Security researchers have discovered a botnet that uses the Tor anonymiser network to hide its command nodes.

Owners of the compromised network of Windows PCs have placed their command-and-control server, which uses the common IRC protocol, as a hidden service inside of the Tor network. Aside from the use of Tor for extra anonymity and stealth, the zombie network is otherwise unremarkable, according to security researchers at German security firm G Data.

The botnet is capable of lending itself towards either running DDoS attacks, adware or secondary malware distribution, among other scams.

Botnet owners have moved from running a central C&C server (subject to takedown) to using a peer-to-peer architecture over recent years. P2P systems give every zombie in a botnet the ability to issue commands to other drones. However, this introduces other problems for cybercrooks because it creates a means for either rival scammers or the authorities to take over their botnet, unless a strong and difficult-to-apply authentication mechanism is built into the systems to thwart potential hijacks.

Cybercrooks have also experimented with Twitter as a control channel, but the approach has not really caught on.

Tor is generally known as a web anonymization service but the technology also creates a handy means to build an IRC server as hidden service, a potential exploited by botherders.

This novel approach brings all sorts of advantages for zombie PC herders, as G-Data explains. Since the server is anonymous, it cannot point towards the botnet owners' identity. Botnet control traffic is encrypted by Tor, so it can't be blocked by Intrusion Detection Systems monitors (a standard component of modern enterprise security systems). Blocking Tor traffic in general is problematic because there are legitimate uses for the technology.

In addition, Tor servers can't easily taken down. Although Tor tends to be slow and unreliable, due to in-built latency, this minor disadvantage is more than offset by the many advantages Tor offers as a venue for a botnet command server.

G-Data's analysis of what it describes as the "latest evolution in botnet C&C" can be found here. ®