The Ubuntu operating system must be configured to prevent unrestricted mail relaying.

From Canonical Ubuntu 16.04 Security Technical Implementation Guide

Associated with: CCI-000366

SV-90571r2_rule The Ubuntu operating system must be configured to prevent unrestricted mail relaying.

Vulnerability discussion

If unrestricted mail relaying is permitted, unauthorized senders could use this host as a mail relay for the purpose of sending spam or other unauthorized activity.

Check content

Determine if "postfix" is installed with the following commands: Note: If postfix is not installed, this is Not Applicable. # dpkg -l | grep postfix ii postfix 3.1.0-3 Verify the Ubuntu operating system is configured to prevent unrestricted mail relaying. If postfix is installed, determine if it is configured to reject connections from unknown or untrusted networks with the following command: # postconf -n smtpd_client_restrictions smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject If the "smtpd_relay_restrictions" parameter contains any entries other than "permit_mynetworks", "permit_sasl_authenticated" and "reject", is missing, or is commented out, this is a finding.

Fix text

If "postfix" is installed, modify the "/etc/postfix/main.cf" file to restrict client connections to the local network with the following command: # sudo postconf -e 'smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject'

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