Computer hackers have found another way to exploit an unpatched hole in Microsoft's Internet Explorer Web browser, using a specially designed attack Web site to install a Trojan horse program on vulnerable Windows machines.

The Trojan program changes the DNS (Domain Name System) configuration on the Windows machine so that requests for popular Web search engines like www.google.com and www.altavista.com bring the Web surfer to a Web site maintained by the hackers instead, according to warnings from leading security companies.

The attacks are just the latest in a string of online scams that rely on an easy-to-exploit flaw in IE known as the "ObjectData" vulnerability. Earlier attacks that relied on the vulnerability include a worm that spreads using America Online's Instant Messenger network.

Microsoft released a patch for the ObjectData vulnerability, MS03-032, in August. However, even machines that applied that patch are vulnerable to the latest attack because of holes in that security patch, according to a bulletin posted by Network Associates.

The Trojan horse program is called Qhosts-1 and rated a "low" threat, Network Associates (NAI) said. Trojan horse programs do not attempt to find and infect other systems. However, they do give attackers access to a compromised computer, often allowing a remote hacker to control the machine as if he or she were sitting in front of it.

Microsoft issued a statement Thursday saying that it was investigating reports of exploits for a variation on a vulnerability originally patched in Microsoft Security Bulletin MS03-032 and would release a fix for that hole shortly. A company spokesman could not say when the patch update will be released.

The Redmond, Washington, company recommended that customers worried about attacks install the latest Windows updates and change their IE Internet security zone settings to notify the user when suspicious programs are being run.

Qhosts-1 was installed on vulnerable Windows machines using attack code planted in a pop-up ad connected to a Web page set up by the hackers on a free Web hosting site, www.fortunecity.com, NAI said. The DNS servers used in the attack resided on systems owned by Houston, Texas hosting firm Everyone's Internet, according to Richard Smith, an independent computer security consultant in Boston.

Those servers, as well as the fortunecity.com site used to install the Trojan, have been taken offline since the attack caught the attention of security experts. That will stop the DNS hijackings, but will also make it impossible for users on infected computers to browse the Web until their DNS configuration is restored, he said. However, as long as the Microsoft hole remains unpatched, similar attacks could be launched.

To be attacked, Windows machines had to be running Internet Explorer versions 5.01, 5.5 or 6.0, which contain the ObjectData vulnerability, and visit the Web site that launched the pop-up. The pop-up ad exploited the ObjectData vulnerability then downloaded the Qhosts-1 Trojan from a Web site in Seattle, Smith said.

There are still questions about how users were lured to the fortunecity.com site that installed the Trojan, but unsolicited commercial ("spam") e-mail with links to the site was a likely suspect and economic gain was a likely motive, Smith said.

Hackers used the DNS changes to drive Web surfers to a site that launched a variety of pop-up advertisements, resulting in increased Web traffic and advertising revenue for the individuals behind the scheme, he said.

The latest attack is an example of the increasingly sophisticated strategies used by malicious hackers, who adopt the strategies of legitimate online businesses, cobbling together available Web technologies in a "Tinker Toy" fashion to create sophisticated attacks, Smith said.

By relying on a network of sites hosted on free and fee-based Internet hosting sites, hackers also make it more difficult for authorities to follow their tracks. Identity theft frequently plays a role in the latest scams as well. Hackers use stolen credit card information to set up hosting accounts which are then used as part of Internet based attacks, he said.