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Title: 13 December 2001 Cumulative Patch for IE
Date: 13 December 2001
Software: Internet Explorer
Impact: Run Code of an Attacker's Choice
Max Risk: Critical
Bulletin: MS01-058

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-058.asp.
- ----------------------------------------------------------------------

Issue:
======
This is a cumulative patch that, when installed, eliminates all
previously discussed security vulnerabilities affecting IE 5.5 and
IE 6. In addition, it eliminates three newly discovered
vulnerabilities.

 - The first vulnerability involves a flaw in the handling of the
   Content-Disposition and Content-Type header fields in an HTML.
   stream. These fields, the hosting URL, and the hosted file data
   determine how a file is handled upon download in Internet
   Explorer. A security vulnerability exists because, if an
   attacker altered the HTML header information in a certain way,
   it could be possible to make IE believe that an executable file
   was actually a different type of file -- one that it is
   appropriate to simply open without asking the user for
   confirmation. This could enable the attacker to create a web
   page or HTML mail that, when opened, would automatically run an
   executable on the user's system. This vulnerability affects
   IE 6.0 only. It does not affect IE 5.5.

 - The second vulnerability is a newly discovered variant of the
   "Frame Domain Verification" vulnerability discussed in Microsoft
   Security Bulletin MS01-015. The vulnerability could enable a
   malicious web site operator to open two browser windows, one in
   the web site's domain and the other on the user's local file
   system, and to pass information from the latter to the former.
   This could enable the web site operator to read, but not change,
   any file on the user's local computer that could be opened in a
   browser window. This vulnerabilty affects both IE 5.5 and 6.0.

 - The third vulnerability involves a flaw related to the display
   of file names in the File Download dialogue box. When a file
   download is initiated, a dialogue provides the name of the file.
   However, in some cases, it would be possible for an attacker to
   misrepresent the name of the file in the dialogue. This could be
   invoked from a web page or in an HTML email in an attempt to
   fool users into accepting unsafe file types from a trusted
   source. This vulnerabilty affects both IE 5.5 and 6.0.

Mitigating Factors:
====================
File Execution Vulnerability:

 - The vulnerability could not be exploited if File Downloads have
   been disabled in the Security Zone from which the file is being
   received. In most attempts to maliciously exploit this
   vulnerability the file would be received from the Internet or
   Intranet zone. Therefore, disabling File Downloads in these zones
   can protect customers. This is not the default setting for either
   of these zone, however.

 - This affects IE 6.0 only.

Frame Domain Verification Variant:
 - The vulnerability could only be used to view files. It could not
   be used to create, delete, modify or execute them.

 - The vulnerability would only allow an attacker to read files that
   can be can be opened in a browser window, such as image files,
   HTML files and text files. Other file file types, such as binary
   files, executable files, Word documents, and so forth, could not
   be read.

 - The attacker would have to have knowledge of the exact file name
   and location in other to successfully read the file on the
   local system.

File Name Spoofing Vulnerability:
 - The determination on choosing to accept a file download from an
   Internet site should always be based on the trustworthiness of
   the source and not on the file type. File downloads should never
   be accepted from an untrusted source, no matter how harmless the
   type may appear to be.

Risk Rating:
============
 - Internet systems: Critical
 - Intranet systems: Critical
 - Client systems: Critical

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the
   Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms01-058.asp
   for information on obtaining this patch.

Acknowledgment:
===============
 - Jouko Pynnonen of Oy Online Solutions Ltd
   (http://www.solutions.fi/index.cgi/?lang=eng)

- ---------------------------------------------------------------------

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