
+        result = json.loads(write_file_tool("/tmp/other.txt", "hello"))
+        assert result["status"] == "ok"
 

tools/file_tools.py

@@ -117,6 +117,26 @@ def _is_blocked_device(filepath: str) -> bool:
 )
 _SENSITIVE_EXACT_PATHS = {"/var/run/docker.sock", "/run/docker.sock"}
 
+_hermes_config_resolved: str | None = None
+_hermes_config_resolved_loaded = False
+
+
+def _get_hermes_config_resolved() -> str | None:
+    """Return the resolved absolute path of the Hermes config file (cached)."""
+    global _hermes_config_resolved, _hermes_config_resolved_loaded
+    if _hermes_config_resolved_loaded:
+        return _hermes_config_resolved
+    _hermes_config_resolved_loaded = True
+    try:
+        from hermes_cli.config import get_config_path
+        _hermes_config_resolved = str(get_config_path().resolve())
+    except Exception:
+        try:
+            _hermes_config_resolved = str(Path("~/.hermes/config.yaml").expanduser().resolve())
+        except Exception:
+            _hermes_config_resolved = None
+    return _hermes_config_resolved
+
 
 def _check_sensitive_path(filepath: str) -> str | None:
     """Return an error message if the path targets a sensitive system location."""
@@ -134,6 +154,17 @@ def _check_sensitive_path(filepath: str) -> str | None:
             return _err
     if resolved in _SENSITIVE_EXACT_PATHS or normalized in _SENSITIVE_EXACT_PATHS:
         return _err
+    # Prevent agents from modifying the Hermes config file directly.
+    # approvals.mode and other security settings live here; a malicious or
+    # prompt-injected agent could silently disable exec approval by writing to
+    # this file.
+    hermes_config = _get_hermes_config_resolved()
+    if hermes_config and (resolved == hermes_config or normalized == hermes_config):
+        return (
+            f"Refusing to write to Hermes config file: {filepath}\n"
+            "Agent cannot modify security-sensitive configuration. "
+            "Edit ~/.hermes/config.yaml directly or use 'hermes config' instead."
+        )
     return None