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Title: Cumulative Patch for Outlook Express (330994)
Date: 23 April 2003
Software: Microsoft (c) Outlook Express
Impact: Run code of the attacker's choice on a user's machine.
Max Risk: Critical
Microsoft encourages customers to review the Security Bulletins at:
MHTML stands for MIME Encapsulation of Aggregate HTML. MHTML is an
Internet standard that defines the MIME (Multipurpose Internet Mail
Extensions) structure used to send HTML content in e-mail message
bodies. The MHTML URL Handler in Windows is part of Outlook Express
and provides a URL type that can be used on the local machine. This
URL type (MHTML://) allows MHTML documents to be launched from a
command line, from Start/Run, using Windows Explorer or from within
A vulnerability exists in the MHTML URL Handler that allows any
file that can be rendered as text to be opened and rendered as part
of a page in Internet Explorer. As a result, it would be possible
to construct a URL that referred to a text file that was stored on
the local computer and have that file render as HTML. If the text
file contained script, that script would execute when the file was
accessed. Since the file would reside on the local computer, it
would be rendered in the Local Computer Security Zone. Files that
are opened within the Local Computer Zone are subject to fewer
restrictions than files opened in other security zones.
Using this method, an attacker could attempt to construct a URL and
either host it on a website or send it via email. In the web based
scenario, where a user then clicked on a 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
the 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
Applying the update listed in Microsoft Security Bulletin MS03-004
- -- Cumulative Patch for Internet Explorer-will help block an
attacker from being able to load a file onto a user's computer and
prevent the passing of parameters to an executable. This means that
an attacker could only launch a program that already existed on the
computer-provided the attacker was aware of the location of the
program-and would not be able to pass parameters to the program for
it to execute.
MHTML is a standard for exchanging HTML content in e-mail and as a
result the MHTML URL Handler function has been implemented in
Outlook Express. Internet Explorer can also render MHTML content,
however the MHTML function has not been implemented separately in
Internet Explorer - it simply uses Outlook Express to render the
- -For the web-based scenario, the attacker would have to host a web
site that contained a web page used to exploit this vulnerability
and entice a user to visit it. An attacker would have no way to
force a user to visit the site. Instead, the attacker would need to
lure the user there, typically by getting the user to click on a
link to the attacker's site.
- -The HTML mail-based attack scenario 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.
- -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 administrative privileges.
- -If the cumulative patch for Internet Explorer MS03-004 has been
installed, known means by which an attacker may place a file onto a
user's computer will be blocked.
- -In order to invoke an executable already present on the local
system, an attacker must know the path to that executable.
- A patch is available to fix this vulnerability. Please read the
Security Bulletins at
for information on obtaining this patch.
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PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS
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WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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