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V-4692

CAT III (Low)

The SMTP service must not have the EXPN feature active.

Rule ID

SV-45872r1_rule

STIG

SUSE Linux Enterprise Server v11 for System z Security Technical Implementation Guide

Version

V1R12

CCIs

CCI-000366

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

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

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