-----BEGIN PGP SIGNED MESSAGE-----
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
Microsoft encourages customers to review the Security Bulletin at: http://www.microsoft.com/technet/security/bulletin/MS02-050.asp.
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.
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:
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.
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.
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.
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
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
iQEVAwUBPdvNiI0ZSRQxA/UrAQGYEwgAien1K9SEpLZ79jcqozLi1d59kkE02SzX jUrIC7vat0npDp8L+Nq8XRMdot68DaGyo7bN2kf3VUwwwU8XJNtDBY/CoXfBgUCh EwPKVmcx69TFpodP6Ul6hC/7Ok2zksQHro2+GBYlEz2pW/km6ZA6woudypY1Dd5K YYA9qfXvP93yuYcvjHa/SLGukTwPDGROrtzIFcmJMdmzg5r4F8LSKYYYHCuVxklN 0VtLGP2RyMvfy4p0iPF6ZdJ/ADC/spDmyvf37/HIQXhLfjGFDoUUAkppqH0fs+oz jQUgjcDQ/O9i9p8+xowJXbJcOeZXdnnoC4EStYIS1zB1U9Z2Vrtz2Q== =b+d7 -----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 below: Reply to this message with the word UNSUBSCRIBE in the Subject line.
For security-related information about Microsoft products, please visit the Microsoft Security Advisor web site at http://www.microsoft.com/security.