For the second time in a week, hackers have discovered a previously unknown bug in Microsoft Corp.'s Internet Explorer (IE) browser that could be exploited to run unauthorized software on a Windows computer.

This latest bug, reported Thursday, could be exploited to seize control of a Windows system and has been given a "high risk" rating by the FrSIRT security Web site.

Although "proof-of-concept" code showing how this vulnerability could be exploited has been published, making the bug a more serious concern, there are some mitigating factors. Attackers would first need to trick users into visiting a specially coded Web page and then somehow get them to perform certain actions, such as writing "specific text in a text field," before they could run their malicious software, FrSIRT said.

The risk is further mitigated by the fact that the bug reportedly does not affect the latest versions of Microsoft's Windows and Windows Server 2003 operating systems, FrSIRT said.

Because of these mitigating factors, Microsoft has decided not to fix the bug in a security update to Internet Explorer.

"The vulnerability cannot be used to execute code on a user's system without multiple user actions that are uncommon in typical Web browsing scenarios," Microsoft said in a statement provided by its public relations agency. "Due to the significant mitigating factors... we have determined that the issue would be most appropriately addressed in a service pack delivery rather than a security update."

Microsoft is not aware of any attacks made so far that were intended to exploit the vulnerability, the statement said.

Users who do not want to wait for the next IE service pack can avoid problems by changing IE's security settings so that the browser no longer prompts users before delivering active content, said Matthew Murphy, the researcher who discovered the bug. "The vulnerability at issue depends fundamentally on a weakness in the browser's method of prompting when warning users of potentially unsafe active content on a Web page," he said in a Full Disclosure mailing list posting.

This workaround may prevent IE from working properly on Web sites that depend on ActiveX controls, however, he warned.

Internet Explorer continues to be a prime target for attackers, with Microsoft having to patch a handful of publicly disclosed bugs in its latest security update for the product, which was released April 11.

Last Sunday, researcher Michael Zalewski posted details of a similarly critical bug in IE to Full Disclosure. Security firm Secunia rates Zalewski's bug as "highly critical."

http://www.computerworld.com/securitytopics/security/holes/story/0,10801,110974,00.html