[THIN] FW: Hacking Citrix Faq
- From: "Sullivan, Glenn" <GSullivan@xxxxxxxxxxxxxx>
- To: "'thin@xxxxxxxxxxxxx'" <thin@xxxxxxxxxxxxx>
- Date: Fri, 27 Sep 2002 14:40:11 -0400
Got this from BugTraq... Thought y'all might want to see what the "other
side" has up their sleeve.
Glenn Sullivan, MCSE+I MCDBA
David Clark Company Inc.
<-------Forwarded Email--------->
Sure, I must admit I forget which list you moderate. If
you would like you can just forward this message, Thanks.
Hacking Citrix:
- 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 built off of Microsoft's
Terminal Services RDP (Remote Desktop Protocol). Unlike
Terminal Services, Citrixs' lines of products allow the
administrator to specify certain applications to be run on
the server. This allows them to control which programs
they want to 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.
In this paper I will be outlining how citrix works, and
how to take advantage of the way citrix handles user
access to programs.
Corrections have been provided to me by Brian Madden
(Author of Citrix MetaFrame XP: Advanced Technical Design
Guide.)
Note: I'm not a Citrix Administrator some of the
descriptions of the product maybe faulty but this is not
the main focus
of this paper. For corrections please email me at
wirepair@xxxxxxxxxxxxx
.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 different 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 by default on
port
1494 and must be connected to using the Citrix ICA client
which can be downloaded for free from their web site.
- Citrix NFuse/Citrix Secure Gateway
Citrix NFuse allows the administrator to lock down
applications even further by only allowing connections in
through
a web browser. NFuse itself sits on IIS 5.0 by default but
I have seen papers detailing how to set it 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 itself. 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 NFuse web server,
then
once the published application is established, the user
makes a direct connection to the Citrix Server. You can
see the problem
with this, the user can still *directly* access the Citrix
Server and after using the published application scanner,
gather their own list of published applications and then
create their own .ica file containing the information they
specify. This also means, if this configuration is for an
external network, two holes are punched through the
firewall,
one for the IIS server *shudder* and one for Citrix. You
can scan port 1494 to see if this holds true.
Citrix MetaFrame usually has a very helpful management
console which enables the administrator to set users and
publish
applications with relative ease. A citrix network without
NFuse uses Independent Computing Architecture (.ICA) files
which is a text based document that contains all of the
configuration options and settings. These files are given
to
the end user who, after having the citrix client
installed, clicks on the file and automatically connects
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 use most is
Citrix published application scanner [you can find these
tools at:
http://www.securiteam.com/exploits/5CP0B1F80S.html].
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 information. 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 citrixtest almost
always
existent.
So lets assume at this point you have access to the citrix
server. And lets suppose you are denied login, maybe guest
doesn't have enough privileges to follow through the login
process. Now, if you used the citrix application 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 when verifying
which
application to run. So here is a regular published
application file:
[WFClinet]
Version=2
TcpBrowserAddress=ip.ip.ip.ip
[ApplicationServers]
word=
[word]
Address=word
InitialProgram=#word
TransportDriver=TCP/IP
WinStationDriver=ICA 3.0
DesiredHRES=800
DesiredVRES=600
So what happens if we modify the InitialProgram from #word
to cmd.exe? or explorer.exe? Well, it happily runs what
ever application we specify in that field. I've used this
tactic numerous times to bypass faulty login processes.
Just
make sure that the application in the brackets and the
Address is legitimate otherwise you'll get some
interesting
errors. So now we have a remote command prompt on the
citrix server. Or if you like a gui and you decide you
want this after you run cmd.exe, no problem just type in
explorer.exe you'll get a little prompt stating that
you are now starting a seamless remote desktop session :).
So now you have a desktop. I'd hope at this point you
would
know enough to elevate your privileges so I will not cover
that aspect. An interesting note, even when 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 firewall?
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 wonderful
piece of
software. If you have access to excel, look into vbscript
macros for starting explorer.exe. Or if your lazy like
me,
just View as Web page. This will open Internet Explorer
and allow you to execute C:\winnt\explorer.exe. This trick
also
works for Microsoft Word and I assume other office
products. In fact, most programs have a help -> View
online help option.
So this will also execute Internet Explorer if it is set
to the default browser. Another issue is 'winhelp32', most
programs have this built in, and guess what? You can "Jump
to URL." Now you can put cmd.exe on a remote 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.
If you compromise a machine that has citrix installed, and
you have sufficient privileges, Citrix stores 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\ or
C:\Documents and Settings\profiles\youruser\Application
Data\ and then open Citrix Program Neighborhood. You'll
notice
all of that users connections. If they are using Citrix
chances are they are lazy as well and most likely 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 lock programs
down.
The goal is to make it so a user may not use any programs
which may be used to upload or transfer files to the
citrix host.
- Securing Externally
1. Use the NFuse / Citrix Secure Gateway configuration
such as the one I displayed earlier in this text.
2. Make sure the IIS/Apache server is completely locked
down and resides in the DMZ. Require .htaccess
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 browser" for
online
help pages. If you install Netscape and set this as the
default browser they will not be able to execute
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
and any other programs that they may execute to transfer
files to the citrix server. I did not test whether or not
this 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 you'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 security
settings under
internet options to restrict to only allow certain web
pages. This will hopefully disallow someone from
downloading their
own cmd.exe.
8. Although I've not attempted this and it may only work
in certain situations. Disallow write access 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 from 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.
ADDITIONS:
The following fixes have been provided to me by Brian
Madden. One of these fixes will disallow the changing of
the
InitialProgram. I thank Brian again for bringing these to
my attention.
1. Disable MetaFrame XP server broadcast response. CMC |
Right-click on farm | MetaFrame Settings tab | 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 published
applications"
in the "Initial Program" section). I'd suggest first
adding a published application for 'remote desktop' and
only
allow administrators to use this feature.
3. This exploit does work on MetaFrame XP with Service
Pack 2 and Feature Release 2. (The most recent version)
- Securing Internally
Try using the same steps as listed for external, if you
can segment your network access via a firewall + citrix
secure gateway you are much better off then leaving it on
the same segment. Other than that I know of 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 additions. Feel free
to email me at wirepair@xxxxxxxxxxxxxx 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 amusing story.
Since I do not have a tech support agreement with citrix,
it would cost me 400$ per incident to report 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, contacting
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 possible
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 fix this issue?
citrix: Yes *nervous giggle again* *mentions something
about going through my reseller*
me: Ok looks like I'll release it, Thank you.
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