__________________________________________________________
The U.S. Department of Energy
Computer Incident Advisory Center
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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.
- ---------------------------------------------------------------------
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MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES
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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
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PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
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