cis-ubuntu2004-v300-2-1-21

name	cis-ubuntu2004-v300-2-1-21
description	Ensure mail transfer agents are configured for local-only mode
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","services"]
cis_id	2.1.21
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

2.1.21 Ensure mail transfer agents are configured for local-only mode (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

Mail Transfer Agents (MTA), including postfix, exim, and sendmail, 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 the MTA is not listening on any non-loopback address:

#!/usr/bin/env bash

{
  l_output="" l_output2=""
  a_port_list=("25" "465" "587")
  for l_port in "${a_port_list[@]}"; do
    if ss -plntu | grep -P -- ':'"$l_port"'\b' | grep -Pvq -- '\h+(127\.0\.0\.1|::1):'"$l_port"'\b'; then
      l_output2="$l_output2\n - Port \"$l_port\" is listening on a non-loopback network interface"
    else
      l_output="$l_output\n - Port \"$l_port\" is not listening on a non-loopback network interface"
    fi
  done
  if [ -z "$l_output2" ]; 
     -e 
  
     -e 
    [ -n  ] &&  -e 
  
}