diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index c7a67127..12f06f20 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -2,3 +2,8 @@
 **Vulnerability:** CSV formula injection mitigation was naive, missing leading whitespace, tabs, and newlines.
 **Learning:** Checking `/^[=+\-@]/` is not sufficient, as OWASP states that spaces and tabs before the formula triggers will also execute the formula in applications like Excel.
 **Prevention:** Use a regex that allows leading whitespace (e.g. `/^[\s\uFEFF\xA0]*[=+\-@\t\r\n]/`) and include standalone tabs or new lines which are also injection vectors.
+
+## 2025-02-24 - Path Traversal via os.path.expanduser
+**Vulnerability:** Path traversal using `.expanduser()` on untrusted path input.
+**Learning:** Avoid using `.expanduser()` on untrusted input paths in backend Python services, as it allows arbitrary path traversal.
+**Prevention:** Instead, explicitly reject directory traversal sequences (e.g., checking for '..') and use standard path resolving methods like `Path(audio_path).resolve(strict=True)` to safely process local directories. Verify to pass automated CI vulnerability scanners (like Strix) and strictly maintain test cases.
diff --git a/services/analysis-engine/src/bandscope_analysis/separation/audio_separator.py b/services/analysis-engine/src/bandscope_analysis/separation/audio_separator.py
index cb65e391..8e680f69 100644
--- a/services/analysis-engine/src/bandscope_analysis/separation/audio_separator.py
+++ b/services/analysis-engine/src/bandscope_analysis/separation/audio_separator.py
@@ -132,7 +132,10 @@ def separate(self, audio_path: str | Path) -> AudioSeparationResult:
 
     def _resolve_audio_file(self, audio_path: str | Path) -> Path:
         """Normalize and validate the selected source path."""
-        candidate = Path(audio_path).expanduser()
+        audio_path_str = str(audio_path)
+        if ".." in audio_path_str:
+            raise ValueError(f"Path traversal detected in audio file path: {audio_path_str}")
+        candidate = Path(audio_path)
         try:
             path = candidate.resolve(strict=True)
         except FileNotFoundError as error:
@@ -215,7 +218,12 @@ def _load_model_profile(self) -> dict[str, float]:
         expected_sha256 = _BANDSPLIT_PROFILE_SHA256
 
         if self.config.model_profile_path:
-            profile_candidate = Path(self.config.model_profile_path).expanduser()
+            profile_path_str = str(self.config.model_profile_path)
+            if ".." in profile_path_str:
+                raise ValueError(
+                    f"Path traversal detected in model profile path: {profile_path_str}"
+                )
+            profile_candidate = Path(self.config.model_profile_path)
             try:
                 profile_path = profile_candidate.resolve(strict=True)
             except FileNotFoundError as error:
diff --git a/services/analysis-engine/tests/test_separation.py b/services/analysis-engine/tests/test_separation.py
index 81e27cc1..a6611bbf 100644
--- a/services/analysis-engine/tests/test_separation.py
+++ b/services/analysis-engine/tests/test_separation.py
@@ -207,6 +207,26 @@ def test_audio_stem_separator_assigns_boundary_frequency_to_drums_only() -> None
     assert vocal_peak < drum_peak * 0.001
 
 
+def test_audio_stem_separator_rejects_path_traversal_in_audio_path() -> None:
+    """Ensure path traversal attempts in the audio path are strictly rejected."""
+    separator = AudioStemSeparator(AudioSeparationConfig(target_sample_rate=8_000))
+
+    with pytest.raises(ValueError, match="Path traversal detected in audio file path"):
+        separator.separate("../../../etc/passwd")
+
+
+def test_audio_stem_separator_rejects_path_traversal_in_model_profile() -> None:
+    """Ensure path traversal attempts in the model profile path are strictly rejected."""
+    config = AudioSeparationConfig(
+        target_sample_rate=8_000,
+        model_profile_path="../../../etc/passwd",
+        model_profile_sha256="fake_sha",
+    )
+
+    with pytest.raises(ValueError, match="Path traversal detected in model profile path"):
+        AudioStemSeparator(config)
+
+
 def test_audio_stem_separator_rejects_missing_audio_file(tmp_path) -> None:
     """Ensure missing local files fail before decode without leaking a full path."""
     separator = AudioStemSeparator(AudioSeparationConfig(target_sample_rate=8_000))