Published:
10 September 2002
Protect yourself against future threats.
-----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 Me Windows NT 4.0 Windows NT 4.0, Terminal Server Edition Windows 2000 Windows XP Office for Mac Internet Explorer for Mac Outlook Express for Mac Vendor: Microsoft Platform: i386 IA-64 Impact: Access Confidential Data Increased Privileges Reduced Security Access Required: Remote Ref: ESB-2002.492 ESB-2002.490 Comment: This ESB also relates to AusCERT Updates AU-2002.004 and AU-2002.005 on SSL Vulnerabilities in Internet Explorer and Konqueror. - --------------------------BEGIN INCLUDED TEXT-------------------- - -----BEGIN PGP SIGNED MESSAGE----- - - ---------------------------------------------------------------------- Title: Certificate Validation Flaw Could Enable Identity Spoofing (Q328145) 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 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 . - - ---------------------------------------------------------------------- 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 possible. The bulletin has been updated to include patch availability for Windows 2000. 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. 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: =================== - - - 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 http://www.microsoft.com/technet/security/bulletin/ms02-050.asp for information on obtaining this patch. - - --------------------------------------------------------------------- THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. 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Previous advisories and external security bulletins can be retrieved from: http://www.auscert.org.au/Information/advisories.html If you believe that your system has been compromised, contact AusCERT or your representative in FIRST (Forum of Incident Response and Security Teams). Internet Email: auscert@auscert.org.au 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----- Version: 2.6.3i Charset: noconv Comment: ftp://ftp.auscert.org.au/pub/auscert/AUSCERT_PGP.key iQCVAwUBPX5IIyh9+71yA2DNAQGnIQQAnbYw1KtS8VJfsiGBnLnxTxEtKh2IIiwD Ln/0oED34FiGL3REbhoUGO2AzvwqV+vDta9+STBJ/ZEJ0YluFjXUdWwSY4IZ0feh Bb/cFF28jms3EkapFpiEoXQl99crhLIJsEBBbAJuo9PmCdUDYuG3ieI+g5xMOlWi rF6zSXHq1HM= =Vwt5 -----END PGP SIGNATURE-----