The migration of the 'Mevade' botnet to use the Tor anonymity network was most likely a botched attempt to hide that has ended up having the opposite effect, security firm Damballa has speculated.
News that something was afoot came after a huge spike on Tor from 19 August onwards, which caused a doubling of traffic on a single day to just over one million connections per day. Three weeks later and Tor's daily connections have hit 4 million per day, with no end to the rise in sight.
At the time, the Project's admins were only able to speculate as to the cause, but eventually researchers traced the likely culprit to the obscure but otherwise well-established Mevade.a bot.
Oddly, this sizable bot had been largely ignored by security firms until its controllers diverted some of its command and control traffic from its usual HTTP to Tor.
"This was a pretty substantial botnet to be so ignored for so long, the hallmark of a successful threat actor. So what changed? Why did a botnet nobody else seemed to notice for a very long time suddenly take front page?," asked Damballa researcher, Mark Gilbert.
The mistake made by the bot was to underestimate the spectacular effect it would have on Tor and then to assume nobody would ask questions about this sudden popularity. Tor offered the lure of a secure if slower connection, a secure C&C, but its very obscurity drew suspicion in the same way using an unusual software port might.
"As the Mevade operators have learned, there are trade-offs for every decision. In this case, the cost of switching to Tor was subsequently unwanted attention, which will likely force the operator to do further work to obscure themselves," he said.
"In the security arms race, sometimes the bad guys screw up too. But you can be sure they've taken the lessons learned from this progression, and will continue to find new ways to remain more elusive going forward."
Criminal use of Tor is nothing new although its adoption by botnets is still a recent trend; in July security firm ESET reported that it had noticed a botnet using it on a more modest scale. Mevade could probably have got away with much the same had it not migrated its entire operation.
If Damballa's analysis of Mevade (aka 'LazyAlienBikers') is correct, the bot is possibly five-million strong and Tor's connection ramp has some way to go before it reaches a peak.
Damballa's research suggests that Mevade.a has been a hugely successful, infecting 80 percent of the enterprise networks monitored by the firm. The behaviour of the bot's malware varies widely from infection to infection but in about 14 percent of cases, was stealing large amounts of data, the firm said.
As to Tor, one recent analysis reckons that criminal activity of one sort or another now makes up most of what travels across it. Mevade is only the latest and most imposing unwanted visitor.