A flaw in the Windows handles digital certificates enables sophisticated crackers to get up to all sorts of mischief on unprotected boxes.
An ActiveX control called Certificate Enrolment Control (which ships with all versions of Windows), which deals with web-based certificate enrolments, can be manipulated to delete certificates on a victim's PC.
"An attacker who successfully exploited the vulnerability could corrupt trusted root certificates, EFS encryption certificates, email signing certificates, and any other certificates on the system, thereby preventing the user from using these features," Microsoft warns in an advisory.
Attacks can be carried out through the ever-popular vectors of either a maliciously constructed Web page or a poisoned HTML email.
Although the flaw is difficult to exploit (as it requires knowledge of digital certificates), Microsoft urges users to consider applying the patch it has supplied as soon as possible by describing it as of critical importance.
In addition, the patch addresses a similar, but less serious vulnerability discovered in the SmartCard Enrolment control, which ships with Windows 2000 and Windows XP.
News of the ActiveX control problem comes as Microsoft continues to grapple with a more serious security vulnerability involving how Internet Explorer identifies secure Web sites.
To recap, a colossal stuff-up in Microsoft's and KDE's implementation of SSL (Secure Sockets Layer) certificate handling made it possible for anyone with a valid VeriSign SSL site certificate to forge any other VeriSign SSL site certificate, and abuse hapless Konqueror and Internet Explorer users with impunity.