function filter_xss

Filters HTML to prevent cross-site-scripting (XSS) vulnerabilities.

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. Defaults to the array returned by _filter_xss_allowed_tags().

Return value

An XSS safe version of $string, or an empty string if $string is not: valid UTF-8.

See also

backdrop_validate_utf8()

Related topics

File

core/includes/common.inc, line 1864

Common functions that many Backdrop modules will need to reference.

Code

function filter_xss($string, $allowed_tags = NULL) {
  if ($allowed_tags === NULL) {
    $allowed_tags = _filter_xss_allowed_tags();
  }
  // Only operate on valid UTF-8 strings. This is necessary to prevent cross
  // site scripting issues on Internet Explorer 6.
  if (!backdrop_validate_utf8($string)) {
    return '';
  }
  $string = (string) $string;
  // Store the text format.
  _filter_xss_split($allowed_tags, TRUE);
  // Remove NULL 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('&', '&', $string);
  // Change back only well-formed entities in our allowlist:
  // Decimal numeric entities.
  $string = preg_replace('/&#([0-9]+;)/', '&#\1', $string);
  // Hexadecimal numeric entities.
  $string = preg_replace('/&#[Xx]0*((?:[0-9A-Fa-f]{2})+;)/', '&#x\1', $string);
  // Named entities.
  $string = preg_replace('/&([A-Za-z][A-Za-z0-9]*;)/', '&\1', $string);

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