SMTP - 25

Simple Mail Transport Protocol (SMTP) is the standard email protocol on the internet.

The SMTP server temporarily stores the messages until they can be relayed to another server or to message store that uses the pop3 or IMAP4 access protocol to communicate with the user's email program.

The SMTP protocol supports commands which allows attacker to enumerate email addresses.

Connect to the SMTP Server:

nc -nv <target> 25 # netcat
telnet <target> 25 # telnet

After establishing the connection its possible to enumerate usernames using the VRFY command

VRFY <username>

Brute force using hydra:

hydra -P <wordlist> <smtp_server_addr> smtp -V

References

Definition of SMTPPCMAG