[NT] Flaw in Windows Script Engine Could Allow Code Execution
From: email@example.com To: firstname.lastname@example.org Date: 23 Mar 2003 12:38:20 +0200
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Flaw in Windows Script Engine Could Allow Code Execution
The Windows Script Engine provides Windows operating systems with the
ability to execute script code. Script code can be used to add
functionality to web pages, or to automate tasks within the operating
system or within a program. Script code can be written in several
different scripting languages, such as Visual Basic Script, or JScript.
A flaw exists in the way by which the Windows Script Engine for JScript
processes information. An attacker could exploit the vulnerability by
constructing a web page that, when visited by the user, would execute code
of the attacker's choice with the user's privileges. The web page could be
hosted on a web site, or sent directly to the user in email.
Although Microsoft has supplied a patch for this vulnerability and
recommends all affected customers install the patch immediately,
additional preventive measures have been provided so that customers can
use to help block the exploitation of this vulnerability while they are
assessing the impact and compatibility of the patch. These temporary
workarounds are discussed in the "Workarounds" section in the FAQ below.
* Microsoft Windows 98
* Microsoft Windows 98 Second Edition
* Microsoft Windows Me
* Microsoft Windows NT 4.0
* Microsoft Windows NT 4.0 Terminal Server Edition
* Microsoft Windows 2000
* Microsoft Windows XP
* For an attack to be successful, the user would need to visit a website
under the attacker's control or receive an HTML e-mail from the attacker.
* Computers configured to disable active scripting in Internet Explorer
are not susceptible to this issue.
* Exploiting the vulnerability would allow the attacker only the same
privileges as the user. Users whose accounts are configured to have few
privileges on the system would be at less risk than ones who operate with
* Automatic exploitation of the vulnerability by an HTML email would be
blocked by Outlook Express 6.0 and Outlook 2002 in their default
configurations, and by Outlook 98 and 2000 if used in conjunction with the
Outlook Email Security Update.
Download locations for this patch
The patches for all Windows systems are available via Windows Update. In
addition, these patches are also available for download to allow the
patches to be manually installed.
* Windows 98 and Windows 98 SE:
* Windows Me:
* Windows Update.
* Windows NT 4.0:
* Windows NT 4.0, Terminal Server Edition:
* Windows 2000:
* Windows XP Home Edition and Professional Edition:
What's the scope of the vulnerability?
This is a buffer overrun vulnerability. An attacker who successfully
exploited this vulnerability could cause code of his or her choice to be
executed as though it originated on the local machine.
What causes the vulnerability?
The vulnerability is caused by a heap overflow in the Windows Script
Engine for the JScript scripting language, JScript.dll.
What is a scripting language?
Scripting languages can be used to add additional functionality to HTML
web pages or operating systems. They can enable a web author to set and
store variables, and work with data in the HTML code. For instance, a
script can be used to check the version of the web browser a user is
running, validate input, work with applets or controls, and communicate to
In addition, scripts can be used in Windows to automate operating system
tasks such as changing settings or mapping a network drive.
What is a scripting engine?
The Windows Scripting Engine serves as the component within Windows that
interprets and executes script code written in scripting languages such as
JScript or VBscript.
What is JScript?
JScript is the Microsoft implementation of the ECMA 262 language
specification (ECMAScript Edition 3).
It is an interpreted, object-based scripting language. In general, JScript
has fewer capabilities than full-fledged object-oriented languages like
C++. Stand-alone applications cannot be written in JScript, for example.
JScript scripts can run only in the presence of an interpreter or "host",
such as Active Server Pages (ASP), Internet Explorer, or Windows Script
What's wrong with the Windows Script Engine for JScript?
There is a flaw in the way the JScript scripting engine processes the
script. It does not correctly size a buffer during a memory operation.
What could this vulnerability enable an attacker to do?
This vulnerability could enable an attacker to cause code of the
attacker's choice to run with user privileges on the system.
If I am not using Internet Explorer do I need the patch?
Yes. The vulnerability exists in the Windows Script Engine. Microsoft
recommends all customers install the patch immediately.
How could an attacker exploit this vulnerability?
The attacker would need to construct a web page that contained specially
formed script code. The attack could then proceed via either of two
vectors. In the first, the attacker could host the web page on a web site;
when a user visited the site, the web page could launch the script and
exploit the vulnerability. In the second, the attacker could send the web
page as an HTML mail. Upon being opened by the recipient, the web page
could attempt to invoke the function and exploit the vulnerability.
In the HTML mail scenario, if the user was using Outlook Express 6.0 or
Outlook 2002 in their default configurations, or Outlook 98 or 2000 in
conjunction with the Outlook Email Security Update, then an attack could
not be automated and the user would still need to click on a URL sent in
e-mail. However if the user was not using Outlook Express 6.0 or Outlook
2002 in their default configurations, or Outlook 98 or 2000 in conjunction
with the Outlook Email Security Update, the attacker could cause an attack
to trigger automatically without the user having to click on a URL
contained in an e-mail.
What does the patch do?
The patch addresses the vulnerability by carrying out proper input
validation on the affected JScript function.
Are there any workarounds that can be used to block exploitation of this
vulnerability while I am testing or evaluating the patch?
Yes. Although Microsoft urges all customers to apply the patch at the
earliest possible opportunity, there are a number of workarounds that can
be applied to help prevent the vector used to exploit this vulnerability
in the interim.
It should be noted that these workarounds should be considered temporary
measures as they simply help block paths of attack rather than correcting
the underlying vulnerability.
The following sections are intended to provide you with information to
protect your computer from attack. Each section describes the workarounds
that you may wish to use depending on your computer's configuration.
* Turn off Active Scripting support in Internet Explorer
You can turn off support for active scripting by performing the steps in
the following knowledge base article:
Note that disabling scripting support in Internet Explorer will affect the
functionality of many websites on the Internet and should be considered a
temporary workaround only.
* Install the Outlook Email Security Update if needed
In the case of an e-mail borne attack, if the user was using Outlook
Express 6.0 or Outlook 2002 in their default configurations, or Outlook 98
or 2000 in conjunction with the Outlook Email Security Update, then an
attack could not be automated and the user would still need to click on a
URL sent in e-mail. However if the user was not using Outlook Express 6.0
or Outlook 2002 in their default configurations, or Outlook 98 or 2000 in
conjunction with the Outlook Email Security Update, the attacker could
cause an attack to trigger automatically without the user having to click
on a URL contained in an e-mail. In both the web based and e-mail based
cases, any limitations on the user's privileges would also restrict the
capabilities of the attacker's script.
* Restrict websites to only your trusted websites
As another workaround for this issue, you can add sites that you trust to
the Internet Explorer Trusted Zone, after disabling Active Scripting in
the Internet Zone. This will allow you to continue using trusted web sites
exactly as you do today, while tightening the restrictions on untrusted
sites. When you are able to deploy the patch, you will be able to
re-enable Active Scripting in the Internet Zone.
To do this, perform the following steps:
* Select "Tools," then "Internet Options." Click the "Security" tab.
* In the box labeled "Select a Web content zone to specify its current
security settings," click "Trusted Sites," then click "Sites."
* If you want to add sites that don't require a secure connection,
de-select the checkbox at the bottom that says "Require server
verification (https:) for all sites in this zone."
* In the box labeled "Add this Web Site to the zone:," type the URL of a
site that you trust, then click the "Add" button. Repeat for each site
that you want to add to the zone.
* Click on OK twice to accept the changes and return to IE.
Add any sites that you trust not to take malicious action on your
computer. One in particular that you may want to add is
http://windowsupdate.microsoft.com. This is the site that hosts the patch,
and it requires Active Scripting in order to install the patch. Note that
there is generally a trade-off between ease-of-use and security; by
selecting a high-security configuration, you could make it extremely
unlikely that a malicious web site could take action against you, but at
the cost of missing a lot of rich functionality. The appropriate balance
between security and ease-of-use is different for everyone, and you should
pick a configuration that fits your needs. The good news is that it is
easy to change your configuration, and you can try different
configurations until you find the right one for you until you can install
The information has been provided by
<mailto:0_45730_E51E4D7D-DECD-43AE-9A29-36080E8D4C3C_US@Newsletters.Microsoft.com> Microsoft Product Security.
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