PoC of python nas heuristic

tools/nasparse.py

@@ -0,0 +1,57 @@
+#!/usr/bin/python3
+import pycrate_mobile
+from pycrate_mobile import NASLTE
+import pycrate_core
+import binascii
+import sys
+import pprint
+from enum import Enum
+
+import pycrate_mobile.TS24301_EMM
+
+EPS_IMSI_ATTACH = 2
+
+def parse_nas_message(buffer, uplink=None):
+    if isinstance(buffer, str): #handle string argument or raw bytes
+        bin = binascii.unhexlify(buffer)
+    else:
+        bin = buffer
+    if uplink:
+        parsed = NASLTE.parse_NASLTE_MO(bin)
+    elif uplink == None: #We don't know if its an up or downlink
+        parsed = NASLTE.parse_NASLTE_MO(bin)
+        if parsed[0] == None:
+            parsed = NASLTE.parse_NASLTE_MT(bin)
+    else:
+        parsed = NASLTE.parse_NASLTE_MT(bin)
+    return parsed[0]
+
+def heur_ue_imsi_sent(msg):
+    output = "device transmitted IMSI to base station!"
+
+    if type(msg) not in [pycrate_mobile.TS24301_EMM.EMMAttachRequest, pycrate_mobile.TS24301_EMM.EMMSecProtNASMessage]: 
+        return (False, None)
+
+    if isinstance(msg, pycrate_mobile.TS24301_EMM.EMMSecProtNASMessage):
+        try:
+            msg = msg['EMMAttachRequest']
+        except pycrate_core.elt.EltErr:
+            return (False, None)
+
+    if msg['EPSAttachType']['V'].to_int() == EPS_IMSI_ATTACH: #EPSAttachType Value is 'Combined EPS/IMSI Attach (2)'
+        return (True, output)
+    return (False, None)
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 2:
+        print("usage: nasparse.py [hex encoded nas message]")
+        exit(1)
+
+    buffer = sys.argv[1]
+    msg = parse_nas_message(buffer)
+    pprint.pprint(msg)
+    (triggered, message)= heur_ue_imsi_sent(msg)
+    if(triggered): 
+        print(message)
+        exit(1)

tools/nasparse_test.py

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+#!/usr/bin/python3
+import unittest
+import nasparse
+
+
+class TestNasparse(unittest.TestCase):
+    imsi_sent_msg = '07412208391185184409309005f0700000100030023ed031d127298080211001000010810600000000830600000000000d00000300ff0003130184000a000005000010005c0a009011034f18a6f15d0103c1000000000000'
+    sec_imsi_sent_msg = '1727db4b7c0207412208391185184409309005f0700000100030023ed031d127298080211001000010810600000000830600000000000d00000300ff0003130184000a000005000010005c0a009011034f18a6f15d0103c1'
+    non_nas_msg = '1312deadbeefcafe'
+    other_nas_msg = '074413780004023fd121'
+    other_nas_mt_msg = "023fd12100000000000000000000000000000000000000000000000000000000"
+    ciphered_nas_msg = "27ed6146bd0162a5d62d62e1ce501720dc8bd84f1167fd"
+
+    def run_heur(self, msg):
+        buf = nasparse.parse_nas_message(msg)
+        return nasparse.heur_ue_imsi_sent(buf)[0]
+
+    def test_imsi_sent(self):
+        self.assertEqual(self.run_heur(self.imsi_sent_msg), True, "imsi_sent_msg should trigger heuristic")
+
+    def test_sec_imsi_sent(self):
+        self.assertEqual(self.run_heur(self.imsi_sent_msg), True, "sec_imsi_sent_msg should trigger heuristic")
+
+    def test_non_nas_msg(self):
+        self.assertEqual(self.run_heur(self.non_nas_msg), False, "non_nas_msg should not trigger heuristic")
+
+    def test_other_nas(self):
+        self.assertEqual(self.run_heur(self.other_nas_msg), False, "other_nas_msg should not trigger heuristic")
+
+    def test_other_nas_mt(self):
+        self.assertEqual(self.run_heur(self.other_nas_mt_msg), False, "other_nas_mt_msg should not trigger heuristic")
+
+    def test_ciphered_nas(self):
+        self.assertEqual(self.run_heur(self.ciphered_nas_msg), False, "ciphered_nas_msg should not trigger heuristic")
+
+if __name__ == '__main__':
+    unittest.main()

tools/pcap_check.py

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+#!/usr/bin/python3
+import nasparse
+from scapy.utils import RawPcapNgReader
+import sys
+
+TYPE_LTE_NAS = 0x12
+UDP_LEN = 28
+
+def process_pcap(pcap_path):
+    print('Opening {}...'.format(pcap_path))
+
+    count = 0
+    for (pkt_data, pkt_metadata,) in RawPcapNgReader(pcap_path):
+        count += 1
+        gsmtap_len = pkt_data[UDP_LEN+1]  * 4 # gsmtap header length is stored in the 2nd byte of GSMTAP as a number of 32 bit words
+        header_end = gsmtap_len + UDP_LEN #length of UDP/IP header plus GSMTAP header
+        gsmtap_end_idx = (len(pkt_data) - header_end) * -1
+
+        gsmtap_hdr = pkt_data[UDP_LEN:gsmtap_end_idx]
+
+        if gsmtap_hdr[2] != TYPE_LTE_NAS:
+            continue
+
+        # uplink status is the 7th bit of the 5th byte of the GSMTAP header. 
+        # Uplink (Mobile originated) = 0 Downlink (mobile terminated) = 1
+        uplink = (gsmtap_hdr[4] & 0b01000000) >> 6
+        buffer = pkt_data[header_end:]
+        msg = nasparse.parse_nas_message(buffer, uplink)
+        (triggered, message)= nasparse.heur_ue_imsi_sent(msg)
+        if triggered:
+            print(f"Frame {count} triggered heuristic: {message}")
+
+if __name__ == "__main__":
+    if len(sys.argv) != 2:
+        print("usage: pcap_check.py [path/to/pcap/file]")
+        exit(1)
+
+    pcap_path = sys.argv[1]
+    process_pcap(pcap_path)