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From:MICROSOFT <secure_(at)_microsoft.com>
Date:21.11.2002
Subject:UPDATE: Microsoft Security Bulletin MS02-050: Certificate Validation Flaw Could Enable Identity Spoofing (Q329115)

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Title:      Certificate Validation Flaw Could Enable Identity
           Spoofing (Q329115)
Released:   04 September 2002
Revised:    20 November 2002 (version 4.0)
Software:   Microsoft Windows, Microsoft Office for Mac, Microsoft
           Internet Explorer for Mac, or Microsoft Outlook Express
           for Mac.
Impact:     Identity spoofing and, in some cases, ability to gain
           control over a user's system.
Max Risk:   Important

Bulletin:   MS02-050

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-050.asp.
- ----------------------------------------------------------------------

Reason for Revision:
====================
The original version of this bulletin was released on 05 September
2002. On 09 September 2002, we updated the bulletin to advise
customers that a Microsoft-issued digital certificate, used to sign
device drivers, did not meet the stricter validation standards
established by the patch. As a result, customers who installed the
patch could see unexpected error messages when installing new
hardware, or in some cases might be unable to install new hardware
altogether. On 20 November 2002, we released an updated version of
the patch that not only eliminates this problem, but also eliminates
a newly discovered variant of the original vulnerability.

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 identified in the original version of the bulletin
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 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.

The newly discovered vulnerability announced on 20 November 2002 is
closely related to the one discussed in the original version of the
bulletin and, like that vulnerability, involves a flaw in the way
certificate validation is performed. However, this vulnerability
could enable an attacker to gain control over a user's system.
Because a fix for this vulnerability was not included in the original
version of the patch, Microsoft strongly recommends that customers
install the new patch, even if they installed the original version
of the patch. Only Microsoft Windows 98, Windows 98 Second Edition,
Windows NT 4.0, and Windows NT 4.0, Terminal Server Edition are
affected by this variant.


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 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:
============
- Important

Note: Responding to customer feedback, Microsoft updated its
severity rating system November 18, 2002. Security bulletins that
originally posted under the old system - before November 18,
2002 - and are later re-released under the new system, will
reflect the severity rating assessed under the new revised
system Severity Rating criteria

Patch Availability:
===================
- A patch is available to fix this vulnerability. Please read the
  Security Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-050.asp
  for information on obtaining this patch.

Acknowledgment:
===============
- UK National Infrastructure Security Co-ordination Centre (NISCC)

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