'Rustock' Trojan A Model For Future Threats

That threat, dubbed "Rustock" by Symantec, is a family of backdoor Trojan horses that first appeared nearly a year ago, says Patrick Martin, a senior product manager with the Cupertino, Calif., company's security response team.

"The techniques that [Rustock] is using will be the baseline for threats in the future," Martin says. "Attackers are looking around to see what techniques are working, then incorporating them. [Things] like this are the threats of the future."

Among Rustock's distinguishing characteristics are its heavy reliance on advanced rootkit technologies to hide from security software and its changeling-like ability to morph itself each time it infects a file.

"A threat has to get on, it has to stay on, and it has to carry out whatever payload its maker assigns," says Martin, spelling out the three most important duties of any malware. Rustock, he says, had all three in spades.

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"It's using techniques that most rootkit detectors aren't looking at or for yet," says Martin. "Once it gets a foothold on a system, it digs itself into that system." The longer a Trojan can remain undetected the longer it can stay on a PC, and the more income it can generate for its owner. Rustock, like other recent in-the-news exploits such as "Stration," is designed to send spam from hijacked computers. Rustock specializes in sending image-based spam, which dramatically jumped in volume during October and November.

Rustock hooks into the Windows 32-bit kernel, and patches several APIs (Applications Programming Interfaces) to hide the new registry keys and files it installs. But it also has some rootkit detector-specific features that make it tougher for security software to sniff it out and eradicate the threat. It will, for example, try to hide from some of the best-known anti-rootkit applications, including F-Secure's BlackLight. Rustock also alters the function of several Windows system components to bypass firewalls.

"Rustock is hooking directly into the network stack," says Martin, to perform packet manipulations, another tactic designed to fool defensive software.

The Trojan also constantly mutates, a once-popular hacker tactic that's been left at the wayside by most modern exploits. "It's a true polymorphic," Martin says, meaning that while the original algorithms remain intact, the code expressing them changes each time it infects a new file.

Polymorphic exploits, which first appeared in 1990, are rarely seen today, Martin says, but Rustock has revived the practice as another defensive strategy against security software, which uses pattern detection and threat-specific signatures to sniff out malware.

"It becomes a matter of who can dig the deepest [into the PC], us or them," says Martin.