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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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