MMA CTF 2015 Splitted writeup Forensics

Download the challenge file from here

The challenge involved, a little bit of thinking to solve in a simple way, if you are unsure about the zip file structure. A zip file was split into 8 different parts and it was sent to the host 192.168.3.10. The task is, reconstruct the fragments of the zip file, arrange it in order adhering to the zip file structure. Once you do it you will get a valid Adobe Photoshop file ( .psd). Then extract the image to view the flag. This works only if you reconstruct the zip file in the order mentioned in the zip file structure. Since it is just a 30 points challenge, I didn’t spend much time in thinking about solving in a proper way. Here is the idea (#lame 😀 :P) I came up with,

1. Reconstruct the fragments from the Wireshark. If you are doing it by hand make sure to strip the HTTP headers in the beginning. The best way is to Export the HTTP objects. Find my reconstructed fragments from here
2. Identify the header and footer of a zip file. Zip file header always starts with the magic number 50 4B 03 04 14 00… (HEX), PK…….(ASCII). Now the footer, it looks something like this,
3. You have identified the header and footer. So the number of remaining fragments from the pcap file is 6 (excluding the header and footer) out of 8. So there are 6! (spelled as six factorial) ways to arrange these fragments to get a valid zip file. We just wrote a script using Python to do complete this task (I owe my sincere thanks 😛 to b3h3m0th for helping me with this script).

#!/usr/bin/env python

import itertools
import zipfile
import os

body_n = [1, 3, 4, 6, 7, 8]
header = "2_header.raw"
footer = "5_footer.raw"

def generate_file(sequence, i):
    final_file = "final_" + str(i) + ".zip"
    open(final_file, "w").close()
    final = open(final_file, "a")

    header_data = open(header, "r").read()
    footer_data = open(footer, "r").read()

    final.write(header_data)
    for item in sequence:
        d = open(item, "r").read()
        final.write(d)
    final.write(footer_data)
    final.close()

    try:
        content = zipfile.ZipFile(final_file)
        content.extractall()
        print "\n\n\n\n [*] permutation", i, "SUCCESS !!! "
        print "\n [i] Successful Sequence:\n\n"
        print header
        for item in sequence:
            print item
        print footer

    except:
        os.remove(final_file)

def main():
    perms = []
    body_list = [str(i) + ".raw" for i in body_n]
    permsobj = itertools.permutations(body_list)
    while True:
        try:
            perms += [permsobj.next()]
        except StopIteration:
            break

    i=1
    for item in perms:
        generate_file(item, i)
        i = i+1

if __name__ == "__main__":
    main()

When you run this script you will get the sequence (2_header.raw, 6.raw, 7.raw, 3.raw.. etc) as well as the valid zip file.

Unzip the valid zip file which is reconstructed using this script. The file name would be final_443.zip. When you unzip you will get a psd file. We used a psd file parser to get the layered images. After running the parser on the psd file we got the flag.

One of the image had our flag,

[EDIT] Another way to reconstruct the ZIP file

Here is a elegant way to solve the challenge. I came to know about this method from the comments.

Filter the http packets. When you click on a GET request packet you can see the “Range” field( in the HTTP section). Now carefully look the range of values(Range field) in all the packets, you will notice a sequence, With that we can reconstruct the zip file.

Packet No: 14  Range : 2345-2813

Packet No: 24  Range : 0 – 468

Packet No: 34  Range : 1407 – 1875

Packet No: 44  Range : 2814 – 3282

Packet No: 54  Range : 3283 – 3744

Packet No: 64  Range : 469 – 937

Packet No: 74  Range : 938 – 1406

Packet No: 84  Range : 1876 – 2344

Now order these packets in increasing order and then write it to a file. So the order should be: Packet number : 24, 64, 74, 34, 84, 14, 44, 54. Or export the data streams in these order, rearrange it to get the valid zip file.

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