Check or Attack Name: smtpexpn

Simple Mail Transfer Protocol (SMTP)-compliant applications, such as the sendmail program EXPN, allow an attacker to determine if an account exists on a system, providing significant assistance to a brute force attack on user accounts. EXPN provides additional information concerning users on the system, such as if they exist and their full names.

If you are running Sendmail, add the line Opnoexpn to your Sendmail configuration file, usually located in /etc/sendmail.cf. For other mail servers, contact your vendor for information on how to disable the expand command.

Newer versions of sendmail are available at http://www.sendmail.org or from ftp://ftp.cs.berkeley.edu/ucb/sendmail.

Sendmail Consortium, Sendmail FAQ, http://www.sendmail.org/faq