NEW YORK (CNNMoney.com) - The new year is off to a rocky start at Microsoft, where security experts are scrambling to confront a potentially massive virus threat to Windows PCs.

According to a report Tuesday in the Financial Times, the latest vulnerability involves a flaw which allows hackers to infect computers using programs inserted into image files. The threat was discovered last week. But it mushroomed over the weekend, when a group of hackers published the source code they used to exploit the flaw.

What makes this threat particularly vicious, according to the Times, is that unwitting victims can infect their computers simply by viewing a web page, e-mail, or instant message that includes a contaminated image. That differs from most virus attacks, which require a user to actually download an infected file.

"The potential [security threat] is huge," Mikko Hypponen, chief research officer at F-Secure, an antivirus company, told the Times. "It's probably bigger than for any other vulnerability we've seen.

"Any version of Windows is vulnerable right now," said Mr. Hypponen, including every Windows system shipped since 1990.

Microsoft said a security patch would be available for the problem on Tuesday, January 10 after it has passed rigorous testing procedures.

Because of the severity of the threat, the SANS Institute, a computer security group, has released a patch for the vulnerability until Microsoft's fix is available next week. It is available here.

Shares in Microsoft (up $0.78 to $26.93, Research) rose nearly 3 percent in mid-day trade on Nasdaq.

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