There's a massive attack currently taking place that's targeting servers running Microsoft's IIS. The attack results in infecting the sites with a malicious script, and has already compromised "tens of thousands" of sites already, according to an article on threatpost.com. The attack was first noticed earlier this week and has affected a number of high-profile sites, including those belonging to The Wall Street Journal, The Jerusalem Post, and the ServiceWomen.org site.
Experts say that an analysis of the attack points to the possibility that it takes advantage of a third-party ad management script used on the sites. The ad management script does not (apparently) properly sanitize incoming variables. According to Google over 114,000 different pages have been infected. The common factor appears to be that they are all hosted on IIS servers and using ASP.net.