(Microsoft Issues Windows 2000 Patch) Microsoft Internet Explorer SSL Implementation Flaw in Following Certificate Chains Allows Remote Users to Conduct Man-in-the-Middle Attacks to Obtain Unencrypted Data from the Browser
SecurityTracker Alert ID: 1005211|
SecurityTracker URL: http://securitytracker.com/id/1005211
(Links to External Site)
Date: Sep 11 2002
Disclosure of user information, User access via network|
Fix Available: Yes Vendor Confirmed: Yes |
Version(s): 98, 98SE, Me, NT 4.0, NT 4.0 Terminal Server Edition, 2000, XP|
A vulnerability was reported in Microsoft Internet Explorer's secure sockets layer (SSL) protocol implementation. A remote user with access to a target user's encrypted data stream could conduct a man-in-the-middle attack to obtain the unencrypted data. A remote user may also be able to take control of the target user's system.|
The vulnerability reportedly exists in the way in which Internet Explorer (IE) follows certificate chains. A browser should verify that the CN field of the certificate matches the server's domain, that it is signed by the intermediate certificate authority (CA), and that the intermediate CA's certificate is signed by a known CA. According to the report, the browser should also check that all intermediate certificates have valid CA Basic Constraints.
It is reported that IE does not check the Basic Constraints. As a result, a server with a valid certificate (signed by a valid CA) for any domain can apparently generate a valid CA-signed certificate for any other domain (from the perspective of the IE browser, that is).
To exploit this, the remote user must generate a valid certificate for a domain within the user's administrative control and obtain a valid signature from a known CA (e.g., VeriSign). Then, the remote user can generate a certificate for an arbitrary domain and sign it with their own CA.
Because IE does not check the Basic Constraints on the certificate for the arbitrary domain, IE will reportedly accept this certificate chain as valid for the arbitrary domain.
A remote user can thus spoof any domain using a man-in-the-middle attack.
It is reported that there will be no browser warnings presented to the target user. However, if the target user chooses to view the certificate of the intended web site, the target user will see the attacker's certificate in the chain.
It is also reported that IE 6 may not follow the chain in certain cases, depending on what content the valid CA places in the Basic Constraint field.
[Editor's note: In November 2002, Microsoft reported that there is a closely related variant (CVE: CVE-2002-1183) to this vulnerability relating to certificate validation. However, the new vulnerability may allow a remote user to gain control over the target user's system. Only Microsoft Windows 98, Windows 98 Second Edition, Windows NT 4.0, and Windows NT 4.0, Terminal Server Edition are affected by this new variant, the vendor reported. The vendor did not provide details of this new variant.]
A remote user with the ability to conduct a man-in-the middle attack can spoof any domain and obtain the target user's unencrypted data.|
A remote user may also be able to take control of the target user's system.
Microsoft has released a patch for several operating systems and plans to release patches for others shortly.|
For Microsoft Windows 98:
For Windows 98 Second Edition:
For Windows Me:
Windows NT 4.0:
For Windows NT 4.0 Terminal Server Edition:
For Windows 2000:
For Windows XP and Windows XP 64 bit Edition:
For Microsoft Office v.X for Mac:
To be released shortly
For Microsoft Office 2001 for Mac:
To be released shortly
For Microsoft Office 98 for the Macintosh:
To be released shortly
For Microsoft Internet Explorer for Mac (for OS 8.1 to 9.x):
To be released shortly
For Microsoft Internet Explorer for Mac (for OS X):
To be released shortly
For Microsoft Outlook Express 5.0.5 for Mac:
To be released shortly
Microsoft notes that the patch for Windows 98 can be installed on Windows 98 Gold. The patch for Windows 98 SE can be installed on Windows 98 SE Gold. The patch for Windows Millennium can be installed on systems running Windows Millennium Gold. The patch for Windows NT 4.0 can be installed on systems running Windows NT 4.0 SP6a. The patch for Windows NT 4.0, Terminal Server Edition, can be installed on systems running Windows NT 4.0, Terminal Server Edition SP6. The patch for Windows 2000 can be installed on Windows 2000 SP2 or SP3. The patch for Windows XP can be installed on systems running Windows XP Gold and the pending Windows XP SP1.
Microsoft plans to include this fix in Windows 2000 SP4 and Windows XP SP2.
Microsoft has issued Knowledge Base article Q328145 regarding this issue, available on the Microsoft Online Support web site:
Vendor URL: www.microsoft.com/technet/security/bulletin/MS02-050.asp (Links to External Site)
|Underlying OS: Windows (Any)|
This archive entry is a follow-up to the message listed below.|
Source Message Contents
Subject: Microsoft Security Bulletin MS02-050: Certificate Validation Flaw Could Enable Identity Spoofing (Q328145) (V3.0)|
-----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-----
You have received this e-mail bulletin because of your subscription to the Microsoft Product Security Notification Service. For more
information on this service, please visit http://www.microsoft.com/technet/security/notify.asp.
To verify the digital signature on this bulletin, please download our PGP key at http://www.microsoft.com/technet/security/notify.asp.
To unsubscribe from the Microsoft Security Notification Service, please visit the Microsoft Profile Center at http://register.microsoft.com/regsys/pic.asp
If you do not wish to use Microsoft Passport, you can unsubscribe from the Microsoft Security Notification Service via email as described
Send an email to unsubscribe to the Service by following these steps:
a. Send an e-mail to email@example.com. The subject line and the message body are not used to process the subscription request,
and can be anything you like.
b. Send the e-mail.
c. You will receive a response, asking you to verify that you really want to cancel your subscription. Compose a reply, and put "OK"
in the message body. (Without the quotes). Send the reply.
d. You will receive an e-mail telling you that your name has been removed from the subscriber list.
For security-related information about Microsoft products, please visit the Microsoft Security Advisor web site at http://www.microsoft.com/security.
Go to the Top of This SecurityTracker Archive Page