Fully patched systems running Windows XP and Windows Server 2003 can be successfully attacked by malicious hackers, various security firms warned Tuesday and Wednesday.

The attacks can be carried out thanks to a newly discovered vulnerability in those operating systems' handling of corrupted .WMF (Windows Metafile) graphic files, the firms said.

The firms describe the exploit as "zero day," because malicious hackers are taking advantage of it while there is no patch or certified workaround against the vulnerability.

Malicious hackers can run the code of their choice on compromised systems, and even machines that have all available patches installed are vulnerable, according to several advisories.

Currently, security firms are warning that machines can be attacked if users do any of the following:

-- visit hostile Web sites hosting exploits;

-- open a malicious .WMF file in Windows Picture and Fax Viewer;

-- or preview a malicious .WMF file in Windows Explorer.

However, the number of attacks could increase dramatically if malicious hackers find more automated ways to target systems, such as using e-mail, instant messages or file sharing, according to Ken Dunham, director of the rapid response team at VeriSign's iDefense.

Attacks so far have been limited to installation of adware and spyware on compromised machines, but "you’re probably going to see Trojans and more sinister code develop and emerge in the next few days," Dunham said in an interview.

There is no patch for the security hole. While some workarounds are being suggested on the Web, Dunham is only validating this one for disabling .WMF file handling: First, users should click on the Start button on the taskbar. Then they should click on Run, type "regsvr32 /u shimgvw.dll" and click "Ok" when the change dialog appears.

However, Dunham warns that recent vulnerabilities related to .WMF have also included .EMF files and that "it is possible that exploitation might still be possible through alternative file types such as EMF," he wrote in an e-mail alert Wednesday afternoon. "For now, the WMF disabling workaround may help mitigate attacks against vulnerable Windows XP/2003 computers. This workaround may impact the display of thumbnails or other images on the computer."

The IDG News Service is a Network World affiliate.