NetSky

NetSky is a prolific family of mass-mailing computer worms targeting Microsoft Windows systems that first emerged in February 2004. It spread primarily through email messages carrying malicious attachments and relied on user execution of the attachment to begin infection. Once active, NetSky harvested email addresses from the compromised host and mailed copies of itself to discovered contacts, enabling rapid self-propagation across organizations and consumer systems.

The family became one of the most prevalent email-borne malware threats of the mid-2000s, with numerous variants appearing in quick succession. NetSky is widely attributed to Sven Jaschan, a German malware author who also confessed to creating Sasser and at least some NetSky variants. Several variants contained taunting messages aimed at the authors of rival malware families, especially Bagle and Mydoom, and some variants attempted to remove those worms from infected systems. This rivalry contributed to a highly visible malware "war" marked by frequent new releases and mutual interference among competing worm families.

Observed behavior associated with NetSky includes address harvesting, autonomous mass-mailing after execution, and user-visible nuisance effects such as beeping on specified dates in some variants. The malware affected Windows environments broadly and achieved sustained global prevalence; the NetSky.P variant remained one of the most widespread email-borne viruses until late 2006. NetSky is historically significant as a defining example of early-2000s Windows email worms and of the competitive dynamics among malware authors during that period.