filter_xss

• Drupal 4.6
• Drupal 4.7
• Drupal 5
• Drupal 6
• Drupal 7

• View documentation
• List references

Definition

filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd'))
modules/filter/filter.module, line 1276

Description

Filters XSS. Based on kses by Ulf Harnhammar, see http://sourceforge.net/projects/kses

For examples of various XSS attacks, see: http://ha.ckers.org/xss.html

This code does four things:

• Removes characters and constructs that can trick browsers
• Makes sure all HTML entities are well-formed
• Makes sure all HTML tags and attributes are well-formed
• Makes sure no HTML tags contain URLs with a disallowed protocol (e.g. javascript:)

Parameters

$string The string with raw HTML in it. It will be stripped of everything that can cause an XSS attack.

$allowed_tags An array of allowed tags.

$format The format to use.

Code

<?php
function filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd')) {
  // Only operate on valid UTF-8 strings. This is necessary to prevent cross
  // site scripting issues on Internet Explorer 6.
  if (!drupal_validate_utf8($string)) {
    return '';
  }
  // Store the input format
  _filter_xss_split($allowed_tags, TRUE);
  // Remove NUL characters (ignored by some browsers)
  $string = str_replace(chr(0), '', $string);
  // Remove Netscape 4 JS entities
  $string = preg_replace('%&\s*\{[^}]*(\}\s*;?|$)%', '', $string);

  // Defuse all HTML entities
  $string = str_replace('&', '&amp;', $string);
  // Change back only well-formed entities in our whitelist
  // Named entities
  $string = preg_replace('/&amp;([A-Za-z][A-Za-z0-9]*;)/', '&\1', $string);
  // Decimal numeric entities
  $string = preg_replace('/&amp;#([0-9]+;)/', '&#\1', $string);
  // Hexadecimal numeric entities
  $string = preg_replace('/&amp;#[Xx]0*((?:[0-9A-Fa-f]{2})+;)/', '&#x\1', $string);

  return preg_replace_callback('%
    (
    <(?=[^a-zA-Z!/])  # a lone <
    |                 # or
    <[^>]*(>|$)       # a string that starts with a <, up until the > or the end of the string
    |                 # or
    >                 # just a >
    )%x', '_filter_xss_split', $string);
}
?>