"These Zbot samples have been crafted to ensure that they only work when executed on one specific machine and from one specific path. Any attempt to execute the sample on a different machine or from a different path will result in early termination of the malware and no impact on the target system," said James Wyke at SophosLabs UK.
"This sophisticated technique is very similar to hardware-based licensing systems employed by major software companies to protect their products from piracy," he said. "But until now I had not seen the technique used to protect malware binaries from analysis."
The advance is likely aimed at preventing rival botnet developers from stealing the Zeus code or exploiting Zeus installations for their own purposes. But it also helps stop security researchers from unraveling precisely how the code works, although many antivirus programs will block it.
Zeus is perhaps the best known of all financially oriented botnets, which aim to exploit individual PCs and harness their power en masse to launch phishing attacks, relay spam, launch denial of service attacks or just steal people's bank account information.
According to managed security services provider SecureWorks, Zeus is available starting at $3,000 on the black market. It can also be customized with a variety of a la carte offerings, including the ability to make financial transactions from zombie PCs, and a plug-in for using instant messaging to send and receive stolen data in real time. According to SohphosLabs' Wyke, the software's creators are nothing if not market-driven. "They appear to respond to customer requests and add new features just like a real software company." For example, Firefox support, formerly a $2,000 add-on by some accounts, is now built in. In addition, the software also allows for multiple infections by different versions of the bot, meaning that "more of the Zeus creators' customers -- the various bot-herders -- can infect the same machine and make money from it," he said.
The prevalence of botnets, as well as the widespread use of spam for distributing botnet-related malware or advertising lucrative online pharmacies, illustrates how most Internet attacks today are after one thing: money. But it's a sign of the times that Zeus's creators may not even be users themselves, but rather distributors.
"This is clear indication of how professional the Zeus operation has become," said Wyke. "The way their business model works -- sell the kit itself and let the buyer worry about getting the bot onto users' machines -- means that they can put all their efforts into development, without spending time and resources on distributing the bot files and infecting machines."