Email Header Injection

When a web application builds email headers from user-supplied data (like a "From" or "Subject" field) without stripping newline characters, attackers can inject additional headers such as CC, BCC, or even a new message body.

Vulnerable PHP mailer:

$from = $_POST["email"];
$subject = $_POST["subject"];
mail("admin@example.com", $subject, $message, "From: " . $from);

An attacker submits email: attacker@evil.com\r\nBcc: victim1@a.com,victim2@b.com

The additional header injects BCC recipients, turning the server into a spam relay. Attackers can also inject Content-Type: text/html to change the message body format.

The primary defense is to validate and strip newline characters from all values that will be used in email headers. Additionally, validate that email addresses match an expected format.

Safe implementation (PHP):

function sanitizeHeader($value) {
    // Remove CRLF characters
    return preg_replace("/[\r\n]/", "", $value);
}

$from = sanitizeHeader($_POST["email"]);
$subject = sanitizeHeader($_POST["subject"]);

// Also validate email format
if (!filter_var($from, FILTER_VALIDATE_EMAIL)) {
    die("Invalid email address");
}

Using a proper mail library:

# Python: use email.message module or a library like sendgrid
from email.message import EmailMessage
msg = EmailMessage()
msg["From"] = user_email  # Library handles header escaping
msg["To"] = "admin@example.com"

Defense checklist:

• Strip \r and \n from all header values
• Validate email addresses with a regex or library
• Use a reputable mail library instead of raw header construction
• Allowlist acceptable subjects and senders