tshark – Cyber Poking

OtterCTF dates from December 2018 and includes reverse engineering, steganography, network traffic, and more traditional forensics challenges. This write-up covers the network forensics portion. I have previously written-up the memory forensics section, and the Birdman’s Data network challenge. The whole CTF is available to play as of the publication of this post.

I managed to complete three of the four challenges in the network traffic section of the CTF. This post is a write-up of the Look At Me challenge.

We start by downloading the challenge PCAP and calculating hashes for reference.

MD5: d7dbb2394596594b438ab3110ace3408
SHA1: fc16f0636a7f141b8b17e0e7a38b776dd5c21c82

Opening up the PCAP in Wireshark, the first thing is that we are not working with TCP/IP traffic; instead this appears to be USB data from a Wacom CTL-471 drawing tablet. After some Google searching I found a write-up of a 2017 BITSCTF challenge featuring USB traffic from a Wacom CTL-460 tablet, which was a huge help in solving this challenge.

With the help of The Blog Post I realised that the packets containing the Leftover Capture Data we need could be filtered within Wireshark based on the Frame Length…

((usb.transfer_type == 0x01) && (frame.len == 37))

Then extract to a separate PCAP with tshark for further processing.

tshark -r look_at_me.pcap -w filtered.pcap -Y '((usb.transfer_type == 0x01) && (frame.len == 37))'

The data we need is a representation of the X/Y coordinates of the pen on the drawing tablet, and the Z value representing the pressure of the pen. Using the packet in the screenshot above as an example:

02:f0:82:0a:ba:05:00:08:00

02:f0 - Header
82:0a - X
ba:05 - Y
00:08 - Z (Pressure)
00    - Padding?

This data is contained within the usb.capdata field; we can use tshark again to extract this to a text file and discard the rest of the PCAP.

tshark -r filtered.pcap -T fields -e usb.capdata -Y usb.capdata > usb.capdata.txt

The next thing to do was to use awk (shamelessly lifted from The Blog Post) to convert these raw hex values to little-endian X/Y and Z coordinates.

awk -F: '{x=$3$4;y=$5$6}{z=$7}$1=="02"{print x,y,z}' usb.capdata.txt > usb.capdata.txt.xyz

After installing the pwntools Python library and running a small Python script “inspired” by The Blog Post, we have the coordinates in a format suitable to be plotted as an image using Gnuplot.

#!/usr/bin/python
from pwn import *

for line in open('usb.capdata.txt.xyz').readlines():
  coord = line.strip().split(' ')
  x = int(coord[0],16)
  y = int(coord[1],16)
  z = int(coord[2],16)

  if z > 0:
    print u16(struct.pack(">H",x)),u16(struct.pack(">H",y))

We feed our newly converted coordinates into Gnuplot…

And out pops an image!

After exporting to a PNG file, we can flip it for easier reading.

convert -flip flag.png flag-flipped.png

There we go! Submit the flag and claim the points, thanks in large part to The Blog Post.

CTF{0TR_U58}

Next up in my OtterCTF series, Otter Leak.

The first CTF created by Security Blue Team was initially for subscribers only, but was made available to the public for a short time at the end of February 2020. While it covered network traffic analysis, password cracking, steganography, forensics, and some general knowledge challenges I didn’t have as much time as I would have liked to spend, so concentrated on the aspects that were most interesting to me personally.

This write-up covers the network analysis challenge – Sneaky Transmission. You can find the rest of my write-ups for Security Blue Team VIP CTF #1 here.

After downloading the PCAP file we can open it in Wireshark to see what we are working with. While the question refers to a DoS attack, and to the possibility of a photo, all we see in the PCAP is ICMP traffic.

Nothing here is obviously an image, but the TTL values of the IMCP requests look a bit strange. Using the following Display Filter we can examine them more easily.

icmp.type == 8

The TTL value changes with each packet, which might be an indication of a covert channel; one byte per packet perhaps? We can easily extract the TTL values using tshark and redirect them to a file.

tshark -r sneaky_transmission.pcapng -Y "icmp.type == 8" -Tfields -e ip.ttl

The data will be much easier to work with if we output it to a file.

tshark -r sneaky_transmission.pcapng -Y "icmp.type == 8" -Tfields -e ip.ttl > ttl.txt

We now have a file containing what we think might be individual bytes, one-per-line, which we need to turn into something more intelligible. One of my favourite tools for playing with data like this is CyberChef, so let’s load our ttl.txt file as input and see what we can make from it.