How Hackers Exploit SMTP Injection t Send Spoofed Email

How Hackers Exploit SMTP Injection to Send Spoofed Email

What is SMTP Injection?​

SMTP Injection is a web security vulnerability that allows an attacker to inject SMTP commands or email headers into an application that sends emails using unsanitized user input. This can lead to email spoofing, mass spam mailing, and sometimes even command injection, depending on the mail server configuration.

Common Areas Where SMTP Injection Happens​

SMTP Injection vulnerabilities usually occur in the following areas:
Contact Us forms
Feedback or support ticket forms
Newsletter subscription forms
Forgot password or email verification features
Signup confirmation emails
Root Cause: These areas accept user input that is directly used in the email headers (To, From, Subject, etc.) without proper sanitization.

Real-World Example (PHP)​

Here’s a vulnerable PHP snippet:

<?php
$to = "admin@example.com";
$subject = $_POST['subject'];
$message = $_POST['message'];
$headers = "From: ".$_POST['email'];
mail($to, $subject, $message, $headers);
?>

If an attacker enters the following as email input:
attacker@evil.com%0ACc:victim@example.com
The resulting email headers become:
From: attacker@evil.com
Cc: victim@example.com
Now the attacker can inject additional recipients and send spoofed or malicious emails using your server!

SMTP Injection Payloads​

Below are common payloads used to test or exploit SMTP Injection:

attacker@example.com%0ACc: victim@example.com
attacker@example.com%0ABcc: boss@example.com
attacker@example.com\r\nSubject: Injected Email
%0A%0DTo: victim@example.com%0ASubject: Hacked

These payloads inject new lines (%0A = LF, %0D = CR) to trick the email header structure.

Tools for Testing​

Burp Suite - Modify HTTP request with payloads
OWASP ZAP - Active scanning and fuzzing
WFuzz - Fuzz headers with SMTP payloads
Python Scripts - Using smtplib or requests for custom testing
Mailtrap.io - For safe testing without sending real emails

Prevention Techniques​

To prevent SMTP Injection, follow these best practices:
1. Sanitize and Validate Inputs
Strip or block newline characters: \n, \r, %0A, %0D
Whitelist valid email formats using:

$email = filter_var($_POST['email'], FILTER_VALIDATE_EMAIL);

2. Use Secure Mailing Libraries
Use libraries or services that handle email headers automatically:
PHPMailer
SendGrid
Mailgun
SMTP2GO

3. Don't Include Raw User Input in Headers
Avoid inserting raw user input directly in headers like From, Cc, Bcc, or Subject.

Penetration Testing Checklist
Test Case Description
Header Injection Inject newline characters and see if additional headers are accepted
Spoofed Headers Try injecting fake From, Cc, or Bcc
Payload Fuzzing Fuzz form inputs with common payloads
Mass Mail Potential Check if multiple recipients can be injected
Response Analysis Analyze server response or email logs

Impact of SMTP Injection​

Email Spoofing
Phishing Campaigns via Trusted Domain
Mass Spamming
Domain Blacklisting
Loss of Reputation & Deliverability

Real Incident:​

> A bug bounty hunter found SMTP Injection in a popular retail site’s feedback form. He was able to send spoofed emails to thousands of users using their trusted domain. The company’s domain got blacklisted, and customer trust was lost - all due to a single unsanitized input field.

Don’t Let This Happen To You!
SMTP Injection is easy to miss, but its impact can be devastating. Always validate user inputs, use secure libraries, and never trust user-controlled data when building emails.