-----BEGIN PGP SIGNED MESSAGE-----
AUSCERT External Security Bulletin Redistribution
ESB-2002.496 -- Microsoft Security Bulletin MS02-050 (version 3.0)
Certificate Validation Flaw Could Enable Identity Spoofing (Q328145)
10 September 2002
AusCERT Security Bulletin Summary
Product: Windows 98
Windows 98 Second Edition
Windows NT 4.0
Windows NT 4.0, Terminal Server Edition
Office for Mac
Internet Explorer for Mac
Outlook Express for Mac
Impact: Access Confidential Data
Access Required: Remote
Comment: This ESB also relates to AusCERT Updates AU-2002.004 and
AU-2002.005 on SSL Vulnerabilities in Internet Explorer and
- --------------------------BEGIN INCLUDED TEXT--------------------
- -----BEGIN PGP SIGNED MESSAGE-----
- - ----------------------------------------------------------------------
Title: Certificate Validation Flaw Could Enable Identity
Released: September 04, 2002
Revised: September 09, 2002 (version 3.0)
Software: Microsoft Windows, Microsoft Office for Mac, Microsoft
Internet Explorer for Mac, or Microsoft Outlook Express
Impact: Identity spoofing.
Max Risk: Critical
Microsoft encourages customers to review the Security Bulletin at:
- - ----------------------------------------------------------------------
Reason for Revision:
Normally, Microsoft releases the patches for all affected products
simultaneously, in order to provide a complete solution. However,
exploit code for this issue has already been posted, and we are
therefore releasing the patches as they become available, in order
to allow customers to begin protecting their systems as quickly as
The bulletin has been updated to include patch availability for
Patches are now available for:
- Windows 98
- Windows 98 Second Edition
- Windows Me
- Windows NT 4.0
- Windows NT 4.0, Terminal Server Edition
- Windows 2000
- Windows XP
- Windows XP 64 bit Edition
Patches will be available shortly for:
- Microsoft Office v.X for Mac
- Microsoft Office 2001 for Mac
- Microsoft Office 98 for the Macintosh
- Microsoft Internet Explorer for Mac (for OS 8.1 to 9.x)
- Microsoft Internet Explorer for Mac (for OS X)
- Microsoft Outlook Express 5.0.5 for Mac
Customers should also review the caveats section of the bulletin
which discusses a warning message that may be displayed after
installing the patch. Updated patches are under development to
eliminate this side effect.
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
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.
- 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
- 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.
- 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.
- 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.
- 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,
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
- - - Patches are available to fix this vulnerability for Windows 98,
Windows 98 Second Edition, Windows Me, Windows NT 4.0,
Windows NT 4.0, Terminal Server Edition, Windows 2000,
Windows XP, and Windows XP 64 bit Edition.
Patches for Windows 2000, Microsoft Office for Mac, Microsoft
Internet Explorer for Mac, and Microsoft Outlook Express
for Mac will be released shortly.
Please read the Security Bulletin at
for information on obtaining this patch.
- - ---------------------------------------------------------------------
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED
"AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL
WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY
DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN
IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.
- -----BEGIN PGP SIGNATURE-----
Version: PGP 7.1
- -----END PGP SIGNATURE-----
- --------------------------END INCLUDED TEXT--------------------
You have received this e-mail bulletin as a result of your organisation's
registration with AusCERT. The mailing list you are subscribed to is
maintained within your organisation, so if you do not wish to continue
receiving these bulletins you should contact your local IT manager. If
you do not know who that is, please send an email to firstname.lastname@example.org
and we will forward your request to the appropriate person.
This security bulletin is provided as a service to AusCERT's members. As
AusCERT did not write the document quoted above, AusCERT has had no control
over its content. The decision to use any or all of this information is
the responsibility of each user or organisation, and should be done so in
accordance with site policies and procedures.
NOTE: This is only the original release of the security bulletin. It may
not be updated when updates to the original are made. If downloading at
a later date, it is recommended that the bulletin is retrieved directly
from the original authors to ensure that the information is still current.
Contact information for the authors of the original document is included
in the Security Bulletin above. If you have any questions or need further
information, please contact them directly.
Previous advisories and external security bulletins can be retrieved from:
If you believe that your system has been compromised, contact AusCERT or
your representative in FIRST (Forum of Incident Response and Security
Internet Email: email@example.com
Facsimile: (07) 3365 7031
Telephone: (07) 3365 4417 (International: +61 7 3365 4417)
AusCERT personnel answer during Queensland business
hours which are GMT+10:00 (AEST). On call after hours
for member emergencies only.
-----BEGIN PGP SIGNATURE-----
-----END PGP SIGNATURE-----