Storm, the Trojan that Hoovers PCs into hacker-controlled botnets, roared back into life last month in several waves, security researchers said Monday, and has blown by 2005's Sober to become the most prolific e-mail-borne malware ever.

"This is the biggest since Sober in mid- to late 2005," said Sam Masiello, the director of threat research at MX Logic, referring to a long-lasting worm whose variants struck repeatedly in the second half of 2005, often in extremely high numbers. In November 2006, for instance, e-mail filtering companies reported malware-laden e-mail counts spiking 1,500 percent in a week, and said they were intercepting four times the usual number of infected messages.

According to MX Logic, Storm -- a bot Trojan that collects compromised computers into large networks of ready-to-use PCs -- has broken Sober's records. Thanks to Storm, the Englewood, Colo., managed e-mail security vendor tracked a July jump in malicious e-mail of 1,700 percent over June.

Storm, however, is much more malevolent than Sober ever dreamed. "Not only is it designed to propagate more copies of Storm, but it releases huge quantities of spam," said Masiello.

Security analysts, Wood and Masiello among them, have been drawing a line between Storm's success and spam outbursts of July and August, including one that dropped impressive quantities of "pump-and-dump" stock scam mail in mailboxes worldwide. "Certain Trojans are specifically written not only to make their own botnet larger, but to propagate specific types of spam," Masiello said.

Other researchers are also convinced that Storm is directly related to the jump in spam users saw in July and are still seeing now. "Looking at the network traffic overall, where [malware] is being hosted, I think it's almost certainly a cause and effect," said Paul Wood, senior analyst with MessageLabs. "The Storm botnet is one of the most successful we've seen in recent times. And now that's paying off, from [the hackers'] perspective."

Storm started to gather steam near the end of June, when several spasms of mail posing as greeting cards reached users, and reach critical mass just before July 4, when holiday subject lines tempted even larger numbers of users to click through. "Storm's using more of a 'pull' than a 'push' model now," said Masiello. Earlier Storm bot-building campaigns had come with attachments that when run hijacked the targeted PC. More recent attacks simply offer up a link in the e-mail; when users click on the link, code on the ensuing site -- actually, often several exploits that try several vulnerabilities until one works -- snatches the PC.

For example, Symantec researchers said on Saturday that their honeypots had snared several examples of mail that tried to dupe users into visiting sites where Mpack, a multiple-exploit hacker kit, awaited. When successful, Mpack then installed Storm, which goes by Peacomm in Symantec's nomenclature. "While Mpack itself is not new, the Peacomm/Mpack link is," said Symantec analyst Anthony Roe in an alert sent to customers of the company's DeepSight threat network. "This is a new propagation vector for the Peacomm Trojan."

But although it had traced a new variation on the Storm saga, Symantec wasn't buying the bot-to-spam link. "Internet news sites are discussing unsubstantiated reports that Trojan.Peacomm [Storm] infections have resulted in a massive attacker-controlled P2P bot network that is accountable for the huge surge in penny stock pumping spam observed lately," said Roe. "This appears to be mostly conjecture."

Masiello was having none of that. "There's definitely a correlation between Storm and recent spam attacks," he said.

In fact, Masiello argued that the large Storm-recruited botnets means the volume of spam, and its percentage of all e-mail, will continue to climb between now and the end of the year. "Spam is generally cyclical," with the peak in the last quarter. "They're setting up the infrastructure now" for that late-year push.