Email injection
Description
This application is vulnerable to email injection, a security flaw that allows attackers to manipulate email headers by injecting malicious content into input fields used by email functions.
Attackers exploit insufficient input validation to insert special characters (carriage return and line feed) that create new email headers, enabling them to modify email recipients, subjects, and content without authorization. This vulnerability is commonly found in contact forms and other features that send automated emails.
Remediation
Implement strict input validation and sanitization for all user-supplied data used in email functions. Follow these steps:
1. Remove or reject newline characters:
Strip carriage return (CR, \r, 0x0D) and line feed (LF, \n, 0x0A) characters from all input fields that will be used in email headers (To, From, Subject, etc.).
Example for PHP:
// Remove CR and LF characters from email inputs
$email = str_replace(array("\r", "\n", "%0a", "%0d"), '', $_POST['email']);
$subject = str_replace(array("\r", "\n", "%0a", "%0d"), '', $_POST['subject']);
// Or use a more comprehensive filter
$email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
// Reject invalid email
die('Invalid email address');
}2. Use email libraries with built-in protection:
Instead of using raw mail() functions, use well-maintained email libraries that automatically handle header injection protection (e.g., PHPMailer, SwiftMailer for PHP).
3. Validate all input fields:
Implement whitelist validation for email addresses, names, and subjects. Reject any input containing unexpected characters.
4. Implement additional headers:
Set additional security headers and use authenticated SMTP when possible to prevent abuse.