Php Email Form Validation - V3.1 Exploit 'link' -

function validate_email($email) if (preg_match('/^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]2,$/', $email)) return true; return false;

filter_var($email, FILTER_VALIDATE_EMAIL) While FILTER_VALIDATE_EMAIL is better, it prevent header injection. An email like "attacker\r\nBcc: spam"@example.com passes validation but still contains CRLF characters after decoding in some PHP edge cases (especially with multibyte strings). php email form validation - v3.1 exploit

Attackers know that this regex allows newlines ( %0a ), carriage returns ( %0d ), and certain special characters inside the local part if URL-encoded. By submitting: function validate_email($email) if (preg_match('/^[a-zA-Z0-9

// 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"); By submitting: // Additional header injection cleanup $email

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:

attacker@example.com%0aCC: victims@example.com The regex sees attacker@example.com and validates. But after PHP urldecodes the input, the mailer sees: