Security firms have warned that patched systems running Windows XP and Windows Server 2003 can be successfully attacked by malicious hackers.

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 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:

  • open a malicious .WMF file in Windows Picture and Fax Viewer
  • 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. "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."

Microsoft is investigating "new public reports of a possible vulnerability in Windows," said a company spokesman. "Upon completion of this investigation, Microsoft will take the appropriate action to protect our customers, which may include providing a fix through our monthly release process or issuing a security advisory, depending on customer needs."

In the meantime, Microsoft encourages its customers follow its recommended security practices, he wrote. Users who believe they have been affected can contact Microsoft's product support team, he wrote. Ways to make this contact vary depending on where users are based. More information can be found on the Microsoft website.

Dunham characterises the threat as "significant," while Secunia rates it "extremely critical." Symantec Corp. labels it as a "level two" threat, on a scale in which "level four" is the most critical.

Secunia lists the vulnerable operating systems as Windows Server 2003 Datacenter Edition, Windows Server 2003 Enterprise Edition, Windows Server 2003 Standard Edition, Windows Server 2003 Web Edition, Windows XP Home Edition and Windows XP Professional.