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Related information

  Buffer overflow in mshtml.dll

  Details and exploitation of buffer overflow in mshtml.dll (and few  sidenotes on Unicode overflows in general)

  Buffer Overflow in Microsoft Internet Explorer

  Advisory CA-2002-04 Buffer Overflow in Microsoft Internet Explorer

  dH & SECURITY.NNOV: buffer overflow in mshtml.dll

From:MICROSOFT <secure_(at)_microsoft.com>
Date:15.02.2002
Subject:Microsoft Security Bulletin MS02-005

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Title:      11 February 2002 Cumulative Patch for Internet Explorer
Date:       11 February 2002
Software:   Internet Explorer
Impact:     Run Code of Attacker's Choice
Max Risk:   Critical
Bulletin:   MS02-005

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-005.asp.
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Issue:
======
This is a cumulative patch that, when installed, eliminates all
previously discussed security vulnerabilities affecting IE 5.01, 5.5
and IE 6. In addition, it eliminates the following six newly
discovered vulnerabilities:

- A buffer overrun vulnerability associated with an HTML directive
  that's used to incorporate a document within a web page. By
  creating a web page that invokes the directive using specially
  selected attributes, an attacker could cause code to run on the
  user's system.

- A vulnerability associated with the GetObject scripting function.
  Before providing a handle to an operating system object,
  GetObject performs a series of security checks to ensure that the
  caller has sufficient privileges to it. However, by requesting a
  handle to a file using a specially malformed representation, it
  would be possible to bypass some of these checks, thereby
  allowing a web page to complete an operation that should be
  prevented, namely, reading files on the computer of a visiting
  user's system.

- A vulnerability related to the display of file names in the File
  Download dialogue box. When a file download from a web site is
  initiated, a dialogue provides the name of the file and lets the
  user choose what action to take. However, a flaw exists in the way
  HTML header fields (specifically, the Content-Disposition and
  Content-Type fields) are handled. This flaw could make it possible
  for an attacker to misrepresent the name of the file in the
  dialogue, in an attempt to trick a user into opening or saving
  an unsafe file.

- A vulnerability that could allow a web page to open a file on the
  web site, using any application installed on a user's system.
  By design, IE should only open a file on a web site using the
  application that's registered to that type of file, and even
  then only if it's on a list of safe applications. However,
  through a flaw in the handling of the Content-Type HTML
  header field, an attacker could circumvent this restriction,
  and specify the application that should be invoked to process
  a particular file. IE would comply, even if the application was
  listed as unsafe.

- A vulnerability that could enable a web page to run a script even
  if the user has disabled scripting. IE checks for the presence of
  scripts when initially rendering a page. However, the capability
  exists for objects on a page to respond to asynchronous events;
  by misusing this capability in a particular way, it could be
  possible for a web page to fire a script after the page has
  passed the initial security checks.

- A newly discovered variant of the "Frame Domain Verification"
  vulnerability discussed in Microsoft Security Bulletin MS01-058.
  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 use the
  Document.open function 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. In addition, this could be used to
  mis-represent the URL in the address bar in a window opened from
  their site.

Mitigating Factors:
====================
Buffer Overrun in HTML Directive:

- The vulnerability could not be exploited if the "Run ActiveX
  Controls and Plugins" security option were disabled in the
  Security Zone in which the page was rendered. This is the default
  condition in the Restricted Sites Zone, and can be disabled
  manually in any other Zone.

- Outlook 98 and 2000 (after installing the Outlook Email Security
  Update), Outlook 2002, and Outlook Express 6 all open HTML mail
  in the Restricted Sites Zone. As a result, customers using these
  products would not be at risk from email-borne attacks.

- The buffer overrun would allow code to run in the security context
  of the user rather than the system. The specific privileges the
  attacker could gain through this vulnerability would therefore
  depend on the privileges accorded to the user.

File Reading via GetObject function:

- This vulnerability could only be used to read files. It could not
  be used to create, change, delete, or execute them.

- The attacker would need to know the name and location of the file
  on the user's computer.

- Some files that would be of interest to an attacker - most
  notably,the SAM Database - are locked by the operating system
  and therefore could not be read even using this vulnerability.

- The email-borne attack scenario would be blocked if the user were
  using any of the following: Outlook 98 or 2000 with the Outlook
  Email Security Update installed; Outlook 2002; or Outlook
  Express 6.

- The web-based attack scenario could be blocked by judicious use of
  the IE Security Zones mechanism such as using the Restricted Sites
  zone.

File Download Dialogue Spoofing via Content-Type and
Content-Disposition fields:

- Exploiting this vulnerability would not give an attacker the
  ability to force code to run on a user's system. It would only
  enable the attacker to misrepresent the file name and type in the
  File Download dialogue. The download operation would not occur
  without the user's approval, and the user could cancel at any
  time.

- The vulnerability could not be exploited if File Downloads have
  been disabled in the Security Zone in which the e-mail is
  rendered. This is not a default setting in any zone, however.

- On versions of IE prior to 6.0, the default selection in the file
  download dialogue is to save, rather than open, the file. (In
  IE 6.0, the default is to open the file; however, this behavior
  is inappropriate, and the patch changes IE 6.0 to conform with the
  behavior of previous versions).

Application invocation via Content-Type field:

- An attacker could only exploit this vulnerability if the
  application specified through the Content-Type field was actually
  installed on the user's system.

- The vulnerability does not provide any way for the attacker to
  inventory the applications installed on the user's system and
  select one, nor does it provide any way to force the user to
  install a particular application.

- The vulnerability would not provide any way to circumvent the
  security features of the application or to reconfigure it.

- Outlook 2002 users who have configured Outlook to render HTML mail
  as plaintext would be at no risk from attack through HTML mail.

Script execution:

- This vulnerability extends only to allowing scripts to run - it
  does not allow any other security restrictions to be bypassed.
  So, for instance, although an attacker could use this
  vulnerability to run a script, the script would still be subject
  to all other expected security settings.

Frame Domain Verification Variant via Document.Open function:

- 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 opened in a browser window, such as image files, HTML files
  and text files. Other file types, such as binary files, executable
  files, Word documents, and so forth, could not be read.

- The attacker would need to specify the exact name and location of
  the file in order to read it.

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/ms02-005.asp
  for information on obtaining this patch.

Acknowledgment:
===============
- The dH team and SECURITY.NNOV (http://www.security.nnov.ru/) team
  for reporting the buffer overrun vulnerability.

- Sandro Gauci of GFI security labs (http://www.gfi.com) for
  reporting the application invocation vulnerability.

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