The Tor network is in danger of being swamped by criminals abusing its anonymity to hide an underworld of parasitic botnets, malicious command and control and ‘darknet’ markets, according to research from Kaspersky Lab.

Tor has long had its dark side but the scale of its use by criminals appears to have expanded quite rapidly in the last year. Kaspersky Lab had uncovered evidence of 900 services using Tor, said researcher Sergey Lozhkin, through its 5,500 plus nodes (server relays) and 1,000 exit nodes (servers from which traffic emerges).

A particular niche was the rise of underground markets that use Tor to hide their activity. “It all started from the notorious Silk Road market and evolved to dozens of specialist markets: drugs, arms and, of course, malware,” said Lozhkin.

It was here that a growing number of forums dealing in stolen card data were finding a home, he said. Predictably, trade on these markets was now based almost entirely on Bitcoins, which can’t be traced to individuals.

“Hosting C&C servers in Tor makes them harder to identify, blacklist or eliminate. Although, creating a Tor communication module within a malware sample means extra work for the malware developers. We expect there will be a rise in new Tor-based malware, as well as Tor support for existing malware,” said Sergey Lozhkin

Examples of Tor abuse are legion and include a recent Android botnet, the first ever discovered to be using Tor from that platform. Conventional botnets have also experimented with Tor. Probably the most infamous Tor parasite yet was the Silk Road market used to trade drugs, weapons, indeed anything illegal.

Tor estimates the number of clients using its relays to be around 2.5 to 3 million at any one time, each one of whch can be used to host multiple malicious services through the system. The number of clients abusing the network is probaby small as a proportion of its user base but the same is true for the rest of the Internet too.

Tor’s traffic has been prone to dramatic surges, including one in September 2013 when traffic rose from under 1 million users per day to well over five million in the space of a week.