Form Hack / XSS / SQL Injection

Question

I got a big problem with a Botnet...I think it is a botnet... What happens? The bot fills out the form and spams the database.

Here is the form:

<form method="POST" action="">
    <textarea name="text2" style="width: 290px; margin-bottom: 10px;"></textarea>
    <center>
    <img id="captcha" alt="Captcha" src="http://www.mysite.de/php/captcha/Captcha_show.php?sid='2d7dd1256d06a724c34b9d703f3733e9">
    <br>
    <a onclick="document.getElementById('captcha').src = 'php/captcha/Captcha_show.php?' + Math.random(); return false" href="#">
    <br>
    <input id="mod" class="inputbox" type="text" alt="Bitte die Zeichen des Bildes eingeben." style="width: 280px" maxlength="15" name="captcha_code" value="">
    <sub>Bitte die Zeichen des Bildes abschreiben</sub>
    <br>
    <br>
    <input class="button" type="submit" value="Hinzufügen" name="submit">
    </center>
    </form>

Here is an array with words that can´t be inserted:

$badWords = array("/delete/i","/deleted/i","/deletee/i", "/update/i", "/updateu/i", "/updateup/i","/union/i","/unionu/i","/unionun/i", "/insert/i","/inserti/i","/insertin/i","/drop/i","/dropd/i","/dropdr/i","/http/i","/httph/i","/httpht/i","/--/i", "/url/i", "/urlu/i", "/urlur/i", "/true/i", "/truet/i", "/truetr/i", "/false/i", "/falsef/i", "/falsefa/i","/!=/i","/==/i", "/insurance/i", "/eating/i", "/viagra/i");


$text3 = preg_replace($badWords, "a12", $text2);

if($text3 != $text2){
    echo "<center><b>No valid data!</b></center> <meta http-equiv=\"refresh\" content=\"2; URL=http://www.mysite.de\">";
    exit;
}

So normally the user should not be able to post any text with e.g. "viagra" in it.

I can´t understand how someone or a bot could insert a text with some of these bad words?

I am using PDO and functions like htmlspecialchars() stripslashes() strip_tags() htmlspecialchars() to prevent the hack...

Any ideas?

Answer 1

Your script can be hacked by HTML entities:

Example:
The input is "Hello" but in code it is Hello. If you now run a preg_match you will not find anything

var_dump(preg_match('/Hello/i','Hello'));    
// returns int 0

If you want to prevent SQL injections: Use prepared statements.
If you not want to be spammed, you have also to look for an other way, as long as I could simply insert a valid string many times.

Notice: I think you can prevent my hack by using html_entity_decode

var_dump(preg_match('/Hello/i',html_entity_decode('Hello'))); 
// returns int 1