HTML contact form with Captra

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     As a relay for sending bulk unsolicited emails

    If you are not validating your form fields (on the serve side) before sending the emails, then hackers can alter your email headers to send the bulk unsolicited emails. (also known as email injection) For example, hackers can place the following code in one of your form fields and make your form processor script send an email to an unintended recipient:

    sender@theirdomain.com%0ABcc:NewRecipient@anotherdomain.com

    The code above is adding another email address to the CC list of the email. Spammers can send thousands of emails using this exploit. Your host will not be happy with this and may warn you or even ban your web site.

    The best way to prevent this spammer exploit is to validate the fields used in the mail() function(fields like email, subject of the email, name etc). Check for the presence of any “new line” (rn) in those fields.

    For Sending spam messages to you

    There are programs known as ‘spam-bots’ that leech through the web pages looking for web forms. When found, those ‘bots’ just fills the fields with a spam message and submits. Eventually you will start getting many hundred submissions send by those spam bots and you will find it difficult to separate genuine submissions from spam messages.

    The solution for this problem is to use a mechanism to identify human submitters from ‘bots’. CAPTCHA is one of such tests.

    Adding Captcha to the form

    Captcha is an image with a code written on it. The website visitor is required to read the code on the image and enter the value in a text field. If the word entered is wrong, the form submission is not processed. As CAPTCHA is a smartly blurred image, the spam bot can’t read it. So the form cannot be auto-submitted by a ‘bot’.

    The contact form with CAPTCHA

    Here is the HTML code for the contact form:

    The HTML form above contains the fields for name, email and message. In addition, we have the CAPTCHA image. The  tag for the CAPTCHA image points to the script captcha_code_file.php. The PHP script in ‘captcha_code_file.php’ creates the image for the captcha and saves the code in a session variable named ’6_letters_code’.

    Validating the CAPTCHA

    When the form is submitted, we compare the value in the session variable(6_letters_code) with the submitted CAPTCHA code( the value in the text field 6_letters_code). If the codes match, then we proceed with emailing the form submission. Else we display an error.

    Here is the code that does the server side processing:

    if(isset($_POST['submit']))
    {
      if(empty($_SESSION['6_letters_code'] ) 					
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