diff --git a/README.md b/README.md
index fd08045..5471cc9 100644
--- a/README.md
+++ b/README.md
@@ -90,7 +90,9 @@ replaced; a
 reported, never run, because an agent may have written it. The pinned program
 must be an absolute path or a root-owned command on a `PATH` entry that root
 owns and no group or other user can write, up to `/`, so a binary planted in a
-user-writable directory such as a virtualenv never runs. A bare name runs as
+user-writable directory such as a virtualenv never runs. The Nix store never
+counts: it is group-writable, and any user can have the daemon build
+root-owned content there, so pin an absolute path on Nix. A bare name runs as
 the resolved file that was checked. Inside a user namespace other than the
 initial one, root means the overflow uid, under which the real root's files
 show there, as do those of every other unmapped owner. Outside such a
diff --git a/tests/unit/pkgmgr/actions/worktree/test_trust.py b/tests/unit/pkgmgr/actions/worktree/test_trust.py
index 6799a12..19abc04 100644
--- a/tests/unit/pkgmgr/actions/worktree/test_trust.py
+++ b/tests/unit/pkgmgr/actions/worktree/test_trust.py
@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import os
-import shutil
 import tempfile
 import unittest
 from types import SimpleNamespace
@@ -113,6 +112,13 @@ class TestRootResolved(unittest.TestCase):
         stats["/usr/bin"] = (0, 0o040757)
         self.assertIsNone(self._resolve(stats))
 
+    def test_sticky_group_writable_ancestor_like_the_nix_store_is_refused(
+        self,
+    ) -> None:
+        stats = self._tree()
+        stats["/usr"] = (0, 0o041775)
+        self.assertIsNone(self._resolve(stats))
+
     def test_stat_error_is_refused(self) -> None:
         stats = self._tree()
         del stats["/usr"]
@@ -131,24 +137,32 @@ class TestRootResolved(unittest.TestCase):
 
 
 class TestTrustedPath(unittest.TestCase):
+    def setUp(self) -> None:
+        trusted = [
+            entry
+            for entry in map(trust.root_resolved, os.defpath.split(os.pathsep))
+            if entry and os.path.isdir(entry)
+        ]
+        if not trusted:
+            self.skipTest("no os.defpath directory is root-controlled on this host")
+        self.system = trusted[0]
+
     def test_symlinked_entry_is_kept_only_resolved(self) -> None:
-        system = os.path.dirname(os.path.realpath(shutil.which("true")))
         with tempfile.TemporaryDirectory() as tmp:
             link = os.path.join(tmp, "bin")
-            os.symlink(system, link)
+            os.symlink(self.system, link)
             with patch.dict(os.environ, {"PATH": link}):
-                self.assertEqual(trust.trusted_path(), system)
+                self.assertEqual(trust.trusted_path(), self.system)
 
     def test_entry_keeps_the_resolution_it_was_checked_under(self) -> None:
-        system = os.path.dirname(os.path.realpath(shutil.which("true")))
         with tempfile.TemporaryDirectory() as tmp:
             link = os.path.join(tmp, "bin")
-            os.symlink(system, link)
+            os.symlink(self.system, link)
             with (
                 patch.dict(os.environ, {"PATH": link}),
-                patch.object(trust.os.path, "realpath", side_effect=[system, tmp]),
+                patch.object(trust.os.path, "realpath", side_effect=[self.system, tmp]),
             ):
-                self.assertEqual(trust.trusted_path(), system)
+                self.assertEqual(trust.trusted_path(), self.system)
 
     def test_no_root_controlled_entry_is_refused(self) -> None:
         with (