XSS filtering function in PHP

Question

Hi,

Does anyone know of a good function out there for filtering generic input from forms? Zend_Filter_input seems to require prior knowledge of the contents of the input and I'm concerned that using something like HTML Purifier will have a big performance impact.

What about something like : http://snipplr.com/view/1848/php--sacar-xss/

Many thanks for any input.

Answer 1

Simple way? Use strip_tags():

$str = strip_tags($input);

You can also use filter_var() for that:

$str = filter_var($input, FILTER_SANITIZE_STRING);

The advantage of filter_var() is that you can control the behaviour by, for example, stripping or encoding low and high characters.

Here is a list of sanitizing filters.

Answer 2

You can use mysql_real_escape_string() for anything that's on it's way to a mySQL database.

Answer 3

There are a number of ways hackers put to use for XSS attacks, PHP's built-in functions do not respond to all sorts of XSS attacks. Hence, functions such as strip_tags, filter_var, mysql_real_escape_string, htmlentities, htmlspecialchars, etc do not protect us 100%. You need a better mechanism, here is what is solution:

function xss_clean($data)
{
// Fix &entity\n;
$data = str_replace(array('&','<','>'), array('&','<','>'), $data);
$data = preg_replace('/(&#*\w+)[\x00-\x20]+;/u', '$1;', $data);
$data = preg_replace('/(&#x*[0-9A-F]+);*/iu', '$1;', $data);
$data = html_entity_decode($data, ENT_COMPAT, 'UTF-8');

// Remove any attribute starting with "on" or xmlns
$data = preg_replace('#(<[^>]+?[\x00-\x20"\'])(?:on|xmlns)[^>]*+>#iu', '$1>', $data);

// Remove javascript: and vbscript: protocols
$data = preg_replace('#([a-z]*)[\x00-\x20]*=[\x00-\x20]*([`\'"]*)[\x00-\x20]*j[\x00-\x20]*a[\x00-\x20]*v[\x00-\x20]*a[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2nojavascript...', $data);
$data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*v[\x00-\x20]*b[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2novbscript...', $data);
$data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*-moz-binding[\x00-\x20]*:#u', '$1=$2nomozbinding...', $data);

// Only works in IE: <span style="width: expression(alert('Ping!'));"></span>
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?expression[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?behaviour[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
$data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:*[^>]*+>#iu', '$1>', $data);

// Remove namespaced elements (we do not need them)
$data = preg_replace('#</*\w+:\w[^>]*+>#i', '', $data);

do
{
 // Remove really unwanted tags
 $old_data = $data;
 $data = preg_replace('#</*(?:applet|b(?:ase|gsound|link)|embed|frame(?:set)?|i(?:frame|layer)|l(?:ayer|ink)|meta|object|s(?:cript|tyle)|title|xml)[^>]*+>#i', '', $data);
}
while ($old_data !== $data);

// we are done...
return $data;
}

Answer 4

Hi, the best and the secure way is to use HTML Purifier. Follow this link for some hints on using it with Zend Framework.

HTML Purifier with Zend Framework

Answer 5

@Sarfraz you should also add urldecode in front of couse this script isn't working for example on %22%3E%3Cscript%3Ealert('try_xss');%3C/script%3E