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.

BitDefender®, a global provider of award-winning antivirus software and data security solutions, announced today that BitDefender antivirus analysts have detected a new trojan, which hijacks Google text advertisements, replacing them with ads from a different provider. The threat, which is identified by BitDefender as Trojan.Qhost.WU, modifies the infected computers' Hosts file (a local storage for domain name / IP address mappings, which is consulted before domain name servers and is considered authoritative).