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In January 2007, a deadly storm hit Europe. Malware authors used the news as an opportunity to practice social engineering. The malicious payload was delivered with subject lines such as "230 dead as storm batters Europe." A thorough writeup by Trend Micro suggests just how effective it was:
The spam attack started just as the storm in Europe was at its strongest on January 18. Over the next few hours and into the next day, as hundreds of thousands of recipients, interested in information about the storm, opened their inboxes, the global computing community found itself in the face of a huge threat attack.
Besides effective social engineering, the Storm worm was among the first widely successful malware examples to use polymorphic techniques, capable of changing its packing code every 10 minutes to frustrate antivirus signatures. It also employed a peer-to-peer network that could rapidly change the IP addresses of its command-and-control servers.
Various members of the Nuwar family continued to deliver malicious payloads throughout 2007. Microsoft added it to the Malicious Software Removal tool in September 2007, and it immediately saw a precipitous decline. It was still active, but at a greatly reduced level, in mid-2008 and had begun using RSA encryption to hide its workings from security researchers.