SimpleEmailSpoofer: A simple Python CLI to spoof emails

by do son · Published May 25, 2018 · Updated October 10, 2021

SimpleEmailSpoofer

A few Python programs designed to help penetration testers with email spoofing.

Email Spoofing 101

Basic Principles

Email spoofing has been an issue since the earliest days of the SMTP protocol. The root cause of email spoofing is that SMTP does not require authentication between mail relays. An attacker can stand up or find an “Open Relay” (i.e. an SMTP server that can send from arbitrary domains), which is the default configuration for SMTP servers, and use that to send arbitrary emails from arbitrary email addresses.

FQDN Requirements

In an effort to combat spam, many SMTP servers now block any mail relay that does not have a Fully-qualified Domain Name (FQDN). An FQDN is a DNS A record that points to the relay’s IP address. This can be either a domain purchased from a domain registrar or by using a domain automatically associated with a virtual private server.

Email Protection

As email spoofing is a serious and widespread issue, over the years several protection mechanisms have been added to combat it. However, all of these protections are opt-in and require significant configuration. As such, as much as 98% of the internet is still vulnerable. For additional information, please see the Bishop Fox blog post on the subject.

To determine if a domain is vulnerable to email spoofing, Bishop Fox has created two tools:

A web interface that produces a report with analysis and recommendations
A command line utility that only performs analysis

Setup

Mail Server

Email servers do not accept connections from normal computers. In an effort to limit the amount of spam, most MTAs will only accept connections from relays that have a fully-qualified domain name (FQDN). As such, the easiest way to use this project is from a Linux Virtual Private Server. There are several free or cheap options available, such as Digital Ocean, Linode, and Amazon EC2.

Once the server is set up, the next step is to install and start an SMTP server. This is required to actually send the spoofed emails. I personally use Postfix, though any will do. This script defaults to using localhost:25 for the mail server.

On Kali Linux, the easiest method of doing this is:

sudo apt-get install postfix sudo service postfix start

When installing postfix, specify Internet-facing and provide the correct FQDN when prompted.