function filter_xss

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

Based on kses by Ulf Harnhammar, see For examples of various XSS attacks, see:

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:).


$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.

Return value

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

See also


Related topics

24 calls to filter_xss()
aggregator_filter_xss in drupal/modules/aggregator/aggregator.module
Renders the HTML content safely, as allowed.
CommentTokenReplaceTestCase::testCommentTokenReplacement in drupal/modules/comment/comment.test
Creates a comment, then tests the tokens generated from it.
comment_tokens in drupal/modules/comment/
Implements hook_tokens().
CommonXssUnitTest::testInvalidMultiByte in drupal/modules/simpletest/tests/common.test
Check that invalid multi-byte sequences are rejected.
DBLogTestCase::assertLogMessage in drupal/modules/dblog/dblog.test
Confirms that a log message appears on the database log overview screen.

... See full list


drupal/includes/, line 1479
Common functions that many Drupal modules will need to reference.


function filter_xss($string, $allowed_tags = array(
)) {

  // 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 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 whitelist:
  // 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);