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Botnet likely caused spike in number of Tor clients

Loek Essers | Sept. 6, 2013
The spike in the number of clients using the Tor anonymity network was likely caused by a botnet, according to Tor and third-party security researchers.

"We have found various references that the malware is internally known as SBC to its operators," they wrote, adding that they assume that it originates from a Russian-speaking area, and is likely to be financial-crime related. The researchers did not specify where they found the references.

Tor also thinks it is plausible that the botnet is running its C&C point as a hidden service, according to Dingledine.

While the Tor network is still working for now, the botnet could cause trouble, according to Tor.

The biggest problems are not caused by the amount of traffic added to the network, but rather by new circuits that are being made, Dingledine wrote.

"Tor clients build circuits preemptively, and millions of Tor clients means millions of circuits. Each circuit requires the relays to do expensive public key operations, and many of our relays are now maxed out on CPU load," Dingledine wrote.

This sets up a possible dangerous cycle. "When a client tries to build a circuit but it fails, it tries again. So if relays are so overwhelmed that they each drop half the requests they get, then more than half the attempted circuits will fail (since all the relays on the circuit have to succeed), generating even more circuit requests," Dingledine wrote.

To deal with these issues, Tor took several temporary measures to mitigate the problem. But for the future, other options need to be explored, Dingledine said. Tor could for example limit the circuit-create requests or learn to recognize the circuit building signature of a bot client.

"It would be great if botnet researchers would identify the particular characteristics of the botnet and start looking at ways to shut it down (or at least get it off of Tor)," Dingledine said.

"And finally, I still maintain that if you have a multi-million node botnet, it's silly to try to hide it behind the 4,000-relay Tor network. These people should be using their botnet as a peer-to-peer anonymity system for itself," Dingledine wrote.

 

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