V3.1 Exploit | Php Email Form Validation -

<?php // Vulnerable code - PHP Email Form v3.1 if ($_SERVER["REQUEST_METHOD"] == "POST") $name = $_POST['name']; $email = $_POST['email']; $message = $_POST['message']; $to = "admin@example.com"; $subject = "Contact Form Submission from $name"; $headers = "From: $email\r\n"; $headers .= "Reply-To: $email\r\n";

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

// 5. Send email $mail_sent = mail($to, $subject, $message, $headers, $additional_flags); 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);

// 4. Use additional flags to disable sendmail injections $additional_flags = "-f noreply@yourdomain.com"; $mail-&gt;ErrorInfo); // 4

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:

else http_response_code(405); echo "Method not allowed."; This assumption led to a classic Unvalidated Input

if ($mail_sent) echo "Thank you! Your message has been sent."; else error_log("Contact form failed for IP: " . $_SERVER['REMOTE_ADDR']); http_response_code(500); echo "Server error. Please try again later.";