New Windows-based "downloader" malware known as Trojan-Downloader.Win32.Diehard has surged to the top of Kaspersky Lab's "Virus Top Twenty" list a for December because of its "explosive propagation," the security firm said Wednesday.

A downloader is a type of malware, which loaded onto a victim's machine, can enable the attacker to download many other types of malicious code to exploit and control it for activities ranging from spam to information theft.