While users wait for a Microsoft fix, many anti-virus products will protect PCs against attacks that exploit a recently disclosed Windows flaw, but not all.

According to a test of a range of anti-virus products published on Wednesday in the United States, Trend Micro was the only major anti-virus vendor that failed to catch a number of malicious files that exploit the new Windows vulnerability.

In the test, administered by independent testing organisation AV-Test, 206 malicious files were pushed through virus shields from a number of vendors. Of the top three anti-virus companies, Symantec and McAfee caught all bad files, while Trend Micro missed 63, according to the test results, which were e-mailed to CNET News.com.

Several smaller providers of anti-virus products also caught all the examples of malicious code, including Sophos, Kaspersky, Computer Associates International, F-Secure and BitDefender. Microsoft's new Windows OneCare, currently available as a test version, also protects against all the attacks, according to AV-Test.

Trend Micro is working to update its product to improve detection, said Raimund Genes, chief technologist for Trend Micro in Europe. "We have the luxury to have some of the biggest customers in the world, but this is also a burden because this means that we have to do a very careful quality assurance," he said.

Still, Genes contends that Trend Micro's product offers good protection. It might not catch all the files used in the test, but it does catch all the malicious files currently found "in the wild" on the Internet, he said.

The Windows flaw is atypical, making it more complicated for most makers of antivirus software to provide protection, said Andreas Marx, an anti-virus software specialist at the University of Magdeburg in Germany and head of the AV-Test.

The flaw lies in the way Windows renders Windows Meta File images. The bug was discovered last week and is being exploited in attacks that compromise a vulnerable PC if the user visits a Web site with a malicious image file.

"Anti-virus companies have the problem that the attacks involve a file format that was not used for previous attacks," Marx said in an interview via e-mail. "The researchers had to dig through the file format, and detection routines had to be carefully tested in order to avoid false positives."

Some providers of anti-virus software are still working on proper detection routines and may offer protection against only the most widespread exploits, Marx said.

"All anti-virus tools are developed in a different way," he said. "Depending on the code, it might be rather easy for some companies to add detection of the exploit codes by simply adding a new signature. In other cases, engine or even program changes need to be made."

In Trend Micro's case the company is working on fine tuning detection capabilities, Genes said. The challenge is finding a balance between detection capabilities of the new file type and speed of the scan engine, he said.

AV-Test also tested free anti-virus products, including Clam AntiVirus and AVG. While Clam AntiVirus stopped all but one file, AVG let through 59 malicious files, according to the test.

The detection in Clam AntiVirus works well, but will result in many false positives and stop almost all WMF files, Marx said. That's not a big problem because Clam AntiVirus is used mostly as a gateway scanner, not on the desktop, he said.

"AVG, on the other hand, is mainly used on PCs. The company has to avoid false positives," Marx said. "I know that the AVG team is working day and night on a solution."

Meanwhile, experts have warned that thousands of malicious Web sites as well as Trojan horses and at least one instant messaging worm that use the WMF flaw as a conduit have surfaced.

Microsoft, however, says it has not seen many attacks on its customers. It plans to deliver a fix on Tuesday in the US as part of its monthly patch cycle. Until then customers can protect themselves using a workaround and by following standard security guidelines, which includes the use of updates anti-virus software, Microsoft said in a security advisory.