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TUCoPS :: Browsers :: ciacl087.txt

CIAC L-087 - Microsoft Internet Explorer Flaws in Certificate Validation




             __________________________________________________________

                       The U.S. Department of Energy
                     Computer Incident Advisory Center
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___
             __________________________________________________________

                             INFORMATION BULLETIN

          Microsoft Internet Explorer Flaws in Certificate Validation
                     [Microsoft Security Bulletin MS01-027]

May 23, 2001 19:00 GMT                                            Number L-087
______________________________________________________________________________
PROBLEM:       Two vulnerabilities in Microsoft's Internet Explorer; one 
               involving certificate validation, the other incorrect url 
               display, could allow an attacker to spoof trusted web sites. 
PLATFORM:      Internet Explorer 5.01 Internet Explorer 5.5 
DAMAGE:        Either of the two vulnerabilities could convince Internet 
               Explorer users that the web site they are viewing is a trusted 
               one, when in reality it is a spoofed site belonging to an 
               attacker. Users could unknowingly pass information to the 
               attacker. 
SOLUTION:      Apply the patch described below. 
______________________________________________________________________________
VULNERABILITY  The risk is LOW: The steps that must be taken by the attacker 
ASSESSMENT:    to actually get a user to the spoofed site are many and rely on 
               several factors falling into place. 
______________________________________________________________________________

[******  Start Microsoft Bulletin ******]
- ----------------------------------------------------------------------
Title:      Flaws in Web Server Certificate Validation Could
             Enable Spoofing
Date:       16 May 2001
Software:   Internet Explorer
Impact:     Spoofing of trusted web site
Bulletin:   MS01-027

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-027.asp.
- ----------------------------------------------------------------------

Issue:
======
A patch is available to eliminate two newly discovered
Vulnerabilities affecting Internet Explorer, both of which could enable
an attacker to spoof trusted web sites. The first vulnerability involves 
how digital certificates from web servers are validated. When CRL checking
for such certificates is enabled, it could be possible for any or all of the
following checks to no longer be performed:
  - Verification that the certificate has not expired
  - Verification that the server name matches the name on the
    certificate
  - Verification that the issuer of the certificate is trusted

The second vulnerability could enable a web page to display the URL
from a different web site in the IE address bar. This spoofing could
occur within a valid SSL session with the impersonated site. Both
vulnerabilities could be used to convince a user that the attacker's
web site was actually a different one - one that the user presumably
trusts and would provide sensitive information to. However, as
discussed in the Mitigating Factors section below, there would be
significant hurdles to exploiting either vulnerability.

In addition to eliminating the two new vulnerabilities, the patch
also eliminates two new variants of a previously discussed vulnerability,
the "Frame Domain Verification" vulnerability, which originally was
discussed in Microsoft Security Bulletin MS00-033. Like the original
version, these new variants 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 any file on the user's local computer that could be opened in
a browser window.

The patch also incorporates the functionality of the patch provided
in Microsoft Security Bulletin MS01-020
(http://www.microsoft.com/technet/security/bulletin/MS01-020.asp).

Mitigating Factors:
====================
Server certificate validation vulnerability:
  - The vulnerability only affects how certificates from web
    servers are validated. It does not affect how code-signing
    certificates or any other type of certificate are validated.
  - The specific checks that might be bypassed vary with both
    the user and the actions she may have taken during the
    current browsing session. An attacker could not predict with
    any degree of certainty which checks might be bypassed in a
    particular case.
  - The vulnerability does not provide any way to force users
    to the attacker's web site. It is likely that this
    vulnerability could only be exploited in conjunction with a
    successful DNS poisoning or similar attack.

Web page spoofing vulnerability:
  - Like the vulnerability above, this vulnerability would not
    provide any way to force users to the attacker's web site,
    and DNS poisoning or other measures would likely be required
    to exploit it.
  - Any hyperlinks within the page would correctly show the target.
    As a result, the attacker would need to point these to bona
    fide locations on the spoofed web site, with the result that
    the attacker would likely only be able to spoof a single web
    page, rather than an entire site.

New variants of "Frame Domain Verification" vulnerability:
  - The vulnerability could only be used to read - not add, delete
    or change - files.
  - The attacker would need to know the exact name and location of
    every file he wished to read.
  - The vulnerability could only be used to read file types that
    can be opened within a browser window - for example, .htm,
    .txt or .doc files, but not .exe or .xls files.

Patch Availability:
===================
  - A patch is available to fix this vulnerability. Please read the
    Security Bulletin
    http://www.microsoft.com/technet/security/bulletin/ms01-027.asp
    for information on obtaining this patch.

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

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED
"AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL
WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
SHALL
MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES
WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL,
LOSS
OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION
OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH
DAMAGES.
SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR
CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY
NOT
APPLY.

[******  End Microsoft Bulletin ******]_______________________________________________________________________________CIAC wishes to acknowledge the contributions of Microsoft for the information contained in this bulletin.
_______________________________________________________________________________


CIAC, the Computer Incident Advisory Center, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC
can be contacted at:
    Voice:    +1 925-422-8193 (7x24)
    FAX:      +1 925-423-8002
    STU-III:  +1 925-423-2604
    E-mail:   ciac@ciac.org

Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.

   World Wide Web:      http://www.ciac.org/
   Anonymous FTP:       ftp.ciac.org

PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins.  If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at http://www.first.org/.

This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial products,
process, or service by trade name, trademark, manufacturer, or
otherwise, does not necessarily constitute or imply its endorsement,
recommendation or favoring by the United States Government or the
University of California. The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government or the University of California, and shall not be used for
advertising or product endorsement purposes.

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