Way back in 1998, Microsoft issued a security bulletin about a component of IIS that ran under Windows NT Server called Microsoft Data Access Components. In the bulletin, MS98-004, Microsoft warned that a part of MDAC called the RDS (Remote Data Service) had a vulnerability that allowed unauthorized people to browse databases.
Flash-forward eight years to the spring of 2006. Microsoft released a security bulletin about a component of MDAC called RDS, which has a vulnerability that permits malicious Web servers to perform drive-by downloads against the unpatched PCs of unsuspecting victims. Eerily familar.
In the later case, it was an ActiveX control that allowed users to connect to RDS through IE and wreak havoc. The ActiveX control doesn't behave as intended, and can be loaded and exploited if you visit the wrong Web site.
Of course, by 2006, MDAC isn't just loaded on servers; you may have it on your PC. Moreover, the bad guys have changed tactics. No longer content to wait patiently for you to happen upon their malicious Web site, they spam you with links, buy ads based on Google searches, and load their pages with SEO (search engine optimization)-rich keywords. The result, however, is the same: Visit and be exploited.
In fact, the bad guys are now using off-the-shelf exploit software to put malware onto your machine. A tool called MPack that's loaded on malicious Web sites can check to see what browser version you're using and what patches you have installed. Based on this analysis, it delivers the exploits that will do the most damage. More galling is that they don't even bother to hide what they're doing, naming the Web page that performs the exploit "mdac4.php."
Upshot: The MDAC RDS is a complex system, with a multitude of patches available depending on which version you have installed. Manually choosing the right patch can be a complicated task. But with such a serious flaw, you can't afford to make a mistake. Patches like these have helped push advancements in Windows Update, which scan your system and pick the right patch automatically, so you don't have to.