The following is a Security Bulletin from the Microsoft Product Security Notification Service. Please do not reply to this message, as it was sent from an unattended mailbox. ******************************** -----BEGIN PGP SIGNED MESSAGE----- - ---------------------------------------------------------------------- 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. 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