Users of the Windows OS should install an unofficial security patch now, without waiting for Microsoft to make its move, security researchers at the SANS Institute's ISC (Internet Storm Center) have warned.

Their recommendation follows a new wave of attacks on a flaw in the way versions of Windows from 98 to XP handle malicious files in the WMF (Windows Metafile) format. One such attack arrives in an email message entitled "happy new year", bearing a malicious file attachment called HappyNewYear.jpg, which is really a disguised WMF file, security research companies including iDefense and F-Secure said on Sunday. Even though the file is labelled as a Jpeg, Windows recognises 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 email, "customers would have to be persuaded to click on a link within a malicious email or open an attachment that exploited the vulnerability." Microsoft's advisory can be found here.

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, Mikko Hypponen, F-Secure's chief research officer, wrote in the company's blog on Sunday.

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 in an email on Sunday.

Alarmed by the magnitude of the threat, 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 scrutinised 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."

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 recognises 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.

As a result of public holidays in Europe, Microsoft representatives could not immediately be reached for comment on Monday morning.

Guilfanov published his patch on his website on Saturday. His introduction to it can be found here.

F-Secure's Hypponen highlighted Guilfanov's patch in the F-Secure company blog on Saturday night, and then on Sunday echoed the ISC's advice to install the patch.

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

According to 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.