Microsoft on Wednesday released a patch that fixes two newly discovered critical vulnerabilities, each of which could allow an attacker to take control of a victim's system. The patch, released in Microsoft Bulletin MS03-020, is a cumulative patch that contains all previous patches for the company's Internet Explorer versions 5.01, 5.5, and 6.0.

The first flaw is the result of a buffer-overrun vulnerability that's made possible because unpatched versions of Internet Explorer 5.01, 5.5, 6.0, and Internet Explorer 6.0 for Windows Server 2003 fail to properly determine an object type returned from a Web server. An attacker who used this vulnerability could potentially run software of their choice on a victim's system, according to Microsoft. It's possible to exploit this vulnerability by simply accessing a maliciously designed Web site or by receiving a specially crafted HTML E-mail.

The second security flaw is made possible, Microsoft said in its bulletin, because unpatched versions of Internet Explorer don't "implement an appropriate block" on a file-download dialog box. As a result, opening an E-mail or visiting a Web site that was designed to exploit this vulnerability could result in the compromise of the user's system.

Microsoft is also asking users to update their HTML Help, if they haven't done so already, as described in Microsoft's Knowledge Base article 811630. Users who haven't updated HTML Help will discover that the window.showHelp() feature won't function after applying this patch and a few previous patches.

More information is about the security bulletin is available on Microsoft's Web site.