Public Vulnerability Reports

Citrix ICA Client ActiveX Memory Corruption Vulnerabillity

03.08.10

BACKGROUND

The Citrix ICA Client is an ActiveX control used to connect to a Citrix XenApp (formerly Citrix Presentation Server) server via the browser. Using this control allows clients to access the various features provided by the application server, such as virtualized applications. For more information, see the vendor's site found at the following link.

http://www.citrix.com/English/ss/downloads/results.asp?productID=186&c1=sot2755

DESCRIPTION

Remote exploitation of a memory corruption vulnerability in Citrix Systems Inc.'s ICA Client could allow an attacker to execute arbitrary code with the privileges of the current user.

The vulnerability occurs in the IICAClient interface within the ICAClient library in the control. This is the primary interface used to implement the client's functionality.

The vulnerability occurs in a certain function, which is accessible through JavaScript. This function is responsible for reading a remote ICA (Independent Computing Architecture) file from the server. Typically, the contents of this file are used to provide options controlling the connection to the application gateway.

During the reading of this file, the vulnerable function fails to check the return value of the realloc() function. It then adds the number of bytes read from the file so far to the value returned, and appends file data at this address. If this allocation fails, a NULL pointer (the value 0) will be returned. This results in "0 + the number of bytes read so far" being used as the destination address for writing file data. This can lead to the corruption of memory at that address, which results in the execution of arbitrary code.

ANALYSIS

Exploitation of this vulnerability results in the execution of arbitrary code with the privileges of the user viewing the web page. To exploit this vulnerability, a targeted user must load a malicious webpage created by an attacker. An attacker typically accomplishes this via social engineering or injecting content into compromised, trusted sites. After the user visits the malicious web page, no further user interaction is needed.

DETECTION

iDefense has confirmed the existence of this vulnerability in ICA Client versions 12.0.0.6410 and 11.2.0.31560. All versions of the Online Plug-in for Windows up to version 12.0.3.

WORKAROUND

Setting the KillBit for the CLSID containing the vulnerable control will prevent exploitation of this vulnerability. However, it will also prevent legitimate use of the control. The CLSID is:

238F6F80-B8B4-11CF-8771-00A024541EE3

VENDOR RESPONSE

Citrix has released a patch which addresses this issue. Information about downloadable vendor updates can be found by clicking on the URLs shown.

http://citrix.com/English/ss/downloads/details.asp?downlaodld=2301299&productId=186

CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not been assigned yet.

DISCLOSURE TIMELINE

04/06/2010 Initial Contact
04/07/2010 Initial Response
08/03/2010 Coordinated public disclosure.

CREDIT

This vulnerability was discovered by Sean Larsson, iDefense Labs.

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LEGAL NOTICES

Copyright © 2010 Verisign, Inc.

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Disclaimer: The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.