An interesting forensic challenge covering a famous method of data exfiltration…

Table of Contents

Challenge description

We are given a packet capture showing lots of Echo Requests / Replies between the same two computers and we’re supposed to investigate.

Wireshark screen capture

Pretty heavy for a ping…

The ICMP protocol is pretty simple and does not contain a lot of information. However, it includes a data field used notably in error messages. This data field can also be used for creating an ICMP tunnel between two remote computers where hosts inject data into icmp echo packets. This method is often used to bypass firewall rules in the aim of exfiltrating data. Such process can be detected by analyzing ICMP echo packets’ size. These packets usually don’t exceed 100 bytes. Here, each of them is 542 bytes long.

ICMP packet size

We definitively should have a look at what takes so much space.

Gimme your data!

I wrote a simple python script extracting the data section of ICMP echo request packets. For this, I used the Scapy module, which is for me the best tool when it comes to handling packet captures, due to its effectiveness and simplicity of use. My script also converts extracted data to ASCII because it occured to be base64 encoded, which is not surprising since it avoids encoding errors during the transmission.

from scapy.all import *
import base64

capture = rdpcap('analysis.pcap')
ping_data = ""

for packet in capture:
   if packet[ICMP].type == 8: # Echo request
       ping_data += packet.load

print base64.b64decode(ping_data)

NOTE: I only kept request packets since replies send back same data and can cause errors when recovering exfiltrated data.

The hidden text reveals to be a part of the very famous “Hacker Manifesto” followed by the man page of the “ping” tool. Right between the two texts lies our reward… :triangular_flag_on_post:

I am a hacker, and this is my manifesto.  You may stop this individual,
but you can't stop us all... after all, we're all alike.

                              +++The Mentor+++

Congratulations, ICMP exfiltatration is awesome! The flag is : ndh2k18_017395f4c6312759

Now let's read the manual of one of the best tools you never had!

PING(8)                                  System Manager's Manual: iputils                                  PING(8)

      ping - send ICMP ECHO_REQUEST to network hosts

BoiteAKlou :hammer: