According to CSIS Security Group, the bug is in an ActiveX control, the "msvidctl.dll" file, that streams video content.

"CSIS has captured more systematic drive-by attacks exploiting a vulnerability in Microsoft DirectShow," the company warned on its Web site. "Thousands of Web sites have been compromised over the weekend and malicious script has been insert[ed]," it added ( Google Translate translation).

The script re-routes users to a malicious site, which in turn downloads and launches a multi-exploit hacker toolkit that includes the DirectShow attack code. DirectShow is a part of Windows' DirectX graphics infrastructure.

Windows 2000, XP and Server 2003 are all vulnerable to attack, CSIS said.

Another Danish security firm, Copenhagen-based Secunia, ranked the vulnerability as "Extremely critical," its highest threat rating. Secunia had no additional information about the bug, however.

This newly-exploited vulnerability is the second unpatched DirectShow bug to surface in the last five weeks. In late May, Microsoft issued a security advisory that reported hackers were exploiting a different DirectShow bug, this one in its QuickTime media parser. A week ago, Symantec said that attack code for the QuickTime parser vulnerability had been added to a multi-exploit toolkit, and that users should expect more attacks.

Hackers have been using the QuickTime parser bug since May, Microsoft has acknowledged.

Today, a Microsoft spokesman said that company security researchers would be posting information about the newest vulnerability soon.

Patches are not available for either vulnerability, although Microsoft has suggested that users disable QuickTime parsing on Windows 2000, XP and Server 2003 machines. To expedite that, Microsoft has posted a tool that automates the process. CSIS recommended that users protect themselves against the newest bug by setting the "kill bit" of the ActiveX control.

Microsoft's next regularly-scheduled security updates are due July 14. While most researches have said they expect the company to patch the DirectShow bug then, it's unclear whether Microsoft will fix the video streaming vulnerability at the same time.