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:
$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.
An XSS safe version of $string, or an empty string if $string is not valid UTF-8.
function filter_xss($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);
}