Secure shell (SSH) ¶

Secure shell (SSH) is the most commonly used protocol to access a remote shell on a remote host.

🐊️ Port: 22 (TCP)

🥊 SSH can be used to create a secure connection for an insecure protocol. For instance, SFTP is for FTP over SSH.

Basic usage:

$ ssh username@IP
$ ssh username@IP -p port

You may use a private key instead of a password (if configured). Note that the key must have at least the permissions 600.

$ ssh username@IP -i /path/to/id_rsa

Foothold ¶

• The .ssh folder may contain a ssh key (often called id_rsa) to connect to a host. This file may be protected by a password, but using offline hash cracking, we may be able to find it.

• Use -v to detect allowed authentication modes and force one that is convenient for us.

$ ssh [...] -v -o PreferredAuthentications=password

• The password may be weak and vulnerable to brute force.

$ hydra -L user.list -P password.list ssh://IP -V -f

Additional Resources ¶

• Run ssh-audit (3.0k ⭐) and analyze the output

• Use puttygen saved_key.ppk -O private-openssh -o id_rsa to convert a Putty key file to a Linux SSH file.

PEM SSH Key Files — Overview ¶

Privacy-Enhanced Mail (PEM) is a file format mostly to share cryptographic keys such as SSH keys.

We can use SSH keys to log in to a host without sending a password over the network. We will generate a private key and a public key.

The target will have our public key in their .ssh/authorized_keys file. They will use it to send us a authentication challenge.

To generate a private key (PEM) and a public key (OpenSSH):

$ ssh-keygen -f mykey      # mykey (private) | mykey.pub
$ ssh username@ip -i mykey # Permissions: 644 or less

PEM SSH Key Files — RSA File Format ¶

For an RSA private key, the format is:

RSAPrivateKey ::= SEQUENCE {
  version           Version,
  modulus           INTEGER,  -- n
  publicExponent    INTEGER,  -- e
  privateExponent   INTEGER,  -- d
  prime1            INTEGER,  -- p
  prime2            INTEGER,  -- q
  exponent1         INTEGER,  -- d mod (p-1)
  exponent2         INTEGER,  -- d mod (q-1)
  coefficient       INTEGER,  -- (inverse of q) mod p
  otherPrimeInfos   OtherPrimeInfos OPTIONAL
}

You can use CyberChef PEM To Hex to get the hexadecimal of a partial SSH key. Let's read 308204be0201000...SNIP:

Additional commands:

$ openssl asn1parse -in key

Related: Bro-key-n, Backup - frank, Corrupted PEM, 1506, X-secrets.

SSH Forwarding ¶

If SSH Forwarding is enabled (/etc/ssh/sshd_config, $HOME/.ssh/config, etc.), then when we SSH to a host using -A, it will create a socket in /tmp and set the variable SSH_AUTH_SOCK accordingly.

There are multiple ways to exploit this:

• As root, we can connect to anyone else SSH session
• As a user, we can connect to any socket we can read, for instance, we compromised a user having a running SSH session as admin.

$ export SSH_AUTH_SOCK=/path/to/agent.xxx; ssh username@0
<no password == it worked>

$ cat /proc/$$/environ | tr '\0' '\n' | grep SSH_AUTH_SOCK
$ cat /proc/*/environ 2> /dev/null | tr '\0' '\n' | grep SSH_AUTH_SOCK

➡️ Article: SSH Agent Hijacking.

SSH Tunneling And Port Forwarding ¶

Please refer to this.

• -R "xxx"
• ssh -X with X11Forwarding enabled
• sshpass -p 'XXX' ssh xxx@IP
• sudo systemctl restart sshd
• ssh-copy-id username@server: add to remote server our public key
• ssh domain\\username@target
• Look for problems in configurations

• /etc/ssh/sshd_config
  • PermitRootLogin no
  • PubkeyAuthentication yes
  • PasswordAuthentication no
  • Hardening
  • forwarding (subsystem filetransfer sftp match group xxx but we must add again no forward)