(Since this story was originally posted, it has been updated to correct the date that\u00a0Microsoft was notified of the flaw.)Excel users have one more reason to worry. On Tuesday, a hacker 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 website.The attack takes advantage of Adobe Systems\u2019 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.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\u2019t 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 said that while it is possible that an attacker could use this approach to load "a vulnerable ActiveX control already present on a user\u2019s 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\u2019s 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 2006 and has come up with a temporary workaround, the posting states.-Robert McMillan, IDG News Service (San Francisco Bureau)Related Links:\n\nSecond Microsoft Excel Attack Discovered by Hacker\n\nMicrosoft Offers Guidance on Excel BugThis article is posted on our Microsoft Informer page.\u00a0For more news on the Redmond, Wash.-based powerhouse, keep checking in.Check out our CIO News Alerts and Tech Informer pages for more updated news coverage.