Well this is a long one, but I feel like I'd be doing a disservice if I didn't share it. All code is taken directly from various parts of the source code of the latest stable release of Drupal and compiled into one area (as shown below). Very effective method of preventing XSS attacks.
Example usage:
$html = file_get_contents('http://example.com');
$output = filter_xss($html);
print $output;
Or:
$html = file_get_contents('http://example.com');
// Allow only <ul></ul>, <li></li>, and <p></p> tags.
$allowed_tags = array('ul', 'li', 'p');
$output = filter_xss($html, $allowed_tags);
print $output;
Here's the code required to run the above examples:
/**
* 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:).
*
* @param $string
* The string with raw HTML in it. It will be stripped of everything that can
* cause an XSS attack.
* @param $allowed_tags
* An array of allowed tags.
*
* @return
* An XSS safe version of $string, or an empty string if $string is not
* valid UTF-8.
*
* @see validate_utf8()
* @ingroup sanitization
*/
function filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'blockquote', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd')) {
// Only operate on valid UTF-8 strings. This is necessary to prevent cross
// site scripting issues on Internet Explorer 6.
if (!validate_utf8($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);
}
/**
* Processes an HTML tag.
*
* @param $m
* An array with various meaning depending on the value of $store.
* If $store is TRUE then the array contains the allowed tags.
* If $store is FALSE then the array has one element, the HTML tag to process.
* @param $store
* Whether to store $m.
*
* @return
* If the element isn't allowed, an empty string. Otherwise, the cleaned up
* version of the HTML element.
*/
function _filter_xss_split($m, $store = FALSE) {
static $allowed_html;
if ($store) {
$allowed_html = array_flip($m);
return;
}
$string = $m[1];
if (substr($string, 0, 1) != '<') {
// We matched a lone ">" character.
return '>';
}
elseif (strlen($string) == 1) {
// We matched a lone "<" character.
return '<';
}
if (!preg_match('%^<\s*(/\s*)?([a-zA-Z0-9]+)([^>]*)>?|(<!--.*?-->)$%', $string, $matches)) {
// Seriously malformed.
return '';
}
$slash = trim($matches[1]);
$elem = &$matches[2];
$attrlist = &$matches[3];
$comment = &$matches[4];
if ($comment) {
$elem = '!--';
}
if (!isset($allowed_html[strtolower($elem)])) {
// Disallowed HTML element.
return '';
}
if ($comment) {
return $comment;
}
if ($slash != '') {
return "</$elem>";
}
// Is there a closing XHTML slash at the end of the attributes?
$attrlist = preg_replace('%(\s?)/\s*$%', '\1', $attrlist, -1, $count);
$xhtml_slash = $count ? ' /' : '';
// Clean up attributes.
$attr2 = implode(' ', _filter_xss_attributes($attrlist));
$attr2 = preg_replace('/[<>]/', '', $attr2);
$attr2 = strlen($attr2) ? ' ' . $attr2 : '';
return "<$elem$attr2$xhtml_slash>";
}
/**
* Processes a string of HTML attributes.
*
* @return
* Cleaned up version of the HTML attributes.
*/
function _filter_xss_attributes($attr) {
$attrarr = array();
$mode = 0;
$attrname = '';
while (strlen($attr) != 0) {
// Was the last operation successful?
$working = 0;
switch ($mode) {
case 0:
// Attribute name, href for instance.
if (preg_match('/^([-a-zA-Z]+)/', $attr, $match)) {
$attrname = strtolower($match[1]);
$skip = ($attrname == 'style' || substr($attrname, 0, 2) == 'on');
$working = $mode = 1;
$attr = preg_replace('/^[-a-zA-Z]+/', '', $attr);
}
break;
case 1:
// Equals sign or valueless ("selected").
if (preg_match('/^\s*=\s*/', $attr)) {
$working = 1; $mode = 2;
$attr = preg_replace('/^\s*=\s*/', '', $attr);
break;
}
if (preg_match('/^\s+/', $attr)) {
$working = 1; $mode = 0;
if (!$skip) {
$attrarr[] = $attrname;
}
$attr = preg_replace('/^\s+/', '', $attr);
}
break;
case 2:
// Attribute value, a URL after href= for instance.
if (preg_match('/^"([^"]*)"(\s+|$)/', $attr, $match)) {
$thisval = filter_xss_bad_protocol($match[1]);
if (!$skip) {
$attrarr[] = "$attrname=\"$thisval\"";
}
$working = 1;
$mode = 0;
$attr = preg_replace('/^"[^"]*"(\s+|$)/', '', $attr);
break;
}
if (preg_match("/^'([^']*)'(\s+|$)/", $attr, $match)) {
$thisval = filter_xss_bad_protocol($match[1]);
if (!$skip) {
$attrarr[] = "$attrname='$thisval'";
}
$working = 1; $mode = 0;
$attr = preg_replace("/^'[^']*'(\s+|$)/", '', $attr);
break;
}
if (preg_match("%^([^\s\"']+)(\s+|$)%", $attr, $match)) {
$thisval = filter_xss_bad_protocol($match[1]);
if (!$skip) {
$attrarr[] = "$attrname=\"$thisval\"";
}
$working = 1; $mode = 0;
$attr = preg_replace("%^[^\s\"']+(\s+|$)%", '', $attr);
}
break;
}
if ($working == 0) {
// Not well formed; remove and try again.
$attr = preg_replace('/
^
(
"[^"]*("|$) # - a string that starts with a double quote, up until the next double quote or the end of the string
| # or
\'[^\']*(\'|$)| # - a string that starts with a quote, up until the next quote or the end of the string
| # or
\S # - a non-whitespace character
)* # any number of the above three
\s* # any number of whitespaces
/x', '', $attr);
$mode = 0;
}
}
// The attribute list ends with a valueless attribute like "selected".
if ($mode == 1 && !$skip) {
$attrarr[] = $attrname;
}
return $attrarr;
}
/**
* Processes an HTML attribute value and strips dangerous protocols from URLs.
*
* @param $string
* The string with the attribute value.
* @param $decode
* (deprecated) Whether to decode entities in the $string. Set to FALSE if the
* $string is in plain text, TRUE otherwise. Defaults to TRUE.
*
* @return
* Cleaned up and HTML-escaped version of $string.
*/
function filter_xss_bad_protocol($string, $decode = TRUE) {
// Get the plain text representation of the attribute value (i.e. its meaning).
if ($decode) {
$string = decode_entities($string);
}
return check_plain(strip_dangerous_protocols($string));
}
/**
* Strips dangerous protocols (e.g. 'javascript:') from a URI.
*
* @param $uri
* A plain-text URI that might contain dangerous protocols.
*
* @return
* A plain-text URI stripped of dangerous protocols. As with all plain-text
* strings, this return value must not be output to an HTML page without
* check_plain() being called on it. However, it can be passed to functions
* expecting plain-text strings.
*
*/
function strip_dangerous_protocols($uri) {
static $allowed_protocols;
if (!isset($allowed_protocols)) {
$allowed_protocols = array_flip(array('ftp', 'http', 'https', 'irc', 'mailto', 'news', 'nntp', 'rtsp', 'sftp', 'ssh', 'tel', 'telnet', 'webcal'));
}
// Iteratively remove any invalid protocol found.
do {
$before = $uri;
$colonpos = strpos($uri, ':');
if ($colonpos > 0) {
// We found a colon, possibly a protocol. Verify.
$protocol = substr($uri, 0, $colonpos);
// If a colon is preceded by a slash, question mark or hash, it cannot
// possibly be part of the URL scheme. This must be a relative URL, which
// inherits the (safe) protocol of the base document.
if (preg_match('![/?#]!', $protocol)) {
break;
}
// Check if this is a disallowed protocol. Per RFC2616, section 3.2.3
// (URI Comparison) scheme comparison must be case-insensitive.
if (!isset($allowed_protocols[strtolower($protocol)])) {
$uri = substr($uri, $colonpos + 1);
}
}
} while ($before != $uri);
return $uri;
}
/**
* Encodes special characters in a plain-text string for display as HTML.
*
* Also validates strings as UTF-8 to prevent cross site scripting attacks on
* Internet Explorer 6.
*
* @param $text
* The text to be checked or processed.
*
* @return
* An HTML safe version of $text, or an empty string if $text is not
* valid UTF-8.
*
* @see validate_utf8()
* @ingroup sanitization
*/
function check_plain($text) {
return htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
}
/**
* Decodes all HTML entities (including numerical ones) to regular UTF-8 bytes.
*
* Double-escaped entities will only be decoded once ("&lt;" becomes "<"
* , not "<"). Be careful when using this function, as decode_entities can
* revert previous sanitization efforts (<script> will become <script>).
*
* @param $text
* The text to decode entities in.
*
* @return
* The input $text, with all HTML entities decoded once.
*/
function decode_entities($text) {
return html_entity_decode($text, ENT_QUOTES, 'UTF-8');
}
/**
* Checks whether a string is valid UTF-8.
*
* All functions designed to filter input should use validate_utf8
* to ensure they operate on valid UTF-8 strings to prevent bypass of the
* filter.
*
* When text containing an invalid UTF-8 lead byte (0xC0 - 0xFF) is presented
* as UTF-8 to Internet Explorer 6, the program may misinterpret subsequent
* bytes. When these subsequent bytes are HTML control characters such as
* quotes or angle brackets, parts of the text that were deemed safe by filters
* end up in locations that are potentially unsafe; An onerror attribute that
* is outside of a tag, and thus deemed safe by a filter, can be interpreted
* by the browser as if it were inside the tag.
*
* The function does not return FALSE for strings containing character codes
* above U+10FFFF, even though these are prohibited by RFC 3629.
*
* @param $text
* The text to check.
*
* @return
* TRUE if the text is valid UTF-8, FALSE if not.
*/
function validate_utf8($text) {
if (strlen($text) == 0) {
return TRUE;
}
// With the PCRE_UTF8 modifier 'u', preg_match() fails silently on strings
// containing invalid UTF-8 byte sequences. It does not reject character
// codes above U+10FFFF (represented by 4 or more octets), though.
return (preg_match('/^./us', $text) == 1);
}
htmlspecialchars()? – Marc B