__________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Center ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Microsoft Internet Explorer Flaws in Certificate Validation [Microsoft Security Bulletin MS01-027] May 23, 2001 19:00 GMT Number L-087 ______________________________________________________________________________ PROBLEM: Two vulnerabilities in Microsoft's Internet Explorer; one involving certificate validation, the other incorrect url display, could allow an attacker to spoof trusted web sites. PLATFORM: Internet Explorer 5.01 Internet Explorer 5.5 DAMAGE: Either of the two vulnerabilities could convince Internet Explorer users that the web site they are viewing is a trusted one, when in reality it is a spoofed site belonging to an attacker. Users could unknowingly pass information to the attacker. SOLUTION: Apply the patch described below. ______________________________________________________________________________ VULNERABILITY The risk is LOW: The steps that must be taken by the attacker ASSESSMENT: to actually get a user to the spoofed site are many and rely on several factors falling into place. ______________________________________________________________________________ [****** Start Microsoft Bulletin ******] - ---------------------------------------------------------------------- Title: Flaws in Web Server Certificate Validation Could Enable Spoofing Date: 16 May 2001 Software: Internet Explorer Impact: Spoofing of trusted web site Bulletin: MS01-027 Microsoft encourages customers to review the Security Bulletin at: http://www.microsoft.com/technet/security/bulletin/MS01-027.asp. - ---------------------------------------------------------------------- Issue: ====== A patch is available to eliminate two newly discovered Vulnerabilities affecting Internet Explorer, both of which could enable an attacker to spoof trusted web sites. The first vulnerability involves how digital certificates from web servers are validated. When CRL checking for such certificates is enabled, it could be possible for any or all of the following checks to no longer be performed: - Verification that the certificate has not expired - Verification that the server name matches the name on the certificate - Verification that the issuer of the certificate is trusted The second vulnerability could enable a web page to display the URL from a different web site in the IE address bar. This spoofing could occur within a valid SSL session with the impersonated site. Both vulnerabilities could be used to convince a user that the attacker's web site was actually a different one - one that the user presumably trusts and would provide sensitive information to. However, as discussed in the Mitigating Factors section below, there would be significant hurdles to exploiting either vulnerability. In addition to eliminating the two new vulnerabilities, the patch also eliminates two new variants of a previously discussed vulnerability, the "Frame Domain Verification" vulnerability, which originally was discussed in Microsoft Security Bulletin MS00-033. Like the original version, these new variants vulnerability could enable a malicious web site operator to open two browser windows, one in the web site's domain and the other on the user's local file system, and to pass information from the latter to the former. This could enable the web site operator to read any file on the user's local computer that could be opened in a browser window. The patch also incorporates the functionality of the patch provided in Microsoft Security Bulletin MS01-020 (http://www.microsoft.com/technet/security/bulletin/MS01-020.asp). Mitigating Factors: ==================== Server certificate validation vulnerability: - The vulnerability only affects how certificates from web servers are validated. It does not affect how code-signing certificates or any other type of certificate are validated. - The specific checks that might be bypassed vary with both the user and the actions she may have taken during the current browsing session. An attacker could not predict with any degree of certainty which checks might be bypassed in a particular case. - The vulnerability does not provide any way to force users to the attacker's web site. It is likely that this vulnerability could only be exploited in conjunction with a successful DNS poisoning or similar attack. Web page spoofing vulnerability: - Like the vulnerability above, this vulnerability would not provide any way to force users to the attacker's web site, and DNS poisoning or other measures would likely be required to exploit it. - Any hyperlinks within the page would correctly show the target. As a result, the attacker would need to point these to bona fide locations on the spoofed web site, with the result that the attacker would likely only be able to spoof a single web page, rather than an entire site. New variants of "Frame Domain Verification" vulnerability: - The vulnerability could only be used to read - not add, delete or change - files. - The attacker would need to know the exact name and location of every file he wished to read. - The vulnerability could only be used to read file types that can be opened within a browser window - for example, .htm, .txt or .doc files, but not .exe or .xls files. Patch Availability: =================== - A patch is available to fix this vulnerability. Please read the Security Bulletin http://www.microsoft.com/technet/security/bulletin/ms01-027.asp 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. [****** End Microsoft Bulletin ******]_______________________________________________________________________________CIAC wishes to acknowledge the contributions of Microsoft for the information contained in this bulletin. _______________________________________________________________________________ CIAC, the Computer Incident Advisory Center, is the computer security incident response team for the U.S. Department of Energy (DOE) and the emergency backup response team for the National Institutes of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. CIAC is also a founding member of FIRST, the Forum of Incident Response and Security Teams, a global organization established to foster cooperation and coordination among computer security teams worldwide. CIAC services are available to DOE, DOE contractors, and the NIH. CIAC can be contacted at: Voice: +1 925-422-8193 (7x24) FAX: +1 925-423-8002 STU-III: +1 925-423-2604 E-mail: ciac@ciac.org Previous CIAC notices, anti-virus software, and other information are available from the CIAC Computer Security Archive. World Wide Web: http://www.ciac.org/ Anonymous FTP: ftp.ciac.org PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing communities receive CIAC bulletins. If you are not part of these communities, please contact your agency's response team to report incidents. Your agency's team will coordinate with CIAC. The Forum of Incident Response and Security Teams (FIRST) is a world-wide organization. A list of FIRST member organizations and their constituencies can be obtained via WWW at http://www.first.org/. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. 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