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Excel users have one more reason to worry. A hacker on Tuesday published code that takes advantage of an unpatched flaw in the Microsoft spreadsheet software, the third such exploit to be disclosed in the past week.

This attack could be used to run unauthorized software on a PC, but it requires that the victim first be tricked into opening an Excel document, according to an alert published on the Securitytracker.com Web site.

The attack takes advantage of Adobe System's Flash technology, which can be used to provide graphics and animation to Excel documents. "When the target user opens the Excel file, the [malicious] Flash code will execute automatically without user interaction," the alert states.

Why Worry?

This latest attack is worrisome because the code can be executed with little user interaction, said Juha-Matti Laurio, an independent security researcher based in Helsinki. "Embedding this type of objects to spreadsheets is not common, but it doesn't matter, because the infection is caused just by opening the file," he said.

Microsoft disputed the notion that this code exploited a vulnerability in its products. The company says that while it is possible that an attacker could use this approach to load "a vulnerable ActiveX control already present on a user's system," it is not aware of any attacks that actually let hackers run their code remotely.

"Microsoft is not aware of any vulnerable ActiveX controls that would lead to remote code execution in this context, attempts to use this method of attack, or of customer impact at this time," the company said in a statement.

Recent versions of Office have a feature that prevents such malicious ActiveX controls from loading once Microsoft has identified them, Microsoft added.

Microsoft's security team had a busy couple of weeks. Last Tuesday, the company patched a critical flaw in its Word software that had been exploited by attackers, and Microsoft is also contending with attack code that exploits two other Excel flaws. All three Excel flaws have cropped up over the past week.

This latest attack was published late Tuesday on the Full Disclosure security discussion list. It was tested on Office 2003 running on Windows XP Professional Edition and Windows 2000, according to the Full Disclosure posting.

Microsoft was notified of the flaw in May of this year and has come up with a temporary workaround to the flaw, the posting states.