Multiple Vendor Invalid X.509 Certificate Chain Vulnerability

A flaw has been reported in the handling of X.509 certificates by a number of products, including several web browsers. It may be possible for a malicious party to create certificates for arbitrary domains, which will be treated as trusted by the vulnerable browser.

The flaw lies in the handling of intermediate certificate authorities. Normally, intermediate certificates should possess a Basic Constraints field which states the certificate may be used as a signing authority.

Vulnerable products do not require the Basic Constraints field be properly defined. A malicious party with one valid certificate may sign a new certificate for an arbitrary domain. This may allow the attacker to spoof a sensitive domain, or to attempt a man-in-the-middle attack against encrypted communications.

This vulnerability was originally reported in Microsoft's Internet Explorer web browser. It has been reported that, in the case of Microsoft Internet Explorer, the flaw lies in some cryptographic functions implemented in the operating system. It should be noted that this flaw has not been reported in the Cryptographic API included with Microsoft Windows.

Reports state that IIS 5.0 under Windows 2000 is also vulnerable. In this case, client certificate chains are not properly verified. Attackers may exploit this vulnerability to bypass some authentication schemes.

This vulnerability also exists in some versions of KDE and the included Konqueror web browser. Versions 3.0.2 and earlier are vulnerable.

** A report suggests that the patch issued by Microsoft may not fully protect against this vulnerability. It may be possible that a malicious site using an invalid certificate may mislead users into believing that a certificate is expired rather than being invalid.

** UPDATE 11/11/03 - Microsoft has updated their bulletin for this issue. Users who installed Internet Explorer 6 after installing Windows 2000 Service Pack 4 may have reintroduced this issue onto their systems. A new patch is available for users who installed Internet Explorer 6 on Windows 2000 SP4 systems.


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