SMTP Penetration Testing: A Comprehensive Guide

Explore this comprehensive guide on SMTP penetration testing, a crucial technique for assessing the security of your email servers.

In this article, Kali is used as a client, and a Windows Server 2012 as an SMTP Server.

SMTP Introduction

Protocol	Port	Description
SMTP	25
SSL/TLS	465
StartTLS	587

MUA → MSA → MTA → internet → MTA → MDA → MUA

Outlook → Exchange → firewall → internet → SMTP-Server of the receiving side → mail-server of the receiving side → Outlook of receiver

SMTP Commands

Command	Description	RFC
HELO	Identifies the sending SMTP server	RFC 5321
EHLO	Extended HELO, provides more information	RFC 5321
MAIL FROM	Specifies the sender’s email address	RFC 5321
RCPT TO	Specifies the recipient’s email address	RFC 5321
DATA	Initiates the message data transmission	RFC 5321
RSET	Resets the session to initial state	RFC 5321
VRFY	Requests verification of an email address	RFC 5321
EXPN	Requests the expansion of a mailing list	RFC 5321
NOOP	No operation, used for testing or keep-alive	RFC 5321
QUIT	Closes the SMTP session	RFC 5321
AUTH	Initiates authentication process	RFC 4954
STARTTLS	Initiates a secure TLS session	RFC 3207
HELP	Requests help information from the server	RFC 5321

RFC 821 – Simple Mail Transfer Protocol

Protocol definition for SMTP. This document covers the model, operating procedure, and protocol details for SMTP.

RFC 1869 – SMTP Service Extensions

Definition of the ESMTP extensions for SMTP. This describes a framework for extending SMTP with new commands, supporting dynamic discovery of the commands provided by the server, and defines a few additional commands.

SMTP Penetration Testing

1. SMTP Tools
2. Footprinting SMTP Service
3. Enumerate SMTP Service
4. SMTP Vulnerabilities
5. SMTP Attacks

SMTP – Pentest Tools

Tool	Description	Download
swaks	Swiss Army Knife for SMTP
smtp-user-enum
nmap	Network Scanner
msfconsole
THC Hydra

SMTP Reconnaissance

Nmap SMTP NSE Scripts

nmap --script smtp-brute,smtp-commands,smtp-enum-users,smtp-vuln-cve2011-1764 -p 25,465,587 <target-ip>

nmap --script smtp-ntlm-info --script-args smtp-ntlm-info.domain=example.com -p 25,465,587 <target-ip>

SMTP Metasploit Modules

auxiliary/scanner/smtp/smtp_enum
auxiliary/scanner/smtp/smtp_relay
auxiliary/scanner/smtp/smtp_version

Footprinting SMTP Service

sudo nmap 10.129.172.83 -sC -sV -p25,465,587

SMTP Enumeration

SMTP Enumeration is a crucial aspect of the cybersecurity landscape that offers detailed insights into a system’s mail server.

This process is particularly significant for network administrators, IT professionals, and ethical hackers who are continuously working to identify and rectify system vulnerabilities.

In this article, we will delve into the world of SMTP Enumeration, its functions, merits, demerits, and the critical role it plays in network security.

Furthermore, we’ll explore different tools used in SMTP enumeration and discuss common strategies for preventing potential exploitation.

nmap -p25 --script smtp-commands 172.180.22.16

auxiliary/scanner/smtp/smtp_enum

Banner Grabbing

nc -vn 172.180.22.16 25

SMTP User Enumeration

nmap –script smtp-enum-users.nse 172.180.22.16

smtp-user-enum -M VRFY -U list.txt -t 10.129.172.83

Open Relay

sudo nmap 10.129.14.128 -p25 --script smtp-open-relay -v

nmap -p25 --script smtp-open-relay 172.180.22.16 -v

SMTP Vulnerabilities

SMTP Injection

Directory harvest attack (DHA)

SMTP Post Exploitation

SWAKS

swaks --to remote-user@example.com --from local-user@<local-ip> --server mail.example.com --body "hello"

sendEmail -t dept@victim.com -f support@rfs.com -s 192.168.8.131 -u Important Upgrade Instructions -a /tmp/BestComputers-UpgradeInstructions.pdf

sudo python -m smtpd -n -c DebuggingServer :25

SMTP Attacks

Attack Type	Description
Email Spoofing	Forging the sender’s email address to appear as if it’s from a different source.
Email Phishing	Sending deceptive emails to trick recipients into revealing sensitive information.
Man-in-the-Middle (MITM)	Intercepting and possibly altering communication between the email client and server.
Brute-Force Attacks	Repeatedly attempting different username and password combinations to gain unauthorized access.
Email Bombing	Flooding an email inbox with an overwhelming number of emails, causing denial of service.
Email Relay Attacks	Exploiting open email relays to send spam or malicious emails through a compromised server.
SMTP User Enumeration	Determining valid email addresses by exploiting SMTP server responses.
SMTP Command Injection	Manipulating SMTP commands to execute arbitrary code on the SMTP server.
SMTP Header Injection	Injecting malicious content into email headers to trick email clients into unintended actions.
Denial of Service (DoS)	Overwhelming SMTP servers with excessive traffic, causing email service disruption.
Email Harvesting	Using automated tools to gather email addresses for spam campaigns or other malicious purposes.
Email Eavesdropping	Intercepting unencrypted emails during transmission to access sensitive information.
Email Attachment Exploits	Exploiting vulnerabilities in email attachments to execute malware on the recipient’s system.
Malicious Email Attachments	Sending attachments or links to infected files or websites to trick recipients into downloading malware or revealing sensitive information.

SMTP Hacking Labs – Join us

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https://cpts-certification.certs-study.com/

https://owasp.org/www-project-web-security-testing-guide/latest/4-Web_Application_Security_Testing/07-Input_Validation_Testing/10-Testing_for_IMAP_SMTP_Injection