Office users face new macro risk

Malformed macros can escape the macro detection in Microsoft Excel and PowerPoint applications, so having the run of the affected PC

A new vulnerability has been detected in systems running Microsoft Excel and PowerPoint, which allows a specially crafted macro to avoid detection and run malicious code on a victim's machine.

The software hole allows a malformed macro to escape the security warnings built into Excel and PowerPoint, so that a user can unknowingly run infected macros when opening a spreadsheet or presentation.

"Macros have their own programming language, and can automate the tasks that a user would normally do," said Mark Read, security analyst at MIS Corporate Defence Solutions. "There is the potential for them to release code that could be very dangerous -- it doesn't matter what the application is."

The macro could be programmed to take any action that the owner of the PC could take. According to US government Internet security organisation CERT, this could include: reading, deleting or modifying data; modifying security settings; sending email; or posting data on Web sites.

The strong potential for macro viruses was demonstrated on a massive scale two years ago with the Melissa virus. The macro virus propagated in the form of an email containing an infected Word document as an attachment. When a user opened an infected .doc file with Microsoft Word97 or Word2000, Melissa immediately executed itself if the macros were enabled.

"We've had macro viruses around for a long time -- since Microsoft built the angle into its products, " said Read. "Microsoft added warnings giving the user an option to back out, but now this safeguard has been removed through this new vulnerability, creating new potentials for script-kiddies."

Microsoft has issued a security bulletin about the Excel and PowerPoint vulnerabilities on its site, but is refusing to make any further comment.

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