Hacking Citrix:

Last Updated: Oct 2nd 9:00PM (Found 'Respond to Broad Cast Packets' to NOT break Ian Viteks scanner,
 so much for that!)

- Oct 1st, 2002 10:36AM (Minor Corrections in Introduction)

- Introduction
- How Citrix Works
- Peeling Citrix Back
- Securing Citrix... Sort of.

.Introduction

Citrix is a Remote Desktop application that is becoming widely popular. It is similar to Microsoft's
 Terminal Services, 
RDP (Remote Desktop Protocol). Unlike Terminal Services, Citrixs' lines of products allow the admini
strator to specify 
certain applications to be run on the server. This allows them to control which programs they want t
o allow the end user 
to execute. There exists an interesting gray line for the security of citrix applications due to the
 mixing of both 
citrix technology, and microsoft technology. With an application that allows users remote access to 
not only published 
programs, but remote desktops, a serious threat arises. Microsoft Terminal Services uses RDP, wherea
s Citrix uses ICA 
(Independent Computing Architecture). 

In this paper I will be outlining how citrix works, and how to take advantage of the way citrix hand
les user access to programs.

Corrections have been provided to me by Brian Madden (Author of Citrix MetaFrame XP: Advanced Techni
cal Design Guide.)
Note: I'm not a Citrix Administrator some of the descriptions of the product maybe faulty but this i
s not the main focus
of this paper. For corrections please email me at wirepair@roguemail.net

.How Citrix Works

There are multiple solutions that citrix has available and I will list them here. 

- Citrix MetaFrame
  Citrix MetaFrame has three different suits in its family; XPs, XPa, and XPe. Each come with a diff
erent set of tools and
applications to help the administrator. XPe is fully featured with many different management options
. XPa and XPe have
slightly less. For this paper I will be discussing just XPe, but most of these tactics also apply to
 the other versions.
Citrix uses XML over whatever port you want usually port 80 to begin communication. Citrix listens b
y default on port 
1494 and must be connected to using the Citrix ICA client which can be downloaded for free from thei
r web site.  

- Citrix NFuse/Citrix Secure Gateway
  Citrix NFuse allows the administrator to lock down applications even further by only allowing conn
ections in through
a web browser. NFuse itself sits on IIS 5.0 by default but I have seen papers detailing how to set i
t up on apache.
You probably understand the concerns regarding NFuse sitting on IIS by default and we will touch on 
that subject later.
All communication is done over SSL at 128-bit encryption, IF specified. 

A common setup with NFuse for remote access allows the administrator to also implement Citrix Secure
 Gateway. If they
properly configure NFuse, the remote user should not be able to directly access the Citrix Server it
self. Instead, 
all connections are filtered through the NFuse server through the Secure Gateway connection. Here's 
a diagram to make this 
a bit easier to understand.

[user]--->[router]--->[firewall]
                      | |  |
                      | |  +-[DMZ]-->[Citrix NFuse Server]
                      | |    | |            [Citrix Secure Gateway]
                      | +----+ |              |
                      |        +--------------+ 
                      | 
               [Citrix Server]  

I hope that makes more sense... and you can follow my horrible diagram I probably confused you more.
 
Now the user does not have a direct connection to the Citrix Server but instead follows through the 
DMZ -> internal network.
In a configuration with the default NFuse setup (without Secure Gateway) the user first hits the NFu
se web server, then
once the published application is established, the user makes a direct connection to the Citrix Serv
er. You can see the problem
with this, the user can still *directly* access the Citrix Server and after using the published appl
ication scanner,
gather their own list of published applications and then create their own .ica file containing the i
nformation they
specify. This also means, if this configuration is for an external network, two holes are punched th
rough the firewall,
one for the IIS server *shudder* and one for Citrix. You can scan port 1494 to see if this holds tru
e.

Citrix MetaFrame usually has a very helpful management console which enables the administrator to se
t users and publish
applications with relative ease. A citrix network without NFuse uses Independent Computing Architect
ure (.ICA) files
which is a text based document that contains all of the configuration options and settings. These fi
les are given to 
the end user who, after having the citrix client installed, clicks on the file and automatically con
nects to the server
specified in the .ica file. 

If NFuse is in use, then these .ICA files are usually not required. A user opens a web browser, goes
 to the NFuse web
server and logs in using his Domain/NT account. After this is successful the user is given a list of
 applications which
they may use to access the servers or Citrix Farm.

.Peeling Citrix Back
There are so many ways to get a remote desktop on a citrix host its impossible for me to know where 
to begin. I imagine 
the first thing people want is remote access so let us begin there. Recently Ian Vitek of ixsecurity
 has released
some very helpful perl tools. I use these a lot now and I must give him props on them. The tool I us
e most is
Citrix published application scanner [you can find these tools at: http://www.cqure.net/itools01.htm
l].
This tool enumerates the published applications that are allowed on the remote server. This comes in
 handy when you're 
'knocked out' of the regular login process, such as a script to automatically log a user out. After 
getting a list of 
the published applications, you can then modify your ICA file with the published application informa
tion.  At this point 
you can attempt to brute force login attempts, I personally find backup accounts to be very helpful.
  If you find a 'test' 
account or backup account has a guess able password you're in luck. I find variants of test and citr
ixtest almost always 
existent. 

Note: If the scanner does not work, you can try my tool which attempts to brute force the published 
applications.
http://sh0dan.org/files/pubappbrute.tar.gz

So lets assume at this point you have access to the citrix server. And lets suppose you are denied l
ogin, maybe guest
doesn't have enough privileges to follow through the login process. Now, if you used the citrix appl
ication scanner
and gathered a list of applications try this trick. I haven't seen this published and maybe this is 
a citrix flaw
or maybe its expected. But citrix appears to only look at the [Application Name] in the .ICA file wh
en verifying which
application to run. So here is a regular published application file:

[WFClient]
Version=2
TcpBrowserAddress=ip.ip.ip.ip

[ApplicationServers]
word=

[word]
Address=word
InitialProgram=#word
ClientAudio=Off
Compress=On
TWIMode=On
DesiredHRES=800
DesiredVRES=600
DesiredColor=4
TransportDriver=TCP/IP
WinStationDriver=ICA 3.0


So what happens if we modify the InitialProgram from #word to cmd.exe? or explorer.exe? Well, it hap
pily runs what
ever application we specify in that field. I've used this tactic numerous times to bypass faulty log
in processes. Just
make sure that the application in the brackets and the Address is legitimate otherwise you'll get so
me interesting
errors. So now we have a remote command prompt on the citrix server. Or if you like a gui and you de
cide you
want this after you run cmd.exe, no problem just type in explorer.exe you'll get a little prompt sta
ting that
you are now starting a seamless remote desktop session :). So now you have a desktop. I'd hope at th
is point you would
know enough to elevate your privileges so I will not cover that aspect. An interesting note, even wh
en I logged in as guest
to a citrix server, I was able to access the Administrative tools. Obviously this is rather alarming
 and to be honest
I myself was rather surprised. I'd suggest looking at the user accounts to see if you can find more 
accounts that would 
most likely have default/easily guessed passwords. Never underestimate the power of shitty password 
choosing. Just so 
you know you are not *really* administrator, just try changing a password.

So what if they are using NFuse with Citrix Secure Gateway and filtering all traffic through a firew
all? 
Then you can not modify your .ica file in this aspect. Once again there are multiple ways of getting
 a remote 
desktop. A lot of times citrix administrators have published applications such as office. What a won
derful piece of 
software. If you have access to excel, look into vbscript macros for starting explorer.exe.  Or if y
our lazy like me, 
just View as Web page. This will open Internet Explorer and allow you to execute C:\winnt\explorer.e
xe. This trick also 
works for Microsoft Word and I assume other office products. In fact, most programs have a help -> V
iew online help option.
So this will also execute Internet Explorer if it is set to the default browser. Another issue is 'w
inhelp32', most 
programs have this built in, and guess what? You can "Jump to URL." Now you can put cmd.exe on a rem
ote web server, jump
to the url and download it. If you have write access your set. Modify your .ica file to execute this
 upon logon and
you now have your shell. 

Here are some other ways of getting a remote desktop, most of these have been submitted to me by Ian
 Vitek so props to him.
By pressing Ctrl + F1 you are given the Windows Security window. From here you start Task Manager st
art -> new task -> 
explorer.exe or cmd.exe. Or you can be lazy and just press Ctrl + F3 and you will automatically get 
Task Manager.
This can be disabled under the Group Policy (Disable Task Manager) so it may not always work.  Most 
programs need to access 
files or have a 'Open' option for accessing their filetypes. Under Windows 2000 go to open a file, m
ove to My Computer, 
here you will get a list of your drives and the servers (if you have access.) Next right click on th
e drive, and select explore, 
once again we have a seemless desktop. Like I said there is a multitude of ways one can get a remote
 desktop and more importantly 
access to a command prompt.


If you compromise a machine that has citrix installed, and you have sufficient privileges, Citrix st
ores all of its connections
for the Citrix Program Neighborhood in a well known directory. 
- In NT4 browse to %systemroot%\profiles\username\Application Data\ 
- In Win2k browse to C:\Documents and Settings\username\Application Data\
steal the ICAClient directory and copy it to your %systemroot%\profiles\youruser\Application Data\ o
r
C:\Documents and Settings\profiles\youruser\Application Data\ and then open Citrix Program Neighborh
ood. You'll notice 
all of that users connections. If they are using Citrix chances are they are lazy as well and most l
ikely cache their 
passwords.  Use Revelation or another 'password reveler' and viola you have their password for that 
connection.

Sniffing is yet another concern, dsniff has citrix sniffing capabilities and this may become a real 
problem when
used in conjunction with arp cache poisoning. 

.Securing Citrix

First off I'd like to say good luck. Here are some tips that may help an administrator locking down 
a Citrix Server.
I spent the last few days attempting to come up with a magical Group Policy for windows that will lo
ck programs down.
The goal is to make it so a user may not use any programs which may be used to upload or transfer fi
les to the citrix host.
- Securing Externally
1. Use the NFuse / Citrix Secure Gateway configuration such as the one I displayed earlier in this t
ext.
2. Make sure the IIS/Apache server is completely locked down and resides in the DMZ. Require .htacce
ss
or NTLM authentication to even access the webroot of the NFuse server.
3. If possible, require remote users to use SecureID for authentication. This gets pricy so is most 
likely
not going to be an option for most people.
4. Use a different browser than Internet Explorer per default. Most applications use the "default br
owser" for online
help pages. If you install Netscape and set this as the default browser they will not be able to exe
cute applications
such as cmd.exe/explorer.exe.
5. Make a group and put all remote citrix users in this group. Deny access (using ntfs) to cmd.exe, 
ftp.exe, tftp.exe, rcp.exe, net.exe,
command.com, iexplorer.exe (if not a published app or required) and any other programs that they may
 execute to transfer files to the 
citrix server. I did not test whether or not disallowing cmd.exe may break login scripts so be sure 
to test this before it is put in to production.
6. Keep up to date on all patches for your citrix server, If you keep up to date on your patching yo
u'll not need to worry about
privilege escalation exploitation. 
7. Since there are ways to bypass the 'default browser' such as winhelp32. You can attempt to set se
curity settings under
internet options to restrict to only allow certain web pages. This will hopefully disallow someone f
rom downloading their
own cmd.exe. 
8. Although I've not attempted this and it may only work in certain situations. Disallow write acces
s to the citrix
server itself. This way they can not upload their own files. This may only be possible to achieve if
 you also disallow
users mapping their drives to the server. Remember, users can map their drives and run their tools f
rom their local machine.
I'm quite sure this runs under the citrix localsystem context, I may be incorrect on this point.
9. If possible, disallow users from mapping their drives. You can do this via the Citrix Connection 
Configuration
double click on ica-tcp, select 'client settings.' And select the necessary boxes.
10. Under Group Policy disable access to TaskMgr.exe.


ADDITIONS:
The following fixes have been provided to me by Brian Madden. One of these fixes will disallow the c
hanging of the
InitialProgram. I thank Brian again for bringing these to my attention. 

FIX #1 DOES NOT PREVENT THE SCANNER FROM WORKING! Sorry, I should have tested this before I posted..
. (XPs FR2 was tested)
1. Disable MetaFrame XP server broadcast response. CMC | Right-click on farm | MetaFrame Settings ta
b | Uncheck the 
two boxes in the "Broadcast Response" section. This will prevent that perl scanner from working.
 
2. Configure your MetaFrame XP servers so that users can only run published applications. 
(Citrix Connection Configuration | double-click "ica-tcp" | Advanced button | Check "Only run publis
hed applications"
 in the "Initial Program" section). I'd suggest first adding a published application for 'remote des
ktop' and only
allow administrators to use this feature.  

3. Ian Vitek suggested a product called SecurEXE, This looks very promising and if I have time to te
st this out
I will post my results. After looking at their live demo (they let you terminal services in and play
 around),
I am simply amazed, I could barely get anywhere, all I could do is right click on an exe and try to 
'run as' a user
and them attempt to brute force the Administrator password. Other than that I was completely locked 
out. I have
yet to test this in a lab, but from that demo I must admit I'm very impressed.
Here is the URL for SecurEXE: http://www.securewave.com/products/secureexe/secure_exe.html 

- Securing Internally
Try using the same steps as listed for external, if you can segment your network access via a firewa
ll + citrix 
secure gateway you are much better off then leaving it on the same segment. Other than that I know o
f no real
good procedures for securing internally.



.Questions Comments?
This is my first paper regarding Citrix and most likely as I play with it more I'll add more additio
ns. Feel free
to email me at wirepair@roguemail.net. Disregard the lame banner underneath my emails, its free, and
 its email...

Hope this helps some people realize the serious flaws with using citrix. It's a good idea, just not 
when used in
conjunction with Microsoft Windows :(.

.Editorial Note:
After attempting to contact Citrix regarding the cmd.exe insert flaw, they told me this rather amusi
ng story.
Since I do not have a tech support agreement with citrix, it would cost me 400$ per incident to repo
rt this.
I was rather astounded and made quite sure this was true. Here is the dialogue:

citrix: We are sorry sir, you do not currently have a tech support agreement with us. Without one, c
ontacting
tech support to report this issue will cost you 400$, would you like to charge this to a credit card
?

me: Wait, so let me get this straight, I pay you 400$ to do you a favor by telling you, you have a p
ossible security
flaw with your program?

citrix: Yes *nervous giggle* we are sorry but it is the only way to contact tech support, they won't
 even let me transfer
you over.

me: Ok so I have to release this to everyone and have your engineers find and possibly scramble to f
ix this issue?

citrix: Yes *nervous giggle again* *mentions something about going through my reseller*

me: Ok looks like I'll release it, Thank you.