diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 157d3ed8..a52fdb9a 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -21,3 +21,7 @@
 **Vulnerability:** `CustomEmployeeManager.Register` accepted arbitrary employee IDs without validation, which were later used directly in `Path.Combine` to construct image loading paths, enabling path traversal (CWE-22).
 **Learning:** Identifiers provided by mods or external sources must be treated as untrusted input and validated before being used in file system operations.
 **Prevention:** Validate input strings that form part of a file path before concatenating them. Reject them if they contain directory traversal characters like `..`, `Path.DirectorySeparatorChar`, `Path.AltDirectorySeparatorChar`, or any invalid filename characters (using `Path.GetInvalidFileNameChars()`).
+## 2024-05-24 - Path Traversal in Mod Entity Registration Defense-in-Depth
+**Vulnerability:** `CustomEmployeeManager.SetPortrait` used `employeeId` to construct image loading paths without validation, relying entirely on upstream validation during registration.
+**Learning:** Relying solely on upstream validation (e.g. during object registration) leaves methods vulnerable to direct calls or alternative execution paths that bypass those checks, breaking the defense-in-depth principle.
+**Prevention:** Every method performing sensitive file system operations (like `Path.Combine` or `File.Exists`) must independently validate dynamic input against path traversal attacks (`IndexOfAny(Path.GetInvalidFileNameChars()) >= 0 || Contains("..")`), even if the input is supposedly validated elsewhere.
diff --git a/src/API/CustomEmployeeManager.cs b/src/API/CustomEmployeeManager.cs
index 9b4512b6..abe87f8d 100644
--- a/src/API/CustomEmployeeManager.cs
+++ b/src/API/CustomEmployeeManager.cs
@@ -891,10 +891,19 @@ private static void SetPortrait(Transform card, string employeeId)
 
             string assetsDir = Path.Combine(MelonEnvironment.UserDataDirectory, "ModAssets");
             string? imagePath = null;
-            foreach (var ext in new[] { ".jpg", ".png" })
+
+            // [Security] Prevent path traversal in case employeeId bypasses registration checks (defense in depth)
+            if (!(string.IsNullOrEmpty(employeeId) || employeeId.IndexOfAny(Path.GetInvalidFileNameChars()) >= 0 || employeeId.Contains("..")))
+            {
+                foreach (var ext in new[] { ".jpg", ".png" })
+                {
+                    string candidate = Path.Combine(assetsDir, employeeId + ext);
+                    if (File.Exists(candidate)) { imagePath = candidate; break; }
+                }
+            }
+            else
             {
-                string candidate = Path.Combine(assetsDir, employeeId + ext);
-                if (File.Exists(candidate)) { imagePath = candidate; break; }
+                CrashLog.Log($"[Security] SetPortrait: Invalid characters in employeeId={employeeId}");
             }
 
             if (imagePath != null)