Kikkert Security Advisory: Potentially serious security flaw in Citrix Client

["Kikkert Security" <unhackables () hotmail com>]

Dear List,

This 'Kikkert Security advisory' has been released after carefull 
consideration and after advising 'Citrix' first. Citrix was initially 
willing to communicate but hasn't responded to any of my emails for the last 
two months. Because there are workarounds for the problem discribed in this 
advisory I decided to release it so people might benefit from these possible 
fixes.
I would like to ask the list to examine the scope of products and OS's 
affected as I have no longer access to a Citrix server to do this myself...

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Serious security flaw in Citrix Client

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Risk: HIGH.
Potentially allowing any possible action on the client machine, including 
reading any file, placing Trojan code or altering data.

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Scope:
Not completely clear. for sure is that Citrix client 6.01 is affected. 
Citrix clients on Apple and MAc seem to be ok, only the Microsoft version is 
affected (according to Citrix, I did not test this). This exploit was tested 
on the following setup:

- Windows 2000 professional + Service pack 2
- Internet explorer 5.5 + SP1 , Q290108, Q299618 (5.50.4522.1800)
- Outlook Express 5.50.4522.1200
- office 2000 SR1
- Citrix ICA client 6.01

Prerequisites: Citrix Client installed (standard install), Internet 
connection with port 1494 open (Citrix port, outbound), Browser or HTML 
email client, windows OS (according to Citrix).

----------------
Background:

Citrix produces Clients which can connect to a terminal server to run thin 
client sessions. A popular use of Citrix client / server is the use of 
published applications that enables thin clients to run 'heavy' 
applications.

A implementation flaw exists in the Citrix client which allows a malicious 
web site owner to perform virtualy any action on the client machine without 
informing the user first or without explicit consent from the user.
This means that anyone with the citrix client installed (and probably with 
IE installed, not sure what the scope is) and who surfs the internet on the 
same machine is in danger of exploitation.

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Technical Details:

When a user has Citrix Client installed and has therefore an extension 
mapping for .ICA files, the user will NOT be warned when downloading an .ica 
file. The user is NOT asked to open or download the file, the ica file will 
just activate the Citrix client and a connection to a remote server can be 
made.

result of this is that any malicious website owner (with access to a Citrix 
terminal server) can place trojan code on a client machine without consent 
of the client.

I created a working demo in the form of a webpage which simply contains an 
Iframe (could also be a hidden frame):

<iframe src="trojan.ica"></iframe>

Trojan.ica will connect to a published application (hosted on a Citrix 
Metaframe XP server) without first asking the user and place a (fake) trojan 
file on the clients' hard drive.
The published application is simply a VBS script that copies the trojan file 
from the local (terminal server's) hard drive to the (mapped) client drive.
After the script ran, the connection to the remote server will be broken.
The client is not in any way warned or promted that the remote server is 
writing anything to the clients hard drive.
Strange enough, the activeX client I tested DOES ask the user for permission 
before the published application can write to the client drive, this is in 
my opinion the way it should work.
Just to make it clear, the malicious website owner can not only write to the 
client, he can also retrieve a complete listing of any file on the machine 
or copy any file/document from the client's machine.


-------------------
Disclosure details:

Citrix was contacted on the 23rd of July and did not take this very serious 
at first. They mentioned that this was a known issue and did not give me the 
idea that they were actively working on a fix.
It is now almost 4 months after I first notified them and they still cannot 
give me a clear indication on what they are planning to do about this. They 
did however give me a few 'workarounds' which are mentioned below. I'm not 
sure how effective these workarounds are as I did not have the opportunity 
to test them in a live environment.

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Possible fixes (as given by Citrix):

* The Citrix ICA Clients for Apple Macintosh and for Unix have
explicit drive mapping dialogs which control client drive mapping, and also
allow read/write selection. Therefore, these clients will only be attacked
if such drive mappings are configured.

* When using the ICA Client for Java, you can set Java security to
prevent file access by Java applications. This will prevent disk access.

* Client Drive Mapping can be disabled in APPSRV.INI by adding the
setting: CDMAllowed=Off  -------[michiel] - Bit of a drastic solution, as 
this just disables the feature.

* In Internet Explorer, the File Download permission can be disabled.
This would avoid the exploit in the form described. ---- [Michiel] - But 
would still be exploitable via email client

And a Microsoft's recommended workaround for Outlook:

it's possible to configure the OESU (Outlook Security Update) to block 
additional file types, including .ICA.





Kind Regards,

Michiel Kikkert - security () kikkert nl
Kikkert security.














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