Security firm ESET has discovered two more bot families using the Tor anonymity network’s hidden service protocol to keep command and control (C&C) traffic away from prying eyes.

The use of Tor by cybercriminals is far from new but so far it has been the exception rather than the rule. The detection of Win32/Atrax and Win32/Agent.PTA using it earlier this month suggests a growing interest.

The firm’s analysis suggests that Atrax is very new, showing a compilation data from this month. Using Tor is going to be cumbersome, which underlines the criminals’ enthusiasm the network, requiring them to cache all data from botted PCs through a server hosted there.

‘Hidden’ it might be but the researchers were still able to access a login screen, complete with its sinister logo for a lethal type of Australian funnel web spider, Atrax Robustus.

The second bot discovered using Tor, Agent.PTA, is less sophisticated but is in fact an older bot form that has recently adopted Tor as an experiment to see if its benefits outweigh the disadvantages.

Using Tor could be seen as a natural evolution for C&C; the main alternative is to use a conventional central server contacted via a standard or proprietary protocol. This is relatively easy to disrupt as numerous ‘bot roasts’ have demonstrated, hence the growing tendency to try alternatives such as P2P, which comes with the disadvantage that bot hosts can be filtered by firewalling.

Tor represents a third possibility which avoids the pitfalls of P2P by allowing conventional servers to advertise themselves through Tor without revealing their real IP addresses. Tor traffic is also encrypted and can potentially beat Intrusion Detection.

So why haven’t criminals rushed to use Tor if it’s that simple? The answer is that as with P2P it introduces unreliability and latency; Tor is more secure but much slower.

The fact that criminals have bothered to channel C&C through Tor at all suggests that they are feeling some pressure from anti-bot activities.

“During the summer we have observed a growth in the numbers of malware families starting to use TOR-based communications,” said ESET’s Alexsandr Matrosov.

“The TOR-based botnets make it really hard to pursue investigation and C&C location tracking. But we have demonstrated with Win32/Atrax.A botnet that ways to analyze communication protocols have not changed and all the old tricks work with addresses in a TOR network too.”

In short, then, Tor hides C&C but it is not invulnerable to detection and analysis by security researchers who can reverse engineer it after detecting the malware it drops. The to and from battle between security firms and criminals shows no sign of abating.

Tor was also a feature of the Skynet botnet last December, which led the firm that publicised it, Rapid7, to predict a growth in the use of the network.  Last September, security firm G Data blogged on an IRC chat server hidden inside Tor.

Although now routinely abused, Tor was originally intended as an anonymity service for whistleblowers and campaigners living in totalitarian countries. Despite its recent adoption by criminals, it remains an important channel for legitimate anti-surveillance.