Exploit kits remain an efficient mechanism for cyber criminals to distribute malware. Such kits include exploits for multiple vulnerabilities within a single malicious webpage. Criminals can check operating systems, web browsers and browser plugins for anything that is not fully patched and launch an exploit specific to the out of date software. Using this technique criminals can maximise their chances of infecting visitors but reduce their exposure to only infect those who are vulnerable; presumably in order to remain inconspicuous.
more here...........http://blogs.cisco.com/talos/evolution-nuclear-ek/
more here...........http://blogs.cisco.com/talos/evolution-nuclear-ek/