Microsoft Terminal Server Client Buffer Overrun (A082802-1)

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                           @stake, Inc.
                        www.atstake.com

                       Security Advisory

Advisory Name: Microsoft Terminal Server Client Buffer Overrun
Release Date: 08/28/2002
Application: Microsoft Terminal Server ActiveX Client v5.02221.1
Platform: Windows 95/98/NT 4.0/2000/XP
Severity: There is a buffer overflow condition that
can result in execution of arbitrary
code.
Author: Ollie Whitehouse [[email protected]]
Vendor Status: Vendor has bulletin and patch
CVE Candidate: CAN-2002-0726
Reference: www.atstake.com/research/advisories/2002/a082802-1.txt

Overview:

Microsoft Terminal Server ActiveX client (http://www.microsoft.com
/windows2000/downloads/recommended/default.asp) is the ActiveX version
of the standard Windows Terminal Services client. It allows a client
to connect to a a Terminal Server from a web page. This allows a web
developer to integrate a Win32-based application into a web page.

There is a buffer overrun vulnerability in one of the parameters used
by the ActiveX component when it is embedded in a web page. An attacker
could exploit this vulnerability to run malicious code on a target
system. The user would need to open a malicious HTML file as
an attachment to an email message, as a file on the local or network
file system, or as a link on a malicious web site. If the malicious
HTML file is opened it will cause the Active X component to execute
the arbitrary computer code contained within the HTML page with the
permissions of the attacker.

Since the Microsoft Terminal Server ActiveX client is signed by
Microsoft and marked safe there is no warning with the default Internet
Explorer security settings if you have previously selected to trust
all controls signed by Microsoft. This is a good example of why not to
trust any ActiveX components from an unknown source. A malicious site
could use an old vulnerable version of the ActiveX control even after
the patched ActiveX component is available from Microsoft. If users
install the latest vendor cumlative patch for Internet Explorer
this problem is eliminated.

Details:

By default the Terminal Server ActiveX client will install itself in a
directory such as 'http://site/tsweb/'. The buffer overrun condition
occurs when a large string is used for the server name field. We were
able to cause an exception to occur with a long string made up of the
letter 'A'. The result was the over writing of EIP with 0x41414141.
ESI will point the buffer of supplied data.

The ID of the component tested was: 1FB464C8-09BB-4017-A2F5-EB742F04392F

Vendor Response:

 Vendor has bulletin and patch for Terminal Server. 
 http://www.microsoft.com/technet/security/bulletin/ms02-046.asp

 Vendor has bulletin and patch for Internet Explorer
 http://www.microsoft.com/technet/security/bulletin/MS02-047.asp

Recommendation:

 You should never open attachments/webpages that come from 

unknown sources no matter how benign they may appear. Be wary of those
that come from known sources.

You should consider the benefits and risks of each attachment file
type or ActiveX control that you let into your organization. Attachment
file types or ActiveX controls that you do not need should be dropped
at your perimeter mail gateway or proxy server. Attachments that you
choose to forward on into your organization should be scanned for
known malicious code using a antivirus product.

End users should install the latest Internet Explorer cumulative patch
which sets the Kill Bit on the vulnerable version of the ActiveX
component so it will not execute.

Terminal Server administrators should install the vendor patch to
update the ActiveX component they have available for download. Until
this patch is installed users who have installed the Internet
Explorer cumulative patch will not be able to access the Terminal
Server via the ActiveX component.

Common Vulnerabilities and Exposures (CVE) Information:

The Common Vulnerabilities and Exposures (CVE) project has
assigned the following names to these issues. These are candidates for
inclusion in the CVE list (http://cve.mitre.org), which standardizes
names for security problems.

CAN-2002-0726 Terminal Server ActiveX Client Buffer Overrun

@stake Vulnerability Reporting Policy:
http://www.atstake.com/research/policy/

@stake Advisory Archive:
http://www.atstake.com/research/advisories/

PGP Key:
http://www.atstake.com/research/pgp_key.asc

Copyright 2002 @stake, Inc. All rights reserved.

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