Windows users should install an unofficial security patch now, without waiting for Microsoft to make its move, advised security researchers at The SANS Institute's Internet Storm Center (ISC).

Their recommendation follows a new wave of attacks on a flaw in the way versions of Windows from 98 through XP handle malicious files in the WMF (Windows Metafile) format. One such attack arrives in an e-mail message entitled "happy new year," bearing a malicious file attachment called "HappyNewYear.jpg" that is really a disguised WMF file, security research companies including iDefense and F-Secure. said Sunday. Even though the file is labelled as a JPEG, Windows recognizes the content as a WMF and attempts to execute the code it contains.

Microsoft advised on 28 December that to exploit a WMF vulnerability by e-mail, "customers would have to be persuaded to click on a link within a malicious e-mail or open an attachment that exploited the vulnerability."

However, simply viewing the folder that contains the affected file, or even allowing the file to be indexed by desktop search utilities such as the Google Desktop, can trigger its payload, F-Secure's Chief Research Officer Mikko Hypponen wrote in the company's blog.

In addition, source code for a new exploit was widely available on the Internet by Saturday, allowing the creation of new attacks with varied payloads.The file "HappyNewYear.jpg," for example, attempts to download the Bifrose backdoor, researchers said.

These factors exacerbate the problem, according to Ken Dunham, director of the rapid response team at iDefense.

"Risk has gone up significantly in the past 24 hours for any network still not protected against the WMF exploit," Dunham warned.

Staff at the ISC worked over the weekend to validate and improve an unofficial patch developed by Ilfak Guilfanov to fix the WMF problem, according to an entry in the Handler's Diary, a running commentary on major IT security problems on the ISC website.

"We have very carefully scrutinized this patch. It does only what is advertised, it is reversible, and, in our opinion, it is both safe and effective," Tom Liston wrote in the diary.

"You cannot wait for the official MS patch, you cannot block this one at the border, and you cannot leave your systems unprotected," Liston wrote.

In the diary, ISC provided a link to the version of the patch it has examined, including a version designed for unattended installation on corporate systems.

While ISC recognizes that corporate users will find it unacceptable to install an unofficial patch, "Acceptable or not, folks, you have to trust someone in this situation," Liston wrote.

Guilfanov published his patch on his website on Saturday.

Not all computers are vulnerable to the WMF threat: those running non-Windows operating systems are not affected.

According to iDefense's Dunham, Windows machines running Windows Data Execution Prevention (DEP) software are at least safe from the WMF attacks seen so far. However, Microsoft said that software DEP offered no protection from the threat, although hardware DEP may help.