diff --git a/net_alerter/net_alerter.py b/net_alerter/net_alerter.py
index a6d54c6..f879d20 100644
--- a/net_alerter/net_alerter.py
+++ b/net_alerter/net_alerter.py
@@ -13,6 +13,7 @@ Three concurrent threads:
 import json
 import logging
 import os
+import re
 import socket
 import struct
 import sys
@@ -365,9 +366,15 @@ def setup_logging():
     return logger
 
 
+# MAC address validation regex: exactly 12 hex digits (after normalization)
+_MAC_RE = re.compile(r'^[0-9A-F]{12}$')
+
 def _normalize_mac(mac: str) -> str:
-    """Normalize MAC address: remove colons/hyphens, uppercase."""
-    return mac.replace(":", "").replace("-", "").upper()
+    """Normalize MAC address: remove colons/hyphens, uppercase. Returns empty string if invalid."""
+    normalized = mac.replace(":", "").replace("-", "").upper()
+    if not _MAC_RE.match(normalized):
+        return ""
+    return normalized
 
 
 def load_personal_macs_from_env() -> None:
@@ -380,6 +387,9 @@ def load_personal_macs_from_env() -> None:
     with personal_lock:
         for mac in macs:
             normalized = _normalize_mac(mac)
+            if not normalized:  # Skip invalid MACs
+                logging.warning(f"Skipping invalid MAC from NET_ALERTER_PERSONAL_MACS: {mac}")
+                continue
             personal_devices.add(normalized)
             logging.info(f"Added personal device MAC: {normalized}")
 
@@ -425,6 +435,9 @@ def is_personal_device(mac: str) -> bool:
     """
     # Normalize MAC
     mac_normalized = _normalize_mac(mac)
+    if not mac_normalized:  # Skip invalid MACs
+        return False
+    
     oui = mac_normalized[:6]
 
     # Check personal_devices set first (manually configured)
@@ -629,46 +642,52 @@ def on_arrival(mac: str, ip: str, hostname: str = "") -> None:
     now = time.time()
 
     # Lock order: known_lock, then departure_lock, then occupancy_lock
+    # Extract timers to cancel OUTSIDE all locks to avoid deadlock
+    timers_to_cancel = []
+    
+    with departure_lock:
+        if mac in departure_timers:
+            timers_to_cancel.append(departure_timers.pop(mac))
+    
+    # Cancel all timers BEFORE acquiring any other locks
+    for timer in timers_to_cancel:
+        timer.cancel()
+    
     with known_lock:
         with departure_lock:
             # Check if device marked departing (rapid flap recovery)
             if mac in known_devices and known_devices[mac].get('departing'):
                 # Device returned during departure window — cancel timer, clear flag, no alert
-                if mac in departure_timers:
-                    departure_timers[mac].cancel()
-                    del departure_timers[mac]
                 known_devices[mac]['departing'] = False
                 with occupancy_lock:
                     _update_occupancy_state_unlocked()
                 last_departed_time.pop(mac, None)
                 logging.debug(f"Device {mac} re-arrived during departure window — suppressed alert")
                 return
-
-            # ALWAYS cancel any pending departure timer when device re-arrives
-            if mac in departure_timers:
-                departure_timers[mac].cancel()
-                del departure_timers[mac]
+            
+            # Log if we cancelled a timer
+            if timers_to_cancel:
                 logging.debug(f"Cancelled departure timer for {mac} — device re-arrived before debounce expired")
 
-            if mac in last_departed_time:
-                time_since_departure = now - last_departed_time[mac]
-                if time_since_departure < 300:  # 5 min
-                    logging.debug(f"Suppressing re-arrival alert for {mac} (departed {int(time_since_departure)}s ago)")
-                    # Re-add to known_devices to track presence
-                    if mac not in known_devices:
-                        vendor = lookup_oui(mac)
-                        known_devices[mac] = {
-                            'ip': ip,
-                            'hostname': hostname,
-                            'vendor': vendor,
-                            'first_seen': now,
-                            'last_seen': now
-                        }
-                    else:
-                        known_devices[mac]['last_seen'] = now
-                    with occupancy_lock:
-                        _update_occupancy_state_unlocked()
-                    return
+        if mac in last_departed_time:
+            time_since_departure = now - last_departed_time[mac]
+            if time_since_departure < 300:  # 5 min
+                logging.debug(f"Suppressing re-arrival alert for {mac} (departed {int(time_since_departure)}s ago)")
+                # Re-add to known_devices to track presence
+                if mac not in known_devices:
+                    vendor = lookup_oui(mac)
+                    known_devices[mac] = {
+                        'ip': ip,
+                        'hostname': hostname,
+                        'vendor': vendor,
+                        'first_seen': now,
+                        'last_seen': now
+                    }
+                else:
+                    known_devices[mac]['last_seen'] = now
+                with occupancy_lock:
+                    _update_occupancy_state_unlocked()
+                return
 
         # Still inside known_lock, but not inside departure_lock now
         if mac in known_devices:
@@ -732,11 +751,18 @@ def on_departure(mac: str) -> None:
         known_devices[mac]['departing'] = True
 
     # Start 15-minute debounce timer
+    # Extract timer to cancel outside of lock to avoid deadlock
+    timer_to_cancel = None
     with departure_lock:
         # Cancel any existing timer for this MAC
         if mac in departure_timers:
-            departure_timers[mac].cancel()
-
+            timer_to_cancel = departure_timers.pop(mac)
+    
+    # Cancel outside lock
+    if timer_to_cancel:
+        timer_to_cancel.cancel()
+    
+    with departure_lock:
         def send_departure_alert():
             """Callback to send alert after debounce expires."""
             # Lock order: known_lock, then departure_lock, then occupancy_lock