diff --git a/tests/tools/test_memory_tool.py b/tests/tools/test_memory_tool.py
index 6ea61b9fe6..55784713fd 100644
--- a/tests/tools/test_memory_tool.py
+++ b/tests/tools/test_memory_tool.py
@@ -1,6 +1,8 @@
 """Tests for tools/memory_tool.py — MemoryStore, security scanning, and tool dispatcher."""
 
 import json
+import os
+import stat
 import pytest
 from pathlib import Path
 
@@ -108,6 +110,45 @@ def store(tmp_path, monkeypatch):
     return s
 
 
+class TestMemoryFileLockPermissions:
+    def test_new_lock_file_is_owner_only_under_permissive_umask(self, tmp_path):
+        memory_path = tmp_path / "MEMORY.md"
+        previous_umask = os.umask(0o002)
+        try:
+            with MemoryStore._file_lock(memory_path):
+                pass
+        finally:
+            os.umask(previous_umask)
+
+        lock_path = tmp_path / "MEMORY.md.lock"
+        assert stat.S_IMODE(lock_path.stat().st_mode) == 0o600
+
+    def test_existing_loose_lock_file_is_tightened(self, tmp_path):
+        memory_path = tmp_path / "MEMORY.md"
+        lock_path = tmp_path / "MEMORY.md.lock"
+        lock_path.write_text("", encoding="utf-8")
+        lock_path.chmod(0o664)
+
+        with MemoryStore._file_lock(memory_path):
+            pass
+
+        assert stat.S_IMODE(lock_path.stat().st_mode) == 0o600
+
+    @pytest.mark.skipif(not hasattr(os, "O_NOFOLLOW"), reason="O_NOFOLLOW unavailable")
+    def test_lock_file_symlink_is_refused(self, tmp_path):
+        memory_path = tmp_path / "MEMORY.md"
+        outside = tmp_path / "outside"
+        outside.write_text("do not touch", encoding="utf-8")
+        lock_path = tmp_path / "MEMORY.md.lock"
+        lock_path.symlink_to(outside)
+
+        with pytest.raises(OSError):
+            with MemoryStore._file_lock(memory_path):
+                pass
+
+        assert outside.read_text(encoding="utf-8") == "do not touch"
+
+
 class TestMemoryStoreAdd:
     def test_add_entry(self, store):
         result = store.add("memory", "Python 3.12 project")
diff --git a/tools/memory_tool_store.py b/tools/memory_tool_store.py
index ccc3d691e8..7098f91899 100644
--- a/tools/memory_tool_store.py
+++ b/tools/memory_tool_store.py
@@ -4,6 +4,7 @@ Module state that tests monkeypatch (``get_memory_dir``, ``fcntl``/``msvcrt``) s
 in ``tools.memory_tool`` and is read lazily."""
 
 import logging
+import os
 import time
 from contextlib import contextmanager, suppress
 from pathlib import Path
@@ -151,7 +152,22 @@ class MemoryStore:
         if fcntl is None and msvcrt is None:
             yield
             return
-        with open(lock_path, "a+", encoding="utf-8") as fd:
+        flags = os.O_RDWR | os.O_CREAT
+        if hasattr(os, "O_NOFOLLOW"):
+            flags |= os.O_NOFOLLOW
+        raw_fd = os.open(lock_path, flags, 0o600)
+        try:
+            # The creation mode is filtered through the process umask and does
+            # not repair a lock left loose by an older Hermes process. Tighten
+            # the opened inode before acquiring the lock so both cases are
+            # owner-only. Operating on the fd avoids a path-swap window.
+            if hasattr(os, "fchmod"):
+                os.fchmod(raw_fd, 0o600)
+            fd = os.fdopen(raw_fd, "r+", encoding="utf-8")
+        except Exception:
+            os.close(raw_fd)
+            raise
+        with fd:
             def _flock(unlock: bool):
                 if fcntl:
                     fcntl.flock(fd, fcntl.LOCK_UN if unlock else fcntl.LOCK_EX)