A computer security researcher and an antivirus company are warning Microsoft customers about an unpatched hole in the company's Internet Explorer Web browser that could allow a remote attacker to bypass security warnings and download malicious content onto vulnerable systems.

The warnings came after the hole was identified on the Bugtraq Internet security discussion list by someone using the name "Rafel Ivgi." The hole affects Internet Explorer (IE) version 6.0.0, including the version released with Windows XP Service Pack 2 (SP2). The vulnerability allows malicious attackers to bypass warnings designed to inform users when a file is being passed to their computer using a specially-crafted HTML (Hypertext Markup Language) Web document.

Microsoft reacted strongly to the warnings Friday, saying that the Bugtraq notice made false claims about Internet Explorer in Windows XP SP2, and claiming that the download blocking feature in that version of the browser is working as designed.

"Microsoft is disappointed that an independent security researcher has posted a false claim on several newsgroups alleging that the automatic blocking feature of Internet Explorer in Windows XP SP2 (also referred to as the Information Bar) fails to function properly. These postings are inaccurate and misleading to customers," the company said in a statement.

Security software company Symantec issued a vulnerability alert about the hole Friday and cited Ivgi, which also provided code proving that the hole existed.

According to the Bugtraq message and Symantec alert, an IE feature designed to catch references to file downloads does not detect a particular HTML event, known as "onclick," when it is combined with the common HTML tag, which designates the beginning and ending of the main part of a Web page.

Malicious Internet users could use the onclick event in combination with another function called "createElement" to create an IFRAME, or "inline frame," which is an HTML element that allows external objects to be inserted into another HTML document. Attackers could link the IFRAME to a malicious Web page that downloaded a malicious file to the user's computer when the page was clicked on, without generating a warning in the Information bar, Symantec said.

There is no patch available for the new hole, and no specific exploit code is required to take advantage of the hole, Symantec said.

According to Microsoft, the issue described in the Bugtraq alert is not a security problem. In fact, Internet Explorer for Windows XP SP2 does display a security warning in the scenario described in the warning: a dialog box instead of the information bar. "And that is what it is supposed to do," said Kevin Kean, director of the Microsoft Security Response Center.

"We have examined the proof of concept code that he included and analyzed that. Internet Explorer does what we would expect it to do, it brings up the dialog box for the download, there is no vulnerability," Kean said.

IE users are advised to avoid links provided by unknown or untrusted sources, to keep from being lured to a malicious Web site. IE users can also configure the browser to disable the execution of script code and active content, though doing so could have adverse effects on the way IE functions, Symantec said.

The news comes just three days after Microsoft issued software patches for several serious Windows security holes and released a new tool that lets users remove malicious software from their PCs, and amid increasing competition in the Web browser market from the Mozilla Foundation's Firefox browser.

On Tuesday, the software company published security bulletins and patches for two critical holes, one in the Windows HTML Help system and the other in Windows code that handles cursor, animated cursor and icon formats.(See: http://www.microsoft.com/technet/security/bulletin/MS05-001.mspx and http://www.microsoft.com/technet/security/bulletin/MS05-002.mspx.)

(Joris Evers in San Francisco contributed to this story.)