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@mbijon
Last active November 1, 2022 03:23
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XSS filtering in PHP (cleans various UTF encodings & nested exploits)
<?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;
}
}
@rudSarkar
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pice of code ;)

@nat4tq
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nat4tq commented Feb 8, 2018

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.

@rusyasoft
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@nat4tq share your paper with us then :)

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

@voku
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voku commented Oct 31, 2022

@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

@mbijon
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mbijon commented Nov 1, 2022

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

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