Microsoft Security Bulletin MS01-027

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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.

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MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES 
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OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION
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APPLY.



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