The creators of a Web-based attack tool called Angler Exploit Kit have added an exploit for a known vulnerability in Microsoft's Silverlight browser plug-in to the tool's arsenal. Exploit kits are essentially malicious Web applications that check if visitors run outdated software on their computers and then exploit vulnerabilities in that software to install malware. They usually target popular applications that are accessible through browser plug-ins, such as Java, Flash Player, and Adobe Reader.