With Microsoft developers scrambling to patch a security hole in Excel, a hacker has now posted code that exploits a second vulnerability in the spreadsheet software.

Microsoft says that criminals are not yet using this code in attacks, but the software could be used to run unauthorised programs on a PC, according to Marc Maiffret, chief hacking officer at security software vendor eEye Digital Security.

It marks the second time in the past week that Excel users have had to worry about this type of malware. On Monday, Microsoft published an alert warning of a similarly critical attack, which was first discovered late last week.

Both attacks rely on users to open maliciously encoded Excel documents, but this new attack is less critical than the first. That's because users must take the additional step of clicking on a specially crafted hyperlink for the attack to work.

On Excel 2003, even more work is required, as users must also dismiss a warning pop-up window before the malicious code can run.

"You'd really have to go out of your way in Excel 2003 to shoot yourself in the foot," Maiffret said.

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Microsoft has published suggested workarounds for the first attack, but the company has had little to say on this latest vulnerability.

"Microsoft is investigating reports of the posting," a spokeswoman for the company's public relations agency said Tuesday in an e-mail.

With Microsoft gradually shoring up the holes in its operating system and browser software, hackers have been looking for new areas in which to poke and prod for security vulnerabilities.

Office is proving to be one such spot. Just last week, Microsoft patched a similarly critical hole in its Word software, and Office and Outlook Express are now listed among the top sources of Internet security holes, as rated by the SANS Institute.