cis-ubuntu1804-v220-2-2-16

name	cis-ubuntu1804-v220-2-2-16
description	Ensure mail transfer agent is configured for local-only mode
category	cis-networking
version	2.2.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-18.04","services","mta"]
cis_id	2.2.16
cis_benchmark	CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

2.2.16 Ensure mail transfer agent is configured for local-only mode (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

Mail Transfer Agents (MTA), such as sendmail and Postfix, are used to listen for incoming mail and transfer the messages to the appropriate user or mail server. If the system is not intended to be a mail server, it is recommended that the MTA be configured to only process local mail.

Rationale

The software for all Mail Transfer Agents is complex and most have a long history of security issues. While it is important to ensure that the system can process local mail messages, it is not necessary to have the MTA's daemon listening on a port unless the server is intended to be a mail server that receives and processes mail from other systems.

Audit Procedure

Command Line

Run the following script to verify that the MTA is not listening on any non-loopback address (127.0.0.1 or ::1):

#!/usr/bin/env bash

{
l_output=""
l_output2=""
a_port_list=("25" "465" "587")

# Check if inet_interfaces is not set to all
if [ "$(postconf -n inet_interfaces)" != "inet_interfaces = all" ]; then
   for l_port_number in "${a_port_list[@]}"; do
      if ss -plntu | grep -P -- ':'"$l_port_number"'\b' | grep -Pvq -- '\h+(127\.0\.0\.1|(?:::1\]?)):'"$l_port_number"'\b'; then
         l_output2="$l_output2\n - Port \"$l_port_number\" is listening on a non-loopback network interface"
      else
         l_output=
      
   

   l_output2=


 a_port_list


 [ -z  ]; 
    -e 

    -e 
   [ -n  ] &&  -e 

}