I want to allow a lot of user submitted html for user profiles, I currently try to filter out what I don't want but I am now wanting to change and use a whitelist approach.
Here is my current non-whitelist approach
function FilterHTML($string) {
if (get_magic_quotes_gpc()) {
$string = stripslashes($string);
}
$string = html_entity_decode($string, ENT_QUOTES, "ISO-8859-1");
// convert decimal
$string = preg_replace('/&#(\d+)/me', "chr(\\1)", $string); // decimal notation
// convert hex
$string = preg_replace('/&#x([a-f0-9]+)/mei', "chr(0x\\1)", $string); // hex notation
//$string = html_entity_decode($string, ENT_COMPAT, "UTF-8");
$string = preg_replace('#(&\#*\w+)[\x00-\x20]+;#U', "$1;", $string);
$string = preg_replace('#(<[^>]+[\s\r\n\"\'])(on|xmlns)[^>]*>#iU', "$1>", $string);
//$string = preg_replace('#(&\#x*)([0-9A-F]+);*#iu', "$1$2;", $string); //bad line
$string = preg_replace('#/*\*()[^>]*\*/#i', "", $string); // REMOVE /**/
$string = preg_replace('#([a-z]*)[\x00-\x20]*([\`\'\"]*)[\\x00-\x20]*j[\x00-\x20]*a[\x00-\x20]*v[\x00-\x20]*a[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iU', '...', $string); //JAVASCRIPT
$string = preg_replace('#([a-z]*)([\'\"]*)[\x00-\x20]*v[\x00-\x20]*b[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iU', '...', $string); //VBSCRIPT
$string = preg_replace('#([a-z]*)[\x00-\x20]*([\\\]*)[\\x00-\x20]*@([\\\]*)[\x00-\x20]*i([\\\]*)[\x00-\x20]*m([\\\]*)[\x00-\x20]*p([\\\]*)[\x00-\x20]*o([\\\]*)[\x00-\x20]*r([\\\]*)[\x00-\x20]*t#iU', '...', $string); //@IMPORT
$string = preg_replace('#([a-z]*)[\x00-\x20]*e[\x00-\x20]*x[\x00-\x20]*p[\x00-\x20]*r[\x00-\x20]*e[\x00-\x20]*s[\x00-\x20]*s[\x00-\x20]*i[\x00-\x20]*o[\x00-\x20]*n#iU', '...', $string); //EXPRESSION
$string = preg_replace('#</*\w+:\w[^>]*>#i', "", $string);
$string = preg_replace('#</?t(able|r|d)(\s[^>]*)?>#i', '', $string); // strip out tables
$string = preg_replace('/(potspace|pot space|rateuser|marquee)/i', '...', $string); // filter some words
//$string = str_replace('left:0px; top: 0px;','',$string);
do {
$oldstring = $string;
//bgsound|
$string = preg_replace('#</*(applet|meta|xml|blink|link|script|iframe|frame|frameset|ilayer|layer|title|base|body|xml|AllowScriptAccess|big)[^>]*>#i', "...", $string);
} while ($oldstring != $string);
return addslashes($string);
}
The above works pretty well, I have never had any problems after 2 years of use with it but for a whitelist approach is there anything similars to stackoverflows C# method but in PHP? http://refactormycode.com/codes/333-sanitize-html
It's a pretty simple aim to achieve actually - you just need to check for anything that's NOT some tags from a list of whitelisted tags and remove them from the source. It can be done quite easily with one regex.
I haven't tested this, and there's probably an error in there somewhere but you get the gist of how it works. You might also want to look at using a formatting language such as Textile or Markdown.
Jamie
For those of you suggesting simply using strip_tags...be aware: strip_tags will NOT strip out tag attributes and broken tags will also mess it up.
From the manual page:
You CANNOT rely on just this one solution.
Maybe it is safer to use DOMDocument to analyze it correctly, remove disallowed tags with removeChild() and then get the result. It is not always safe to filter stuff with regular expressions, specially if things start to get such complexity. Hackers can find a way to cheat your filters, forums and social networks do know that very well.
For instance, browsers ignore spaces after the <. Your regex filter <script, but if I use < script... big FAIL!
Try this function "getCleanHTML" below, extract text content from the elements with exceptions of elements with tag name in the whitelist. This code is clean and easy to understand and debug.
Hope this helps.
HTML Purifier is the best HTML parser/cleaner out there.