function filter_xss in Drupal 6

Same name and namespace in other branches

1. 4 modules/filter.module \filter_xss()
2. 5 modules/filter/filter.module \filter_xss()
3. 7 includes/common.inc \filter_xss()

Filters an HTML string 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

8 calls to filter_xss()

aggregator_filter_xss in modules/aggregator/aggregator.module

Safely render HTML content, as allowed.

dblog_overview in modules/dblog/dblog.admin.inc

Menu callback; displays a listing of log messages.

drupal_error_handler in includes/common.inc

Log errors as defined by administrator.

drupal_html_to_text in includes/mail.inc

Transform an HTML string into plain text, preserving the structure of the markup. Useful for preparing the body of a node to be sent by e-mail.

filter_xss_admin in modules/filter/filter.module

Very permissive XSS/HTML filter for admin-only use.

... See full list

1 string reference to 'filter_xss'

drupal_error_handler in includes/common.inc

Log errors as defined by administrator.

File

modules/filter/filter.module, line 992

Framework for handling filtering of content.

Code

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('&', '&', $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);
}