diff --git a/tests/core/test_sanitize.py b/tests/core/test_sanitize.py
new file mode 100644
index 00000000..0d1ddeba
--- /dev/null
+++ b/tests/core/test_sanitize.py
@@ -0,0 +1,25 @@
+"""Regression tests for peer-input sanitizers (``core/api/sanitize.py``)."""
+
+from __future__ import annotations
+
+from unshackle.core.api.sanitize import safe_cache_key
+
+
+def test_safe_cache_key_rejects_backslash_traversal():
+    # On POSIX, backslash is a plain character, so pathlib.Path treats
+    # "..\\..\\secret" as one filename; written as "<key>.json" it escapes the
+    # cache dir on Windows. PureWindowsPath handling must reject it everywhere.
+    assert safe_cache_key("..\\..\\secret") is None
+    assert safe_cache_key("sub\\dir") is None
+
+
+def test_safe_cache_key_rejects_forward_slash_and_dotdot():
+    assert safe_cache_key("../secret") is None
+    assert safe_cache_key("a/b") is None
+    assert safe_cache_key("..") is None
+    assert safe_cache_key(".") is None
+    assert safe_cache_key("") is None
+
+
+def test_safe_cache_key_accepts_plain_filename():
+    assert safe_cache_key("valid-key_123") == "valid-key_123"
diff --git a/unshackle/core/api/sanitize.py b/unshackle/core/api/sanitize.py
index f62bcf33..fd516db0 100644
--- a/unshackle/core/api/sanitize.py
+++ b/unshackle/core/api/sanitize.py
@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from pathlib import Path
+from pathlib import PureWindowsPath
 from typing import Optional
 
 
@@ -17,9 +17,12 @@ def safe_cache_key(key: object) -> Optional[str]:
     Cache keys are written as ``<key>.json`` inside a fixed directory. A key containing a
     path separator, ``..``, or an absolute path would let the peer write outside that
     directory, so anything that is not already a plain filename is rejected.
+
+    PureWindowsPath treats both ``/`` and ``\\`` as separators, so a key like
+    ``..\\..\\secret`` is rejected even on POSIX (where backslash is a plain character).
     """
     text = str(key)
-    name = Path(text).name
+    name = PureWindowsPath(text).name
     if not name or name != text or name in {".", ".."}:
         return None
     return name