The SMTP service must not have the EXPN feature active.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-4692	GEN004660	SV-45872r1_rule		Low

Description
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.

STIG	Date
SUSE Linux Enterprise Server v11 for System z Security Technical Implementation Guide	2018-09-19

Details

Check Text ( C-43189r1_chk )
This vulnerability is applicable only to sendmail. If Postfix is the SMTP service for the system this will never be a finding. Procedure: Determine if EXPN is disabled. # grep -v "^#" /etc/mail/sendmail.cf |grep -i PrivacyOptions If nothing is returned or the returned line does not contain "noexpn", this is a finding.

Fix Text (F-39250r1_fix)
Rebuild /etc/mail/sendmail.cf with the "noexpn" Privacy Flag set. Procedure: Edit /etc/mail/sendmail.mc resetting the Privacy Flags to the default: define('confPRIVACYFLAGS', 'authwarnings,novrfy,noexpn,restrictqrun')dnl Rebuild the sendmail.cf file with: # make -C /etc/mail Restart the sendmail service. # service sendmail restart