Php Email Form Validation - V3.1 Exploit Review
The \r\n characters terminate the From: header prematurely and inject a new Bcc: header. The PHP mail() function (especially on older Unix sendmail systems) will honor this injected header, causing the server to send blind carbon copies of the contact form message to every address in the Bcc list. The "v3.1 exploit" is not just a theoretical vulnerability. It enables four distinct attack chains: 1. Spam Relay (Most Common) Attackers use the vulnerable form to send thousands of spam emails. Because the email originates from your trusted server IP, your domain's reputation is destroyed, leading to blacklisting by Spamhaus, Barracuda, and Microsoft. 2. Phishing via Trusted Domain An attacker injects:
else http_response_code(405); echo "Method not allowed."; php email form validation - v3.1 exploit
// 5. Send email $mail_sent = mail($to, $subject, $message, $headers, $additional_flags); The \r\n characters terminate the From: header prematurely
// 3. Hardcoded headers (no user input allowed in headers!) $to = "admin@example.com"; $subject = "Contact Form: " . mb_substr($name, 0, 50); // Truncate to prevent overflow $headers = "From: noreply@yourdomain.com\r\n"; $headers .= "Reply-To: " . $email . "\r\n"; // Email already validated $headers .= "Content-Type: text/plain; charset=UTF-8\r\n"; $headers .= "X-Sender-IP: " . $_SERVER['REMOTE_ADDR'] . "\r\n"; It enables four distinct attack chains: 1
if ($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.";
// 4. Use additional flags to disable sendmail injections $additional_flags = "-f noreply@yourdomain.com";
This article is written for security researchers, system administrators, and legacy system maintainers. It covers the technical nature of the exploit, the vulnerable code pattern, and remediation strategies. Introduction In the archive of web security vulnerabilities, certain version numbers become infamous. The search query "php email form validation - v3.1 exploit" points directly to a specific, highly reproducible attack vector that plagued countless small business websites and portfolio contact forms between 2012 and 2018.
