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PHP E-mail Injection

E-mail injection is a critical security vulnerability that can affect PHP applications handling email functionality. It occurs when an attacker manipulates input fields to inject malicious content into email headers or bodies.

Understanding the Threat

This attack exploits poorly sanitized user input in email forms. Attackers can modify email recipients, add hidden recipients, or alter the message content. The consequences can be severe, potentially leading to spam distribution or data breaches.

How E-mail Injection Works

Attackers typically insert additional email headers or SMTP commands into form fields. When processed by the PHP mail() function or similar email-sending mechanisms, these injected elements can alter the email's behavior.

Example of Vulnerable Code


$to = $_POST['to'];
$subject = $_POST['subject'];
$message = $_POST['message'];
$headers = "From: " . $_POST['from'];

mail($to, $subject, $message, $headers);

In this example, user input is directly used in the email function without proper validation or sanitization.

Preventing E-mail Injection

To protect against e-mail injection, implement these security measures:

  • Validate and sanitize all user inputs
  • Use PHP's filter_var() function with FILTER_SANITIZE_EMAIL
  • Implement strict input validation for email addresses and other fields
  • Avoid using user input directly in email headers

Secure Code Example


$to = filter_var($_POST['to'], FILTER_SANITIZE_EMAIL);
$subject = filter_var($_POST['subject'], FILTER_SANITIZE_STRING);
$message = filter_var($_POST['message'], FILTER_SANITIZE_STRING);
$from = filter_var($_POST['from'], FILTER_SANITIZE_EMAIL);

$headers = "From: " . $from;

if (filter_var($to, FILTER_VALIDATE_EMAIL) && filter_var($from, FILTER_VALIDATE_EMAIL)) {
    mail($to, $subject, $message, $headers);
} else {
    echo "Invalid email address";
}

This improved version sanitizes inputs and validates email addresses before sending the email.

Best Practices

Enhance your PHP application's security by following these guidelines:

  • Use PHP's built-in email validation functions
  • Implement PHP Form Validation techniques
  • Consider using secure email libraries instead of the native mail() function
  • Regularly update your PHP version to benefit from security patches
  • Employ PHP Prepared Statements when working with databases

Related Security Concepts

E-mail injection is just one aspect of web application security. To build robust PHP applications, also familiarize yourself with:

By implementing these security measures and staying informed about potential vulnerabilities, you can significantly reduce the risk of e-mail injection attacks in your PHP applications.