Attackers know that this regex allows newlines ( %0a ), carriage returns ( %0d ), and certain special characters inside the local part if URL-encoded. By submitting:
The only safe approach is trusting validation alone—you must sanitize for the context of use . Part 5: Patching the v3.1 Vulnerability – A Hardening Guide If you find a script referencing "v3.1" or using ancient patterns, here is your patch strategy: Step 1: Remove the mail() function entirely. Use PHPMailer or SwiftMailer instead. These libraries automatically escape headers. Step 2: Validate and then also sanitize. $email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL); if (!filter_var($email, FILTER_VALIDATE_EMAIL)) die("Invalid email"); php email form validation - v3.1 exploit
From: legit@example.com%0aBcc: spamlist@example.com%0aContent-Type: text/html%0a%0a<script>malicious payload</script> The server becomes an open relay for spam, phishing, or malware distribution. The original contact form now sends thousands of emails without the owner's knowledge. Stage 3: Remote Code Execution (The Grand Prize) This is where "v3.1" becomes a true exploit. Some versions of this legacy library allowed "attachment uploads" or "log file writing" based on the email input. If the script writes logs to a .php file using the email address as part of the filename or content: Attackers know that this regex allows newlines (
// Additional header injection cleanup $email = str_replace(array("\r", "\n", "%0a", "%0d"), '', $email); If you must, use mb_encode_mimeheader() or a safe wrapper. Step 4: Disallow null bytes and control characters. if (preg_match('/[\x00-\x1F\x7F]/', $input)) http_response_code(400); exit("Invalid characters"); Use PHPMailer or SwiftMailer instead
email = "shell.php%00.jpg" Due to PHP's old %00 (null byte) injection (fixed in PHP 5.3.4+ but still present on outdated hosts), the file becomes logs/shell.php . Then, they inject PHP code via the message field:
$to = "admin@example.com"; $subject = $_POST['subject']; $message = $_POST['message']; $headers = "From: " . $_POST['email']; // Exploit here mail($to, $subject, $message, $headers); Using the injected newline, an attacker adds arbitrary SMTP commands: