diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index d3e2b0a..35d589d 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -75,3 +75,8 @@
 **Vulnerability:** Python's `ipaddress.ip_address()` function accepts both `str` and `bytes`. When passing an extremely large bytes object (e.g., `b"A" * 10**8`), the module can take several seconds to raise a `ValueError` due to inefficient internal parsing logic, leading to a CPU exhaustion Denial of Service (DoS).
 **Learning:** Checking the length of `str` inputs before passing them to `ipaddress.ip_address()` is not sufficient to prevent DoS, as an attacker could pass a massive `bytes` object if the function accepts polymorphic types.
 **Prevention:** Always enforce strict length limits (e.g., <= 100 characters/bytes) on *both* `str` and `bytes` inputs before attempting to parse them using the `ipaddress` module. Use `isinstance(ip, (str, bytes))` and check `len()`.
+
+## 2024-05-11 - SSRF Bypass via ISATAP IPv6 Addresses
+**Vulnerability:** The application was vulnerable to an SSRF bypass when passed ISATAP tunneling addresses (e.g., `2001:db8::5efe:127.0.0.1` or `2001:db8::200:5efe:127.0.0.1`), which encapsulate a blocked internal IPv4 address.
+**Learning:** Python's `ipaddress` module evaluates these addresses as `is_global = True` and does not provide an `isatap` property to easily extract the embedded IPv4 address.
+**Prevention:** To prevent SSRF bypasses via ISATAP (RFC 5214) tunneling addresses, manually identify and unwrap the embedded IPv4 address. Extract the 32-bit ISATAP identifier using `(ip_int >> 32) & 0xFFFFFFFF` and check for `0x00005efe` or `0x02005efe`, then validate the underlying IPv4 address against SSRF rules.
diff --git a/test_testping1.py b/test_testping1.py
index 9f39c61..d6f136c 100644
--- a/test_testping1.py
+++ b/test_testping1.py
@@ -281,6 +281,20 @@ def test_is_reachable_ssrf_bypass_teredo(self, mock_call):
                 self.assertIn("IP address not allowed for scanning", log.output[0])
                 mock_call.assert_not_called()
 
+    @patch('testping1.subprocess.call')
+    def test_is_reachable_ssrf_bypass_isatap(self, mock_call):
+        """Test is_reachable prevents SSRF bypass via ISATAP addresses."""
+        ssrf_ips = [
+            '2001:db8::5efe:127.0.0.1',
+            'fe80::5efe:192.168.1.1',
+            '2001:db8::200:5efe:127.0.0.1'
+        ]
+        for ip in ssrf_ips:
+            with self.assertLogs(level='ERROR') as log:
+                self.assertFalse(is_reachable(ip))
+                self.assertIn("IP address not allowed for scanning", log.output[0])
+                mock_call.assert_not_called()
+
     @patch('testping1.subprocess.call')
     def test_is_reachable_ssrf_bypass_nat64_and_compat(self, mock_call):
         """Test is_reachable prevents SSRF bypass via NAT64 and IPv4-compatible addresses."""
diff --git a/testping1.py b/testping1.py
index aa0ff6e..941e600 100644
--- a/testping1.py
+++ b/testping1.py
@@ -128,14 +128,18 @@ def is_reachable(ip, timeout=1):
                 not t_cli.is_global or t_cli.is_multicast
             )
         else:
-            # 🛡️ Sentinel: Unpack NAT64 (RFC 6052) and IPv4-compatible (RFC 4291) addresses manually
-            # as Python's ipaddress module does not natively unwrap them for SSRF checks.
+            # 🛡️ Sentinel: Unpack NAT64 (RFC 6052), IPv4-compatible (RFC 4291), and ISATAP (RFC 5214)
+            # addresses manually as Python's ipaddress module does not natively unwrap them for SSRF checks.
             ip_int = int(ip_obj)
             unwrapped = None
             if ip_int >> 32 == 0x0064ff9b0000000000000000: # NAT64 64:ff9b::/96
                 unwrapped = ipaddress.IPv4Address(ip_int & 0xFFFFFFFF)
             elif ip_int < 2**32 and ip_int not in (0, 1): # IPv4-compatible ::w.x.y.z
                 unwrapped = ipaddress.IPv4Address(ip_int)
+            else:
+                isatap_identifier = (ip_int >> 32) & 0xFFFFFFFF
+                if isatap_identifier == 0x00005efe or isatap_identifier == 0x02005efe: # ISATAP
+                    unwrapped = ipaddress.IPv4Address(ip_int & 0xFFFFFFFF)
 
             if unwrapped is not None:
                 is_blocked = not unwrapped.is_global or unwrapped.is_multicast