diff --git a/ui/tests/conftest.py b/ui/tests/conftest.py
index 5dd29006b3..9930e138c2 100644
--- a/ui/tests/conftest.py
+++ b/ui/tests/conftest.py
@@ -10,6 +10,51 @@
 # Global flag to track initialization
 _initialized = False
 
+# Isolation: the tests write QSettings (~/.config/opensnitch/settings.conf),
+# delete settings.conf, and toggle the autostart .desktop file. Redirect HOME
+# and the XDG dirs to a throwaway directory *before* opensnitch or Qt cache
+# any path, so a test run never touches an installed OpenSnitch UI.
+import hashlib
+import os
+import pwd
+import shutil
+import sys
+import tempfile
+
+if any(m == "opensnitch" or m.startswith("opensnitch.") for m in sys.modules):
+    raise RuntimeError("opensnitch was imported before tests/conftest.py could isolate HOME; refusing to run")
+
+_REAL_HOME = pwd.getpwuid(os.getuid()).pw_dir
+_SANDBOX = tempfile.mkdtemp(prefix="opensnitch-ui-tests-")
+for _var, _sub in (
+        ("HOME", ""),
+        ("XDG_CONFIG_HOME", ".config"),
+        ("XDG_DATA_HOME", ".local/share"),
+        ("XDG_CACHE_HOME", ".cache"),
+        ("XDG_STATE_HOME", ".local/state"),
+        ("XDG_RUNTIME_DIR", "run")):
+    _path = os.path.join(_SANDBOX, _sub)
+    os.makedirs(_path, mode=0o700, exist_ok=True)
+    os.environ[_var] = _path
+# no desktop notifications or other D-Bus traffic to the real session
+os.environ.pop("DBUS_SESSION_BUS_ADDRESS", None)
+
+# files of a real installation that a test run must never modify
+_REAL_FILES = [
+    os.path.join(_REAL_HOME, ".config/opensnitch/settings.conf"),
+    os.path.join(_REAL_HOME, ".config/autostart/opensnitch_ui.desktop"),
+]
+
+def _snapshot_real_files():
+    snap = {}
+    for path in _REAL_FILES:
+        try:
+            with open(path, "rb") as f:
+                snap[path] = hashlib.sha256(f.read()).hexdigest()
+        except FileNotFoundError:
+            snap[path] = None
+    return snap
+
 def init_test_environment():
     """Initialize database and config after QApplication exists."""
     global _initialized
@@ -35,6 +80,27 @@ def init_test_environment():
 
     _initialized = True
 
+@pytest.fixture(scope="session", autouse=True)
+def isolated_home():
+    """Fail loudly if anything resolves outside the sandbox, or if the
+    real installation's files changed during the run."""
+    try:
+        from PyQt6 import QtCore
+        from opensnitch.utils import xdg
+    except ImportError:
+        # tests that run without Qt: only the check of the real files applies
+        pass
+    else:
+        settings_file = QtCore.QSettings("opensnitch", "settings").fileName()
+        for path in (settings_file, xdg.xdg_config_home, xdg.xdg_runtime_dir, xdg.Autostart().userAutostart):
+            assert path.startswith(_SANDBOX + os.sep), f"not isolated: {path} is outside {_SANDBOX}"
+
+    before = _snapshot_real_files()
+    yield _SANDBOX
+    after = _snapshot_real_files()
+    shutil.rmtree(_SANDBOX, ignore_errors=True)
+    assert before == after, f"test run modified the real installation: {before} -> {after}"
+
 @pytest.fixture(scope="session")
 def qapp():
     """Create QApplication for the entire test session."""