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.
Return value
An XSS safe version of $string, or an empty string if $string is not valid UTF-8.
See also
Related topics
25 calls to filter_xss()
- aggregator_filter_xss in modules/
aggregator/ aggregator.module - Renders the HTML content safely, as allowed.
- CommentTokenReplaceTestCase::testCommentTokenReplacement in modules/
comment/ comment.test - Creates a comment, then tests the tokens generated from it.
- comment_tokens in modules/
comment/ comment.tokens.inc - Implements hook_tokens().
- CommonXssUnitTest::testInvalidMultiByte in modules/
simpletest/ tests/ common.test - Check that invalid multi-byte sequences are rejected.
- DBLogTestCase::assertLogMessage in modules/
dblog/ dblog.test - Confirms that a log message appears on the database log overview screen.
File
-
includes/
common.inc, line 1538
Code
function filter_xss($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) $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);
}
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