proof of concept pcap reader for nas heuristic

tools/nasparse.py

@@ -9,8 +9,13 @@ from enum import Enum
 
 import pycrate_mobile.TS24301_EMM
 
+EPS_IMSI_ATTACH = 2
+
 def parse_nas_message(buffer, uplink=None):
-    bin = binascii.unhexlify(buffer)
+    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
@@ -33,7 +38,7 @@ def heur_ue_imsi_sent(msg):
         except pycrate_core.elt.EltErr:
             return (False, None)
 
-    if msg['EPSAttachType']['V'].to_int() == 2:
+    if msg['EPSAttachType']['V'].to_int() == EPS_IMSI_ATTACH: #EPSAttachType Value is 'Combined EPS/IMSI Attach (2)'
         return (True, output)
     return (False, None)
 
@@ -46,7 +51,7 @@ if __name__ == "__main__":
     buffer = sys.argv[1]
     msg = parse_nas_message(buffer)
     pprint.pprint(msg)
-    res = heur_ue_imsi_sent(msg)
-    if(res[0]): 
-        print(res[1])
+    (triggered, message)= heur_ue_imsi_sent(msg)
+    if(triggered): 
+        print(message)
         exit(1)

tools/pcap_check.py

@@ -0,0 +1,39 @@
+#!/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)