Spam Trojan Installs Own Anti-Virus Scanner

by Ryan Naraine - eWEEK

Veteran malware researcher Joe Stewart was fairly sure he'd seen it all until he started poking at the SpamThru Trojan -- a piece of malware designed to send spam from an infected computer.

The Trojan, which uses peer-to-peer technology to send commands to hijacked computers, has been fitted with its own anti-virus scanner -- a level of complexity and sophistication that rivals some commercial software.

"This the first time I've seen this done. [It] gets points for originality," says Stewart, senior security researcher at SecureWorks, in Atlanta, Ga.

"It is simply to keep all the system resources for themselves; if they have to compete with, say, a mass-mailer virus, it really puts a damper on how much spam they can send," he added.

Most viruses and Trojans already attempt to block anti-virus software from downloading updates by tweaking hosts file to the anti-virus update sites to the localhost address.

Malicious hackers battling for control over an infected system have also removed competing malware by killing processes, removing registry keys, or setting up mutexes that fool the other malware into thinking it is already running and then exiting at start.

But, as Stewart discovered during his analysis, SpamThru takes the game to a new level, actually using an anti-virus engine against potential rivals.

At start-up, the Trojan requests and loads a DLL from the author's command-and-control server.

This then downloads a pirated copy of Kaspersky AntiVirus for WinGate into a concealed directory on the infected system.

It patches the license signature check in-memory in the Kaspersky DLL to avoid having Kaspersky refuse to run due to an invalid or expired license, Stewart said.

Ten minutes after the download of the DLL, it begins to scan the system for malware, skipping files which it detects are part of its own installation.

"Any other malware found on the system is then set up to be deleted by Windows at the next reboot," he added.

At first, Stewart said he was confused about the purpose of the Kaspersky anti-virus scanner.

"I theorized at first that distributed scanning and morphing of the code before sending the updates via P2P would be a clever way to evade detection indefinitely," he said, but it wasn't until he looked closely at the way rival malware files were removed that he realized this was a highly sophisticated operation working hard to make full use of stolen bandwidth for spam runs.

Stewart also found SpamThru using a clever command-and control structure to avoid shutdown.

The Trojan uses a custom P2P protocol to share information with other peers including the IP addresses and ports and software version of the control server.

"Control is still maintained by a central server, but in case the control server is shut down, the spammer can update the rest of the peers with the location of a new control server, as long as he/she controls at least one peer," he said.

Stewart found that the network generally consists of one control server (running multiple peer-nets on different ports), several template servers, and around 500 peers per port.

There appears to be a limit to how many peers each port can effectively control, as the overhead in sharing information between hosts is fairly large, he added.

"The estimated number of infected hosts connected to the one control server we looked at was between one and two thousand across all open ports," Stewart added.

The operation uses template-based spam, setting up a system where each SpamThru client is its own spam engine, downloading a template containing the spam, random phrases to use as hash-busters, random "from" names, and a list of several hundred e-mail addresses to send advertising.

The templates are encrypted and use a challenge-response authentication method to prevent third parties from being able to download the templates from the template server.

Stewart also found that the Trojan was randomizing the GIF files -- changing the width and height of the images -- to defeat anti-spam solutions that reject e-mail based on a static image.

"Although we've seen automated spam networks set up by malware before, this is one of the more sophisticated efforts. The complexity and scope of the project rivals some commercial software. Clearly the spammers have made quite an investment in infrastructure in order to maintain their level of income," Stewart said.

During his analysis, Stewart found that SpamThru was being used to operate a spam-based pump-and-dump stock scheme.

Check out's Security Center for the latest security news, reviews and analysis. And for insights on security coverage around the Web, take a look at Security Center Editor Larry Seltzer's Weblog.

Copyright 2006 Ziff Davis Inc.

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