diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 339c53a..1e6421a 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -18,3 +18,7 @@
 **Vulnerability:** Comparing `ipaddress` objects of different versions (e.g., IPv4 and IPv6) raises a `TypeError`, which if unhandled, causes the application to crash abruptly (Denial of Service risk).
 **Learning:** `ipaddress` module's comparison operators (`<`, `>`, `<=`, `>=`) are strictly typed by IP version. They do not implicitly convert or handle cross-version comparisons securely.
 **Prevention:** Always validate that `ipaddress` objects share the same `.version` before comparing them, and catch `TypeError` alongside `ValueError` when parsing or manipulating generic IP address inputs.
+## 2024-05-24 - Unsafe Relative Path Execution
+**Vulnerability:** The application used `shutil.which("ping") or "ping"`. If `ping` was not found in the system `PATH`, it fell back to executing the relative string `"ping"`. This could allow arbitrary code execution or local privilege escalation if run from a directory containing a malicious executable named `ping`.
+**Learning:** Never fallback to relative command names when a system binary is expected. If a required binary is missing from the system path, the application should fail securely rather than attempting a risky, unverified local execution.
+**Prevention:** Remove fallback logic for critical system commands. Use `shutil.which()` and raise an exception (e.g., `RuntimeError`) if the expected binary is `None`.
diff --git a/testping1.py b/testping1.py
index b64f065..389823f 100644
--- a/testping1.py
+++ b/testping1.py
@@ -9,7 +9,11 @@
 # Calling shutil.which() once at module load avoids the overhead of traversing
 # the system PATH environment variable during every subprocess.call() execution.
 # This yields a measurable speedup when firing thousands of concurrent pings.
-PING_PATH = shutil.which("ping") or "ping"
+PING_PATH = shutil.which("ping")
+if not PING_PATH:
+    # 🛡️ Sentinel: Fail securely if the required system binary is missing, rather than
+    # falling back to a relative path ("ping") which could execute a malicious local file.
+    raise RuntimeError("Required 'ping' binary not found in system PATH.")
 
 def is_reachable(ip, timeout=1):
     """Checks if a device at the given IP address is reachable with a ping.