Paper: How to Fix a XSS Vulnerability in PHP Source Codes

Links:
http://www.gnucitizen.org/xssdb/application.htm (Attack Database)
http://www.xssed.com (Mirror Archive of Vulnerable Websites)
http://ha.ckers.org/xss.html (XSS Cheat sheet)
http://software.graflex.org/dexss/ (Removing JavaScript from HTML)

http://en.wikipedia.org/wiki/Cross-site_scripting

http://it.php.net/htmlentities
http://it2.php.net/htmlspecialchars
http://it2.php.net/strip_tags
-------------------------------

For fix the problem of cross site injection we have to use one of the 3 functions php.

These functions clean up the HTML tags, so is not possible inject into the code.

The function more used is htmlspecialchars() that transmutes all the characters "<" and ">" into "&lt;" and "&gt".

Another option is htmlentities(), which replaces all the characters in the corresponding entities.

PHP Code:

<?
// This page shows an example
// of the differences in output between 2 functions

$input = '<script>alert(1);</script>';

echo htmlspecialchars($input) . '<br />';
echo htmlentities($input);

?>

An example of htmlentities()

PHP Code:

<?php
$str = "A 'quote' is <b>bold</b>";

echo htmlentities($str);
echo htmlentities($str, ENT_QUOTES);
?>

The first show --> A 'quote' is &lt;b&gt;bold&lt;/b&gt;
The second --> A &#039;quote&#039; is &lt;b&gt;bold&lt;/b&gt;

An example of htmlspecialchars()

PHP Code:

<?php
$new = htmlspecialchars("<a href='test'>Test</a>", ENT_QUOTES);
echo $new;
?>

This show --> &lt;a href='test'&gt;Test&lt;/a&gt;

The funztion strip_tags(), instead, deletes all HTML elements, except certain elements that need to specify permitted such as <i>, <b> or <p>.

An example of strip_tags()

PHP Code:

<?php
$text = '<p>Test paragraph.</p><!-- Comment --> Other text';
echo strip_tags($text);

echo "\n";
// allow <p>
echo strip_tags($text, '<p>');
?>

Now that we know at least that there are these functions, we will to apply into the code when we find a xss in our web application.

I have recently found a xss on my website in Video section of GoogleBig which is a plugin of Mybb forum, I have placed a piece of code to make the idea of how I had to apply the function to fix the search bug.

First of all I have found the php page in question: search.php

Now let's look for the portion of code that makes available research, query and output the result of the query:

PHP Code:

function search($query, $page)

{

    global $db, $bgcolor2, $bgcolor4, $sitename, $io_db, $module_url, $list_page_items, $hm_index;

    $option = trim($option);

    $query = trim($query);

    $query = FixQuotes(nl2br(filter_text($query)));

    $db->escape_string($query);

    $db->escape_string($option);

        alpha_search($query);
    ...


In this case the variable that passes the values is $query then we apply the function htmlentities():

PHP Code:

    $query = FixQuotes(nl2br(filter_text(htmlentities($query))));


If you have problems you can post here, or consult the manuals on these 3 php functions that we saw:

http://it.php.net/htmlentities
http://it2.php.net/htmlspecialchars
http://it2.php.net/strip_tags

Posted in: XSS