Security watchers Marshal claim the infamous Storm botnet is no more, after waning spam emails finally dried up altogether last month.

Other security researchers have noted a similar decline, but warn that while the botnet is currently inactive it may yet return, possibly in a more potent form.

Storm front

The malware used to create zombie clients within the Storm botnet first began circulating in January 2007 in emails referencing the lethal storms ravaging Europe at the time. Prospective marks were invited to click links that directed them towards booby-trapped websites that infected Windows PCs.

Over time the gang behind the Storm worm - actually more accurately described as a Trojan - developed a variety of different social engineering lures, including fake electronic greeting cards. No new campaigns have been launched for a month.

At the peak of its activity in September 2007, compromised proxies in the Storm worm botnet were throwing out 20 per cent of the world's junk emails, according to Marshal. Industry estimates at the time (since revised) suggested anywhere between one and ten million Windows PCs had become zombie drones within the Storm worm botnet at the peak of its activity. More detailed studies have since put the figure of compromised clients in the Storm botnet somewhere between 500,000 and one million.

The success of the unknown bot-herders behind the worm inspired similar attacks by other groups, likewise designed to establish a network of compromised machines. These zombie clients were used to distribute junk mail or launch denial of service attacks, either by their owners themselves or (more commonly) by spammers and other miscreants who rented access to compromised machines via underground forums.

"Storm was one of the first botnets to use these tactics on a mass scale," explained Phil Hay, a lead threat analyst at Marshal. "It became the most successful botnet of its type and established the basic template for developing a spam empire that other botnets have since copied."

"Whoever was behind Storm really set the benchmark at the time for the kind of scale that was achievable with a spambot. They also led the way in using self-perpetuating malicious spam to grow the botnet. They utilised every social engineering trick in the book and invented quite a few of their own."

Storm’s success ultimately led to its undoing, especially after Microsoft targeted Storm through the Malicious Software Removal Tool in September 2007. Redmond reported that 274,372 Windows PCs were cleaned up using its tool during the first month alone.

By January 2008 the Storm worm was pumping out two per cent of the world's junk mail as Microsoft's clean-up efforts, as well as competition from rival botnets, ate into the malware's "market share".