The Trojan that pumped up spam volumes in January is at it again, and is now spreading over instant messaging and even attacking rival malware.

Symantec researchers said that the "Storm Trojan," aka "Peacomm", is now spreading via AOL Instant Messenger (AIM), Google Talk and Yahoo Messenger.

An alert to some Symantec customers pegged the new infection vector as "insidious" because the message - such as the cryptic "LOL ;)" - and the included URL can be dynamically updated by the attacker. Even worse, according to Alfred Huger, senior director of Symantec's security response team, "it injects a message and URL only into already-open windows. It's not just some random message that pops up, but it appears only to people [you are] already talking to. That makes the approach very effective."

Moreover, the server from which the malware is downloaded to the victim's PC can be quickly changed by the attacker using the Trojan's peer-to-peer (P2P) control channel. "Everything can constantly change," said Huger.

The newest attack by Peacomm follows an earlier campaign in January, when the Trojan got its nickname from e-mail subject headings that touted news of massive storms throughout Europe.

But one researcher traced the Trojan further back than that. According to an analysis by Joe Stewart, a SecureWorks senior security researcher, Peacomm is actually a spin-off of last year's "Nuwar" worm. "It's pretty much the same code," said Stewart.

Both Stewart and Huger also noted that the Trojan has been behind several recent distributed denial-of-service (DDoS) attacks against anti-spam Web sites, as well as servers supporting rival malware.

Among the multiple second-stage components downloaded to Windows PCs compromised by Peacomm, said Stewart, is a DDoS module that can be enabled at will by the attacker and aimed at any site. The January target list included spamnation.info, which was knocked offline for eight days starting January 12. The better-known spamhaus.org was an indirect victim, too.

Systems hijacked by Peacomm have also conducted DDoS attacks against at least five domains used by the creators of the noted Warezov (or Stration) worm. After a busy September and October, Warezov was credited by some analysts as the genesis of 2006's massive fourth-quarter spike in spam volume.

"It seems that this spam group is prone to attack anyone that interferes with its business model, be it anti-spammer or spammer," said Stewart.

Symantec's Huger agreed: "Malware groups compete with each other [over compromised systems] as much as for uninfected machines." In fact, according to Symantec's data, the bulk of DDoS activity is due to hacker internecine warfare as one group tries to blunt another's attempt to corral large numbers of PCs in botnets.

But the two researchers disagree when it comes to pegging Peacomm with a label. "This guy is pulling from the standard playbook, taking the best techniques and using off-the-shelf protocols," Stewart said as he characterised the malware's maker as persistent, not technically sophisticated. "It's very basic stuff, but it works."

Huger sees it differently. "Whether he's technically sophisticated or not, the point is moot," said Huger. "This is very well thought out and very well staged."