Advisory: Cross-site Scripting in Tapatalk Plugin for WoltLab Burning Board 4.0

RedTeam Pentesting discovered a cross-site scripting (XSS) vulnerability
in the Tapatalk plugin for the WoltLab Burning Board forum software,
which allows attackers to inject arbitrary JavaScript code via URL
parameters.

### Details

- Product: Tapatalk Plugin com.tapatalk.wbb4 for WoltLab Burning Board 4.0
- Affected Versions: \>= 1.0.0
- Fixed Versions: 1.1.2
- Vulnerability Type: Cross-Site Scripting
- Security Risk: high
- Vendor URL: `https://tapatalk.com`
- Vendor Status: fixed version released
- Advisory URL: `https://www.redteam-pentesting.de/advisories/rt-sa-2014-015`
- Advisory Status: published
- CVE: CVE-2014-8869
- CVE URL: `https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-8869`

### Introduction

"Tapatalk is an app built for interacting with discussion forums on
mobile devices. It differs from a forum’s mobile web skin in that it
offers the speed of a native app and a streamlined unified interface for
every forum a user subscribes to. Tapatalk also creates a unique
eco-system that allows forums to be searched and discovered by millions
of Tapatalk users which in turn promotes content, new memberships, and
interactions."

(from Tapatalk's Homepage)

### More Details

The Tapatalk extension includes the PHP script welcome.php at the path

com.tapatalk.wbb4/files/mobiquo/smartbanner/welcome.php

which is accessible via the URL

`http://www.example.com/mobiquo/smartbanner/welcome.php`

on systems using the plugin. It outputs JavaScript code that includes
improperly encoded values from the two URL parameters "app_android_id"
and "app_kindle_url". Depending on which parameters is used, one of
their values is assigned to the PHP variable \$byo:

``` 
<?php
[...]
else if (isset($_GET['app_android_id']))
{
  $app_android_id = $_GET['app_android_id'];
  if ($app_android_id && $app_android_id != '-1')
    $byo = "&app_android_id=$app_android_id";
}
else if (isset($_GET['app_kindle_url']))
{
  $app_kindle_url = $_GET['app_kindle_url'];
  if ($app_kindle_url && $app_kindle_url != '-1')
    $byo = "&app_kindle_url=$app_kindle_url";
}
```

Later the \$byo variable is used to build a URL without URL encoding it
and the URL is used without further encoding in a script element:

``` 
<?php
[...]
$ads_url = $protocol.'tapatalk.com/welcome_screen.php'
    .'?referer='.urlencode($referer)
    .'&code='.urlencode($code)
    .'&board_url='.urlencode($board_url)
    .'&lang='.urlencode($lang)
    .$byo
    .'&callback=?';
[...]
?>[...]

<script>$.getJSON("<?php echo $ads_url; ?>",function(data){
[...]
```

### Proof of Concept

The following URL can be used to demonstrate the vulnerability:

`http://www.example.com/mobiquo/smartbanner/welcome.php`  
?app_kindle_url=");alert('RedTeam Pentesting');\</script\>\<!--

The result is a notification showing the text "RedTeam Pentesting".

### Workaround

The PHP function urlencode() should be used to encode the \$byo variable
before building a URL with it.

### Fix

Update the plugin to version 1.1.2.

### Security Risk

This security vulnerability is rated as a high risk. It allows to
execute arbitrary JavaScript code in users' browsers if they access URLs
prepared by attackers. This provides many different possibilities for
further attacks against these users. Since the plugin is used for a
bulletin board, the vulnerability could be exploited to display a fake
login page and obtain credentials from users or administrators. The
vulnerability also affects other web applications hosted on the same
domain.

### Timeline

- 2014-10-20 Vulnerability identified
- 2014-10-29 CVE number requested
- 2014-11-14 CVE number assigned
- 2014-11-26 Vendor notified via `https://tapatalk.com/security.php`
- 2014-12-16 Vendor notified again, received reply from vendor
- 2014-12-16 Vulnerability patched in SCM  (<https://github.com/tapatalk/tapatalk-wbb/commit/71024545904024cea9d04a887fdc64b9a9b85871>)
- 2014-12-23 Updated plugin released by vendor  (<https://github.com/tapatalk/tapatalk-wbb/commit/31472f6fcfffacd698b0c20809c4a8fb3c4f32f9>)
- 2015-01-08 Vendor updated release notes to mention XSS (<https://support.tapatalk.com/threads/19540/#post-146253>)
- 2015-01-12 Advisory released

### RedTeam Pentesting GmbH

RedTeam Pentesting offers individual penetration tests, short pentests,
performed by a team of specialised IT-security experts. Hereby, security
weaknesses in company networks or products are uncovered and can be
fixed immediately.

As there are only few experts in this field, RedTeam Pentesting wants to
share its knowledge and enhance the public knowledge with research in
security-related areas. The results are made available as public
security advisories.

More information about RedTeam Pentesting can be found at
`https://www.redteam-pentesting.de`.