Fix SMB2 compoud response signing (#1834)

* Fix SMB2 compoud response signing

Fix the signing the logic when responding with an SMB2 compount
response. The signature will include the padding of each compound
element and include the next offset value before signing the data.

* Pad all SMB2 packets, even the last in a compound response

Author: jborean93

impacket/smbserver.py

@@ -4403,10 +4403,11 @@ def signSMBv1(self, connData, packet, signingSessionKey, signingChallengeRespons
         packet['SecurityFeatures'] = m.digest()[:8]
         connData['SignSequenceNumber'] += 2
 
-    def signSMBv2(self, packet, signingSessionKey):
+    def signSMBv2(self, packet, signingSessionKey, padLength=0):
         packet['Signature'] = b'\x00' * 16
         packet['Flags'] |= smb2.SMB2_FLAGS_SIGNED
-        signature = hmac.new(signingSessionKey, packet.getData(), hashlib.sha256).digest()
+        packetData = packet.getData() + b'\x00' * padLength
+        signature = hmac.new(signingSessionKey, packetData, hashlib.sha256).digest()
         packet['Signature'] = signature[:16]
         # print "%s" % packet['Signature'].encode('hex')
 
@@ -4624,34 +4625,29 @@ def processRequest(self, connId, data):
                         else:
                             respPacket['Data'] = str(respCommand)
 
-                        if connData['SignatureEnabled']:
-                            self.signSMBv2(respPacket, connData['SigningSessionKey'])
-
                         packetsToSend.append(respPacket)
             else:
                 # The SMBCommand took care of building the packet
                 packetsToSend = respPackets
 
         if isSMB2 is True:
-            # Let's build a compound answer
-            finalData = b''
-            i = 0
-            for i in range(len(packetsToSend) - 1):
-                packet = packetsToSend[i]
-                # Align to 8-bytes
-                padLen = (8 - (len(packet) % 8)) % 8
-                packet['NextCommand'] = len(packet) + padLen
+            # Let's build a compound answer and sign it
+            finalData = []
+            totalPackets = len(packetsToSend)
+            for idx, packet in enumerate(packetsToSend):
+                padLen = -len(packet) % 8
+                if idx + 1 < totalPackets:
+                    packet['NextCommand'] = len(packet) + padLen
+
+                if connData['SignatureEnabled']:
+                    self.signSMBv2(packet, connData['SigningSessionKey'], padLength=padLen)
+
                 if hasattr(packet, 'getData'):
-                    finalData += packet.getData() + padLen * b'\x00'
+                    finalData.append(packet.getData() + padLen * b'\x00')
                 else:
-                    finalData += packet + padLen * b'\x00'
+                    finalData.append(packet + padLen * b'\x00')
 
-            # Last one
-            if hasattr(packetsToSend[len(packetsToSend) - 1], 'getData'):
-                finalData += packetsToSend[len(packetsToSend) - 1].getData()
-            else:
-                finalData += packetsToSend[len(packetsToSend) - 1]
-            packetsToSend = [finalData]
+            packetsToSend = [b"".join(finalData)]
 
         # We clear the compound requests
         connData['LastRequest'] = {}