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IDG News Service - The first widespread attack to leverage a recently patched flaw in Microsoft's Internet Explorer browser has surfaced.

Starting late Wednesday, researchers at antivirus vendor Symantec's Security Response group began spotting dozens of Web sites that contain the Internet Explorer attack, which works reliably on the IE 6 browser, running on Windows XP.

The attack installs a Trojan horse program that is able to bypass some security products and then give hackers access to the system, said Joshua Talbot, a security intelligence manager with Symantec.

Once it has infected a PC, the Trojan sends a notification e-mail to the attackers, using a U.S.-based, free e-mail service that Symantec declined to name.

As of midday Thursday, Symantec had spotted hundreds of Web sites that hosted the attack code, typically on free Web-hosting services or domains that the attackers had registered themselves.

The IE flaw being leveraged in these attacks was also used to hack into Google's corporate network last December. It has been linked to similar incidents at 33 other companies, including Adobe Systems. Microsoft patched the vulnerability in an emergency security update Thursday morning.

The Google attack hit IE 6 on Windows XP, but over the past week hackers have found ways to exploit the flaw on more recent versions of the browser as well. These latest techniques do not appear to be used on the Web sites Symantec has uncovered. They use the IE 6 exploit code, Talbot said.

Still, with IE 6 still being widely used, the move to more widespread attacks is worrying. "It may be an indication that attackers have finally ramped up their attack toolkits and are now ready to launch widespread attacks," Talbot said.

He believes that the criminals are tricking victims into visiting their Web sites by sending spam e-mail or instant messages with links to sites.

On Thursday, Websense published some sample e-mails used in targeted attacks that exploit the IE bug. A typical subject line is "Helping You Serve Your Customers." The e-mail reads, "I just heard the news: Helping you serve your customers" and includes a link to the malicious Web site.

The e-mails contain spoofed e-mail addresses, designed to fool victims into thinking that they were sent by a colleague. The malicious Trojan used in the attack is not the same one that was used in the Google attack, however.

Websense has seen these e-mails sent to targeted companies in the U.S. and the U.K., said Patrik Runald, a security research manager with Websense. "These attacks are actually continuing; they happened today; they happened yesterday and they happened the day before."

However, Websense believes that the e-mails it has tracked are part of a small-scale targeted attack, similar to those used on Google and Adobe in attacks that are ongoing. Websense has counted only about 25 malicious Web sites to date, but the number is rising fast, Runald said.

Security experts believe this more targeted technique is used as part of a systematic cyber-espionage campaign, which some have linked to China.