Fixed stack trace bug & verified logic

- Fixed stack trace bug when value of "packet" is nill.
- Verified logic of Oracle TNS Listener poisoning which requires an ACCEPT response to be marked as vulnerable.

modules/auxiliary/scanner/oracle/tnspoison_checker.rb

@@ -42,19 +42,23 @@ class MetasploitModule < Msf::Auxiliary
       send_packet = tns_packet("(CONNECT_DATA=(COMMAND=service_register_NSGR))")
       sock.put(send_packet)
       packet = sock.read(100)
+      if packet != nil
           hex_packet = Rex::Text.to_hex(packet, prefix = ':')
           split_hex = hex_packet.split(":")
           find_packet = /\(ERROR_STACK=\(ERROR=/ === packet
           #find_packet == true ? print_error("#{ip}:#{rport} is not vulnerable ") : print_good("#{ip}:#{rport} is vulnerable")
-      if find_packet == true
+          if find_packet == true #TNS Packet returned ERROR
                 print_error("#{ip}:#{rport} is not vulnerable")
-      elsif split_hex[5] == "02"
+          elsif split_hex[5] == "02" #TNS Packet Type: ACCEPT
     	        print_good("#{ip}:#{rport} is vulnerable")
-      elsif split_hex[5] == "04"
+          elsif split_hex[5] == "04" #TNS Packet Type: REFUSE
                 print_error("#{ip}:#{rport} is not vulnerable")
-      else
+          else #All other TNS packet types or non-TNS packet type response cannot guarantee vulnerability
                 print_error("#{ip}:#{rport} might not be vulnerable")
           end
+      else
+          print_error("#{ip}:#{rport} is not vulnerable")
+      end
       # TODO: Module should report_vuln if this finding is solid.
       rescue ::Rex::ConnectionError, ::Errno::EPIPE
       print_error("#{ip}:#{rport} unable to connect to the server")