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Bot software looks to improve peerage

P2P botnets will be 'hard to stop'

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Peer-to-peer technology appears to have resurfaced in a worm last weekend.

The worm, dubbed Nugache and classified also as bot software, attempts to infect systems through email, America Online's instant messaging network, and network shares on vulnerable computers. Once it compromises a computer, the program uses a seed list of 22 different internet addresses to establish connections to other victims' computers in a peer-to-peer network.

The program appears to encrypt - or at least obfuscate - the data it sends to other servers, possibly making it harder for intrusion detection systems (IDSs) to detect the program, according to an analysis posted to a security mailing list by university network administrator Brian Eckman.

"The 'bot' - for lack of a better term - does not use DNS (the domain name system) to find any (command and control network); it also does not use any human readable string in its communication," Eckman, a security analyst at the University of Minnesota, wrote in his analysis. "Therefore, many IDS measures will not help you detect infected hosts on your network."

The techniques represent the latest improvements for bots - the tools of choice for many online criminals aiming to turn compromised computers into cash. Typically, the programs allow a bot master to control a large network of infected systems - or botnet - by sending commands through an internet relay chat (IRC) system, the still extant precursors to the major IM networks. This latest variant of bot software shows that - threatened by investigators' ability to tap into command-and-control networks built on top of internet relay chat - bot masters are looking to peer-to-peer communications, encryption and other technologies to hide their tracks.

Botnets that take their commands through peer-to-peer channels will make defenders' jobs much more difficult, said Joe Stewart, senior security researcher with network protection firm Lurhq.

"If done properly, it makes it near impossible to shut down the botnet as a whole," Stewart said. "It also provides anonymity to the controller, because they can appear as just another node in the network."

The current lack of anonymity has obviously worried many attackers that run botnets, because the latest tools are focusing on ways of keeping communications private, said Vincent Weafer, senior director for security software maker Symantec's response team (SecurityFocus is owned by Symantec.)

"IRC is really disappearing," Weafer said. "There is a big movement towards alternative channels - at least encrypted channels - to prevent people from eavesdropping."

While the analysis of Nugache is still preliminary, research shows that it infects systems by sending executable files attached to email messages, by attempting to convince victims to download the program by clicking on a link sent through AOL's instant messaging client, and by exploiting two vulnerabilities in unpatched systems running Microsoft Windows. Currently, the program does not seem to have spread far, Weafer said.

Using peer-to-peer communications to create an ad-hoc network for controlling compromised computers is not new, but the functionality within bot software has steadily improved.

The Slapper worm, which started infecting certain varieties of Linux in September 2002, used a peer-to-peer protocol to command other worm-infected computers to carry out one of three types of denial-of-service attacks. A year after Slapper, another bot - dubbed Sinit - used peer-to-peer communications to create a network of connected compromised PCs that could be updated with additional software using its own protocol. In 2004, security researchers warned that Phatbot - a variant of the prolific Agobot codebase - used a peer-to-peer system created by America Online as part of an open source project to send commands to other compromised systems.

Whether Nugache's techniques for communicating over peer-to-peer networks will become part of other bot master's toolboxes depends largely on whether the malicious software writer that created the program shares his code, said Mikko Hyppönen, chief research officer for anti-virus firm F-Secure. At least two other families of bot code have seen widespread development because the source code is widely available.

"It all comes down to whether the code ends up being shared or not,"Hyppönen said. "We have not seen it out on the internet yet."

A flaw in Nugache could make it easy to shut down, according to Lurhq's Stewart. The initial list of servers appears to be hard coded into the worm, suggesting that blocking those initial 22 internet addresses could stop the botnet from growing.

"As it stands, with this one, all that has to be done is shut down a couple dozen infected home-user systems," he said.

Yet, the push towards peer-to-peer botnets will be harder to stop.

This article originally appeared in Security Focus.

Copyright © 2006, SecurityFocus

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