The notorious Mpack hacker toolkit is installing malware that carries out all its chores -- including spewing spam -- from within the Windows kernel, making it extremely difficult for some security software to detect, Symantec said Thursday.

The Trojan horse that Symantec has dubbed "Srizbi" is being dropped onto some PCs by the multi-exploit Mpack, a ready-to-use attack application that until recently has been selling for around US$1,000. Responsibility for a large-scale attack launched from thousands of hijacked Web sites last month was pinned on Mpack, as was a follow-up campaign waged from compromised Internet porn sites.

Although Mpack can force-feed any malicious code to a commandeered PC, Symantec researchers said Srizbi stands out. Rather than follow the current practice of hiding only some activities with rootkit cloaking technologies, Srizbi goes completely undercover. The new Trojan, said Symantec, works without any user-mode payload and does everything from kernel-mode, including its main task: sending spam.

"When Trojans go to spam, they go out to userland," said Dave Cole, director of Symantec's security response team, referring to malware writers injecting their code into visible Windows processes to carry out spam-sending tasks. "Srizbi, though, is basically using kernel libraries [to send spam]," said Cole. "It's calling the kernel libraries it needs."

A blog entry by Kaoru Hayashi, a senior security response engineer at Symantec, spelled out the technical details. "Srizbi seems to move a step forward by working totally in kernel-mode without the need to inject anything into user-mode," wrote Hayashi. "To manipulate the network connection directly in kernel-mode, it attaches NDIS and TCP/IP drivers and gets all the Ndis* and Zw* functions that it needs. This technique also allows the Trojan to bypass firewall and sniffer tools, and to hide all its network activities."

Translation: Only advanced security tools will detect Srizbi.

It's also the first full-kernel malware spotted in the wild. "It's evolutionary, not revolutionary, but it's indicative of what we see in the threat landscape," Cole said. "And if your security software is relying only on [Windows'] APIs to detect malware, you won't be protected. You need something that uses modern, anti-rootkit techniques."

Cole added other information to the Srizbi story, in particular his suspicion that its author is probably the same hacker who created Rustock, a dangerous polymorphic Trojan from 2006 that also relied on rootkit technology to avoid detection. "It appears to be from the same author," said Cole. "He's been quiet for a while, and everyone has been waiting for Rustock 2.0. Some thought he'd maybe went away, but [Srizbi] uses some of the same components as Rustock."

Cole expects Srizbi to follow a similar trajectory. "The pattern with Rustock seemed to be to get something out there that might be sloppy at first, but to evolve it over time. It's reasonable to believe that [Srizbi] will do the same."

According to Symantec's research, the new Trojan is sloppy. "Users may be able to detect the presence of this malicious code via a registry key that happens to not be hidden by the rootkit component," said an alert to Symantec's DeepSight customers. The registry key to look for, said Symantec, is:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RcpApi\"MachineNum" = "[six random digits-six random digits-two random digits]"

In other Mpack-related news, a third Symantec security researcher said today that he's spotted the toolkit selling for as little as $150, an 85 percent reduction from its previous $1,000 price tag.

"Considering the toolkit is written in a script language, it is easy to redistribute and modify," said Eric Chien, a Symantec security response engineer. "The sellers likely didn't even need to buy it themselves, but rather probably found some of the multiple Web sites that did not employ standard Web site protections, allowing them to download the whole kit for free.

"Talk about clearance sales."

Mpack's wider "distribution" spells trouble not only for the hacker -- suspected to be Russian -- who created the multiple-strike attack kit, but for users, too. "Its usage is likely to grow," said Chien. "In addition, without a single author controlling the distribution of the toolkit, we also expect to see forks of the source code with additional exploits, bug fixes, and other feature enhancements."