• Implement strong password policies:

  Enforce complex passwords with a minimum length of 12 characters, including uppercase and lowercase letters, numbers, and special characters.

  # Example PAM configuration for password complexity
  password requisite pam_cracklib.so retry=3 minlen=12 dcredit=-1 ucredit=-1 ocredit=-1 lcredit=-1

• Use SSH key-based authentication for SFTP:

  Configure SFTP to use SSH keys instead of passwords for stronger authentication.

  # In sshd_config
  PasswordAuthentication no
  PubkeyAuthentication yes

• Implement multi-factor authentication (MFA):

  Use tools like Google Authenticator or YubiKey to add an extra layer of security.

  # Install Google Authenticator PAM module
  sudo apt-get install libpam-google-authenticator
  
  # Configure PAM to use Google Authenticator
  auth required pam_google_authenticator.so

• Disable anonymous FTP access:

  Unless absolutely necessary, disable anonymous access to prevent unauthorized users from accessing your FTP server.

  # In vsftpd.conf
  anonymous_enable=NO

To test these authentication measures using NetExec, you can use the following command:

netexec ftp 192.168.1.100 -u users.txt -p passwords.txt

This command attempts to brute-force FTP credentials, which should be unsuccessful if strong authentication measures are in place.