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... ----------- Serious security flaw in Citrix Client ----------- Risk: HIGH. Potentially allowing any possible action on the client machine, including reading any file, placing Trojan code or altering data. --------- 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. ------------------- 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. ------------- 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 - securityat_private Kikkert security. _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.
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