Title

The SMTP service must not have the EXPN feature active. (Cat III impact)

Discussion

The SMTP EXPN function allows an attacker to determine if an account exists on a system, providing significant assistance to a brute force attack on user accounts. EXPN may also provide additional information concerning users on the system, such as the full names of account owners.

Check Content

Perform the following to determine if EXPN is disabled: # telnet localhost 25 expn root If the command does not return a 500 error code (command unrecognized), this is a finding. OR Check the sendmail.cf configuration file by: # cat /etc/mail/sendmail.cf | tr '\011' ' ' | tr -s ' ' | sed -e 's/^[ \t]*//' | grep -v "^#" |\ grep -i privacyoptions | egrep -c -i "noexpn|goaway" The O PrivacyOptions should have the noexpn or the goaway option (covering both noexpn and novrfy). If the EXPN command is not disabled, this is a finding.

Fix Text

Edit the /etc/mail/sendmail.cf file and add or edit the following line: O PrivacyOptions=goaway Then restart the Sendmail service.

Additional Identifiers

Rule ID: SV-35076r1_rule

Vulnerability ID: V-4692

Group Title: GEN004660

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.