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| A security vulnerability in the Citrix client results in attackers being able to perform any possible action on the client machine, including reading any file, placing Trojan code or altering data. |
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Credit:
The information has been provided by Kikkert Security.
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Vulnerable systems:
Citrix client version 6.01
Citrix produces clients that 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.
An implementation flaw exists in the Citrix client that allows a malicious web site owner to perform virtually 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) and who surfs the internet on the same machine is in danger of exploitation.
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.
This results in 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.
Example:
<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 prompted that the remote server is writing anything to the client's hard drive. Strange enough, the ActiveX client that was tested does ask the user for permission before the published application can write to the client drive (this is 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.
Vendor status:
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.
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
* In Internet Explorer, the File Download permission can be disabled. This would avoid the exploit in the form described.
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.
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