John Pierce

. SLAE, Security+

PANW CTF Labyrenth Unix Track Challenge 1

I looked at the sourcecode for and found a bunch of input requests and string match checks.  The answers were generally yes or no.  Response is "You're doing a thing" until the final run.  After each correct answer, a portion of a gif was decoded from base64 and stored to a variable.  Here's a code snippet showing a test for a "no" response:

print "You're doing a thing...\n";
my $input = <STDIN>;
$input = trim($input);
if ($input eq ("\x6E" . chr(4423 - 4312))) {

 After several rounds, there's another base64 string that contains more script code.  These have to be decoded in order to retrieve the entire list of tests that will get one to the flag.

So, what I want to do is capture all the cleartext if statements down to the base64 string with more script (marked by "eval"), decode it, capture the if ($input ) line, isolate the base64 string after eval MIME::Base64::decode(" down to "); and do it again.  I'm going to write a program to decode and reassemble the important lines of the entire program (if or eval), modify it so the answers are supplied at runtime, and see what happens.  Luckily all the lines I needed to work on had a common format, so simple tests and stripping of characters sufficed.  I am most comfortable in Python, so I translated the perl strings in the if statements into Python form and ran the eval on the new string.  To make the iterate function generic, I read the entire file into a variable as a single string, then broke it up based on occurrences of newline.  This way, the same logic would apply whether it was the initial file read or a decode of the base64 string.  It's written in Python 2.7.  If using Python 3, I would have to import base64 and call the decode function differently

Here's the code:


# recursively work through the many rounds of base64 encoding
def iterate(a):
    iNl = a.find('\n')
    iStart = 0
    while iNl != -1:
        line = a[iStart:iNl]
        iStart = iNl+1
        iNl = a.find('\n',iStart)
        if line.find('if (') != -1:
            line = line.strip()+'\n'
            line = line[line.find('eq (')+3:len(line)-4]
            line = line.replace('.','+')
            print  eval(line)
            if line.find('eval') != -1:
                b64start = line.find('(') + 1
                b64end = line.find(')')

fIn = open("")
a =

Ran the program and got response:
./ |perl
You're doing a thing...
You're doing a thing...
You're doing a thing...
You're doing a thing...
... (a bunch more of these were removed)

You're doing a thing...
You're doing a thing...
You're doing a thing...
You did a thing!


ls -l *.gif
-rw-r--r-- 1 john john 110403 Aug 19 11:02 entrevue.gif

A gif (entrevue.gif) was created giving the password: PAN{L3ts_533_h0W_U_deal_w_th1s_little_511CE}

Most Recent Articles

First bit::

This is a writeup of the format string vulnerability in level 4 of the 64bitprimer VM from vulnhu

First bit::

Installation of the software to make a yubikey 4 work in FIDO U2F mode on Debian Jessie i386

First bit::

Lesson(s) learned

First bit::

This one stumped me. Overall, it was a great competition for me as I got to learn a whole lot of new things. I had never worked on a Mac, other than as a user, had never used Hopper, lldb or any of the other tools for reversing on a Mac, and haven't got any experience in the Objective C/Swift framework.

First bit::

4 rounds, lots of debugging


Categories: Network security, Videos
First bit::

Explains the workings of a DMZ, walks through setting up and testing of a DMZ in a virtual machine lab environment

Categories: Network security, Videos
First bit::

In this video I go through the process of setting up an SSH tunnel to hide an IP and also setting

Categories: Exploits, Videos
First bit::

Useful for someone who is interested in what a buffer overflow is. Does not go into the details of development, just explains generally and demonstrates the use of one.

Categories: Exploits, Videos
First bit::

a demonstration of a vulnerability discovered and published by Muts in 2004, exploited on a Windows XP SP3 machine using Python, Immunity Debugger, and Metasploit.

Categories: Network security, Videos
First bit::

In this video I demo some simple iptables rules and show them how to perform network traffic analysis to test them out.