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Title:      Certificate Validation Flaw Could Enable Identity 
            Spoofing (Q328145)
Date:       September 04, 2002
Software:   Microsoft Windows, Microsoft Office for Mac, Microsoft
            Internet Explorer for Mac, or Microsoft Outlook Express
            for Mac. 
Impact:     Identity spoofing. 
Max Risk:   Critical
Bulletin:   MS02-050

Microsoft encourages customers to review the Security Bulletin at: 
http://www.microsoft.com/technet/security/bulletin/MS02-050.asp .
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Issue:
======
The IETF Profile of the X.509 certificate standard defines several
optional fields that can be included in a digital certificate. One
of these is the Basic Constraints field, which indicates the maximum
allowable length of the certificate's chain and whether the
certificate is a Certificate Authority or an end-entity certificate.
However, the APIs within CryptoAPI that construct and validate
certificate chains (CertGetCertificateChain(), 
CertVerifyCertificateChainPolicy(), and WinVerifyTrust()) do not
Check the Basic Constraints field. The same flaw, unrelated to
CryptoAPI, is also present in several Microsoft products for
Macintosh. 

The vulnerability could enable an attacker who had a valid
end-entity certificate to issue a subordinate certificate that,
although bogus, would nevertheless pass validation. Because
CryptoAPI is used by a wide range of applications, this could
enable a variety of identity spoofing attacks. These are discussed
in detail in the bulletin FAQ, but could include: 

 - Setting up a web site that poses as a different web site, and
   "proving" its identity by establishing an SSL session as the
   legitimate web site. 

 - Sending emails signed using a digital certificate that
   purportedly belongs to a different user. 

 - Spoofing certificate-based authentication systems to gain
   entry as a highly privileged user. 

 - Digitally signing malware using an Authenticode certificate
   that claims to have been issued to a company users might trust. 


Mitigating Factors:
====================
Overall: 

 - The user could always manually check a certificate chain, and
   might notice in the case of a spoofed chain that there was an
   unfamiliar intermediate CA. 

 - Unless the attacker's digital certificate were issued by a CA
   in the user's trust list, the certificate would generate a 
   warning when validated. 

 - The attacker could only spoof certificates of the same type as
   the one he or she possessed. In the case where the attacker
   attempted an attack using a high-value certificate such as
   Authenticode certificates, this would necessitate obtaining
   a legitimate certificate of the same type - which could
   require the attacker to prove his or her identity or
   entitlement to the issuing CA. 

Web Site Spoofing: 

 - The vulnerability provides no way for the attacker to cause the
   user to visit the attacker's web site. The attacker would need
   to redirect the user to a site under the attacker's control
   using a method such as DNS poisoning. As discussed in the
   bulletin FAQ, this is extremely difficult to carry out in
   practice. 

 - The vulnerability could not be used to extract information from
   the user's computer. The vulnerability could only be used by an
   attacker as a means of convincing a user that he or she has
   reached a trusted site, in the hope of persuading the user to
   voluntarily provide sensitive data. 

Email Signing: 

 - The "from" address on the spoofed mail would need to match the
   one specified in the certificate, giving rise to either of two
   scenarios if a recipient replied to the mail. In the case where
   the "from" and "reply-to" fields matched, replies would be sent
   to victim of the attack rather than the attacker. In the case
   where the fields didn't match, replies would obviously be
   addressed to someone other than ostensible sender. Either case
   could be a tip-off that an attack was underway. 

Certificate-based Authentication: 

 - In most cases where certificates are used for user 
   authentication, additional information contained within the
   certificate is necessary to complete the authentication. The
   type and format of such data typically varies with every
   installation, and as a result significant insider information
   would likely be required for a successful attack. 

Authenticode Spoofing: 

 - To the best of Microsoft's knowledge, such an attack could not
   be carried out using any commercial CA's Authenticode
   certificates. These certificates contain policy information
   that causes the Basic Constraints field to be correctly
   evaluated, and none allow end-entity certificates to act as CAs. 

 - Even if an attack were successfully carried out using an
   Authenticode certificate that had been issued by a corporate
   PKI, it wouldn't be possible to avoid warning messages, as trust
   in Authenticode is brokered on a per-certificate, not per-name,
   basis. 

Risk Rating:
============
Microsoft Windows platforms:
 - Internet systems: Critical
 - Intranet systems: Critical
 - Client systems: Critical

Microsoft programs for Mac:
 - Internet systems: None
 - Intranet systems: None
 - Client systems: Moderate

Patch Availability:
===================
 - A patch is available to fix this vulnerability for Windows NT 
   4.0, Windows NT 4.0, Terminal Server Edition, Windows XP, and
   Windows XP 64 bit Edition.
   Please read the Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms02-050.asp
   for information on obtaining this patch.

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