diff --git a/repeater/web/http_server.py b/repeater/web/http_server.py
index 93c8e6e..337ca7a 100644
--- a/repeater/web/http_server.py
+++ b/repeater/web/http_server.py
@@ -286,7 +286,7 @@ class StatsApp:
             raise cherrypy.NotFound()
         root = Path(root_dir).resolve()
         target = (root.joinpath(*relative_parts)).resolve()
-        if not str(target).startswith(str(root)) or not target.is_file():
+        if not target.is_relative_to(root) or not target.is_file():
             raise cherrypy.NotFound()
         guessed_type, _ = mimetypes.guess_type(str(target))
         cherrypy.response.headers["Content-Type"] = guessed_type or "application/octet-stream"
diff --git a/tests/test_frontend_static_confinement.py b/tests/test_frontend_static_confinement.py
new file mode 100644
index 0000000..547ca24
--- /dev/null
+++ b/tests/test_frontend_static_confinement.py
@@ -0,0 +1,81 @@
+"""Static frontend roots must use path components, not string prefixes."""
+
+from http.client import HTTPConnection
+from threading import Thread
+from types import SimpleNamespace
+from wsgiref.simple_server import make_server
+
+import cherrypy
+import pytest
+
+from repeater.web.http_server import StatsApp
+
+
+@pytest.mark.parametrize("directory", ["assets", "_next"])
+@pytest.mark.parametrize("symlink", [False, True])
+def test_static_rejects_prefix_sharing_sibling(tmp_path, monkeypatch, directory, symlink):
+    root = tmp_path / directory
+    root.mkdir()
+    private = tmp_path / (directory + "-private")
+    private.mkdir()
+    (private / "probe.txt").write_text("OUTSIDE-ROOT")
+    parts = ("..", private.name, "probe.txt")
+    if symlink:
+        (root / "linked").symlink_to(private, target_is_directory=True)
+        parts = ("linked", "probe.txt")
+    monkeypatch.setattr(cherrypy, "response", SimpleNamespace(headers={}))
+    app = object.__new__(StatsApp)
+    with pytest.raises(cherrypy.NotFound):
+        app._serve_static_file(str(root), parts)
+
+
+def test_static_keeps_nested_assets(tmp_path, monkeypatch):
+    (tmp_path / "nested").mkdir()
+    (tmp_path / "nested" / "app.js").write_text("PUBLIC-SCRIPT")
+    monkeypatch.setattr(cherrypy, "response", SimpleNamespace(headers={}))
+    app = object.__new__(StatsApp)
+    assert app._serve_static_file(str(tmp_path), ("nested", "app.js")) == b"PUBLIC-SCRIPT"
+
+
+@pytest.mark.parametrize("directory", ["assets", "_next"])
+def test_public_dispatch_rejects_encoded_sibling_paths(tmp_path, monkeypatch, directory):
+    root = tmp_path / directory
+    root.mkdir()
+    (root / "app.js").write_text("PUBLIC-SCRIPT")
+    private = tmp_path / (directory + "-private")
+    private.mkdir()
+    (private / "probe.txt").write_text("OUTSIDE-ROOT")
+    monkeypatch.setattr(cherrypy, "request", cherrypy._ThreadLocalProxy("request"))
+    monkeypatch.setattr(cherrypy, "response", cherrypy._ThreadLocalProxy("response"))
+    app = object.__new__(StatsApp)
+    app.html_dir = str(tmp_path)
+    monkeypatch.setattr(app, "_resolve_html_dir", lambda: app.html_dir)
+    application = cherrypy.Application(app, script_name="")
+    with make_server("127.0.0.1", 0, application) as server:
+        worker = Thread(target=server.serve_forever, daemon=True)
+        worker.start()
+        try:
+            for path in (
+                f"/{directory}/../{directory}-private/probe.txt",
+                f"/{directory}/..%2f{directory}-private%2fprobe.txt",
+                f"/{directory}/%2e%2e/{directory}-private/probe.txt",
+            ):
+                connection = HTTPConnection("127.0.0.1", server.server_port, timeout=3)
+                try:
+                    connection.request("GET", path)
+                    response = connection.getresponse()
+                    assert response.status == 404, path
+                    assert b"OUTSIDE-ROOT" not in response.read()
+                finally:
+                    connection.close()
+            connection = HTTPConnection("127.0.0.1", server.server_port, timeout=3)
+            try:
+                connection.request("GET", f"/{directory}/app.js")
+                response = connection.getresponse()
+                assert response.status == 200
+                assert response.read() == b"PUBLIC-SCRIPT"
+            finally:
+                connection.close()
+        finally:
+            server.shutdown()
+            worker.join(timeout=3)