Microsoft Security Bulletin MS03-006

Technical description:

Help and Support Center provides a centralized facility through which users can obtain assistance on a variety of topics. For instance, it provides product documentation, assistance in determining hardware compatibility, access to Windows Update, online help from Microsoft, and other assistance. Users and programs can execute URL links to Help and Support Center by using the "hcp://" prefix in a URL link instead of "http://".

A security vulnerability is present in the Windows Me version of Help and Support Center, and results because the URL Handler for the "hcp://" prefix contains an unchecked buffer.

An attacker could exploit the vulnerability by constructing a URL that, when clicked on by the user, would execute code of the attacker's choice in the Local Computer security context. The URL could be hosted on a web page, or sent directly to the user in email. In the web based scenario, where a user then clicked on the URL hosted on a website, an attacker could have the ability to read or launch files already present on the local machine. 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.

Mitigating factors:

• The Help and Support Center function could not be started automatically in Outlook Express or Outlook if the user is running Internet Explorer 6.0 Service Pack 1.
• 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.
• 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.

Severity Rating:

The above assessment is based on the types of systems affected by the vulnerability, their typical deployment patterns, and the effect that exploiting the vulnerability would have on them.

Vulnerability identifier: CAN-2003-0009

Tested Versions:

Microsoft tested Windows Me and Windows XP to assess whether they are affected by these vulnerabilities. Previous versions of Windows do not contain the code in question and are not affected by this vulnerability.

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. Such code could provide the attacker with the ability to take any desired action on the machine, including adding, deleting or modifying data on the system or running any code of the attacker's choice.

What causes the vulnerability?
The vulnerability results because of an unchecked buffer in the URL Handler for Help and Support Center.

What's Help and SupportCenter?
Help and Support Center (HSC) is a feature in Windows that provides help on a variety of topics. For instance, HSC enables users to learn about Windows features, download and install software updates, determine whether a particular hardware device is compatible with Windows, get assistance from Microsoft, and so forth.

What's wrong with Help and SupportCenter?
The HSC URL Handler has a function that allows pages to be opened using the "hcp://" prefix. However, this function does not properly check the parameters of the input it receives. As a result, a buffer is unchecked, and could allow an attacker to craft a URL that could allow code execution.

What is a URL handler?
A URL handler lets an application register a new URL type that, when invoked by a web page, starts the application automatically. For example, when Outlook 2002 is installed on a system, it registers "outlook://" as a custom URL handler. Outlook can then be invoked by typing this URL in IE, in the "Run" box, or by clicking on a hyperlink.
In the case of Help and Support Center, the custom URL prefix is "hcp://".

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 additional privileges on the system. This could allow the attacker to add, delete or modify data on the system, or take any other action of the attacker's choice.

How could an attacker exploit this vulnerability?
The attacker would need to construct a web page that launches a specially crafted URL. 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 would attempt to launch the URL 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 send 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 correcting the unchecked buffer in the URL handler.

Download locations for this patch

• Microsoft Windows Me:

  http://windowsupdate.microsoft.com