function filter_xss

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7 common.inc filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'blockquote', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd'))
4.6 filter.module filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd'))
4.7 filter.module filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd'))
5 filter.module filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd'))
6 filter.module filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd'))
8 common.inc filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'blockquote', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd'))

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.

Return value

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

See also

drupal_validate_utf8()

Related topics

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

... See full list

2 string references to 'filter_xss'
_options_prepare_options in modules/field/modules/options/options.module
Sanitizes the options.
_options_properties in modules/field/modules/options/options.module
Describes the preparation steps required by each widget.

File

includes/common.inc, line 1430
Common functions that many Drupal modules will need to reference.

Code

function filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'blockquote', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd')) {
  // Only operate on valid UTF-8 strings. This is necessary to prevent cross
  // site scripting issues on Internet Explorer 6.
  if (!drupal_validate_utf8($string)) {
    return '';
  }
  // Store the 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);
}

Comments

Hi,
I understand the importance of the xss filtering. On one project we use the fieldset and legend element on Views. Currently I need to edit the common.inc on each update.

Would be possbile to include the fieldset and legend element by default? I don't think they would provide any danger as they only act as wrapper elements.

If not, can I add custom options using a hook?

Thanks