An attacker injects:
: Vulnerabilities with high CVSS v3.1 scores (like 9.8) are actively hunted by bots within hours of disclosure. Use Modern Filters : Rely on built-in functions like the PHP filter_var and keep libraries updated to avoid "legacy" exploits.
$email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL); if (filter_var($email, FILTER_VALIDATE_EMAIL)) // Additional security checks if (preg_match('/[\r\n%0A%0D]/', $email)) // Reject email containing line breaks
Due to PHP's old %00 (null byte) injection (fixed in PHP 5.3.4+ but still present on outdated hosts), the file becomes logs/shell.php .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: php email form validation - v3.1 exploit
// Additional header injection cleanup $email = str_replace(array("\r", "\n", "%0a", "%0d"), '', $email);
Never concatenate user input into SQL strings. Use PDO prepared statements.
This adds BCC headers to the email, allowing the attacker to use the contact form for spam distribution. More sophisticated payloads can inject additional headers that modify the email's envelope, recipient list, and message content. An attacker injects: : Vulnerabilities with high CVSS v3
PHPMailer < 5.2.18 Remote Code Execution exploit ... - GitHub
attacker@example.com%0aCC: victims@example.com
The vulnerable code extracted from mmex.php shows the critical flaw: Then, they inject PHP code via the message
// Remove malicious newlines completely $clean_name = preg_replace("/[\r\n]+/", " ", $_POST['name']); $clean_subject = preg_replace("/[\r\n]+/", " ", $_POST['subject']); Use code with caution. 2. Use Built-in PHP Filters
The core flaw in PHP Email Form Validation v3.1 lies in and unsafe functions within the core validation routine.
Check your server for signs of the v3.1 exploit: