The DirectX software development kit Microsoft issued in 2002 contains a critical vulnerability, a Polish researcher claimed as he released attack code that can hijack Windows PCs by tempting Internet Explorer users to malicious sites.

According to Krystian Kloskowski, who posted exploit code on the milw0rm.com site, the FlashPix ActiveX control included with DirectX Media 6.0 SDK contains a buffer overflow bug that can be exploited. More importantly, according to an advisory issued by US-CERT on Sunday, "because the FlashPix ActiveX control is marked 'Safe for Scripting,' Internet Explorer can be used as an attack vector for this vulnerability."

Internet Explorer 6 (IE 6) can be leveraged to exploit the flaw, noted Kloskowski, but there was no word on whether the newer IE 7 is also a workable attack vector. Microsoft did not immediately reply to queries on the severity of Kloskowski's vulnerability.

The likely attack scenario, said US-CERT, would be a malicious site that includes the exploit, and spam that tries to dupe users into clicking on a link to that site. Alternately, an HTML e-mail message -- with the exploit buried in the HTML -- could also be used. In that case, infection would occur as soon as the recipient viewed the message.

Danish bug tracker Secunia rated the vulnerability as "highly critical," its second-highest threat ranking in its five-step scoring system. US-CERT, meanwhile, recommended taking the somewhat-extreme steps of either disabling all ActiveX controls or setting what's called a "kill bit" using the registry to disarm only the FlashPix control. US-CERT's warning included the string to add to the Windows registry to set the FlashPix kill bit.

Although Microsoft has added additional security features to both IE 6 and IE 7 over the years to clamp down on threats posed by buggy ActiveX controls, they remain a problem. Late last month, for example, Yahoo Widgets, a platform that runs small, Web-based gadgets on a Windows machine's desktop, was tagged with a critical vulnerability in an associated ActiveX control.