XSS filtering in PHP (cleans various UTF encodings & nested exploits)

xss_clean.php

<?php
/*
 * XSS filter, recursively handles HTML tags & UTF encoding
 * Optionally handles base64 encoding
 *
 * ***DEPRECATION RECOMMENDED*** Not updated or maintained since 2011
 * A MAINTAINED & BETTER ALTERNATIVE => kses
 * https://github.com/RichardVasquez/kses/
 *
 * This was built from numerous sources
 * (thanks all, sorry I didn't track to credit you)
 *
 * It was tested against *most* exploits here: http://ha.ckers.org/xss.html
 * WARNING: Some weren't tested!!!
 * Those include the Actionscript and SSI samples, or any newer than Jan 2011
 *
 */

class xssClean {

    /*
     * Recursive worker to strip risky elements
     *
     * @param   string  $input      Content to be cleaned. It MAY be modified in output
     * @return  string  $output     Modified $input string
     */
    public function clean_input( $input, $safe_level = 0 ) {

        $output = $input;
        do {
            // Treat $input as buffer on each loop, faster than new var
            $input = $output;
            
            // Remove unwanted tags
            $output = $this->strip_tags( $input );
            $output = $this->strip_encoded_entities( $output );

            // Use 2nd input param if not empty or '0'
            if ( $safe_level !== 0 ) {
                $output = $this->strip_base64( $output );
            }

        } while ( $output !== $input );

        return $output;

    }

    /*
     * Focuses on stripping encoded entities
     * *** This appears to be why people use this sample code. Unclear how well Kses does this ***
     *
     * @param   string  $input  Content to be cleaned. It MAY be modified in output
     * @return  string  $input  Modified $input string
     */
    private function strip_encoded_entities( $input ) {

        // Fix &entity\n;
        $input = str_replace(array('&amp;','&lt;','&gt;'), array('&amp;amp;','&amp;lt;','&amp;gt;'), $input);
        $input = preg_replace('/(&#*\w+)[\x00-\x20]+;/u', '$1;', $input);
        $input = preg_replace('/(&#x*[0-9A-F]+);*/iu', '$1;', $input);
        $input = html_entity_decode($input, ENT_COMPAT, 'UTF-8');

        // Remove any attribute starting with "on" or xmlns
        $input = preg_replace('#(<[^>]+?[\x00-\x20"\'])(?:on|xmlns)[^>]*+[>\b]?#iu', '$1>', $input);

        // Remove javascript: and vbscript: protocols
        $input = preg_replace('#([a-z]*)[\x00-\x20]*=[\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', '$1=$2nojavascript...', $input);
        $input = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\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', '$1=$2novbscript...', $input);
        $input = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*-moz-binding[\x00-\x20]*:#u', '$1=$2nomozbinding...', $input);

        // Only works in IE: <span style="width: expression(alert('Ping!'));"></span>
        $input = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?expression[\x00-\x20]*\([^>]*+>#i', '$1>', $input);
        $input = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?behaviour[\x00-\x20]*\([^>]*+>#i', '$1>', $input);
        $input = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:*[^>]*+>#iu', '$1>', $input);

        return $input;

    }

    /*
     * Focuses on stripping unencoded HTML tags & namespaces
     *
     * @param   string  $input  Content to be cleaned. It MAY be modified in output
     * @return  string  $input  Modified $input string
     */
    private function strip_tags( $input ) {
        // Remove tags
        $input = preg_replace('#</*(?:applet|b(?:ase|gsound|link)|embed|frame(?:set)?|i(?:frame|layer)|l(?:ayer|ink)|meta|object|s(?:cript|tyle)|title|xml)[^>]*+>#i', '', $input);

        // Remove namespaced elements
        $input = preg_replace('#</*\w+:\w[^>]*+>#i', '', $input);

        return $input;

    }

    /*
     * Focuses on stripping entities from Base64 encoded strings
     *
     * NOT ENABLED by default!
     * To enable 2nd param of clean_input() can be set to anything other than 0 or '0':
     * ie: xssClean->clean_input( $input_string, 1 )
     *
     * @param   string  $input      Maybe Base64 encoded string
     * @return  string  $output     Modified & re-encoded $input string
     */
    private function strip_base64( $input ) {

        $decoded = base64_decode( $input );

        $decoded = $this->strip_tags( $decoded );
        $decoded = $this->strip_encoded_entities( $decoded );

        $output = base64_encode( $decoded );

        return $output;

    }

}

Hi,

I was testing your filter against a set of XSS test inputs. It seems that your filter is still vulnerable to XSS such as with inputs that contain XSS payloads in the comment-type, anchor and image tags. Examples of one of each are:

<!--#exec cmd=""/usr/X11R6/bin/xterm ?display 127.0.0.1:0 &""-->
<a href="jAvAsCrIpT&colon;alert&lpar;1&rpar;">X</a>
/><img/onerror=\x09javascript:alert(1)\x09src=xxx:x />

A full report can be read in our paper, "Assessment of Dynamic Open-source Cross-site Scripting Filters as Security Devices in Web Applications". I kindly suggest that you add these tags to the blacklist to make it more robust against XSS.

Thank you.

@nat4tq share your paper with us then :)

@nat4tq I tried the code you provided on the local server. This is really a problem. Do you have any solutions?

@nat4tq I tried the code you provided on the local server. This is really a problem. Do you have any solutions?

You could try this library : https://github.com/voku/anti-xss

My method is deprecated. I now recommend http://htmlpurifier.org/