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Since August 19, 2013, there has been remarkable growth in the number of Tor users, which caused much speculation. Was August 19 the starting date to run en masse from the NSA’s PRISM project? Were European internet users downloading the latest American cable TV series via Tor only, thus overcoming blockades of sites like the Pirate Bay by European ISPs? Neither was very likely, so some thought a botnet abusing the Tor network to hide its command and control server must be the reason of the sudden increase of Tor users.

Yesterday, Fox-IT published evidence for this plausible explanation. The Mevade malware family downloaded a Tor component, possibly as a backup mechanism for its C&C communications. (We will release a second blog post describing in more detail the behavior of the Mevade variants we have encountered.)

Feedback provided by the Smart Protection Network shows that the Mevade malware was, indeed, downloading a Tor module in the last weeks of August and early September. Tor can be used by bad actors to hide their C&C servers, and taking down a Tor hidden service is virtually impossible.

The actors themselves, however, have been a bit less careful about hiding their identities. They operate from Kharkov, Ukraine and Israel and have been active since at least 2010. One of the main actors is known as “Scorpion”. Another actor uses the nickname “Dekadent”. Together, they are part of a well organized and probably well financed cybercrime gang.

We strongly associate these actors with installations of adware and hijacking search results. Therefore, we suspect that one of the ways the Mevade botnet is monetized is by installing adware and toolbars onto affected systems. In fact, we have seen Mevade downloading adware. Adware and toolbars might seem less harmful than e.g. data stealing malware, but the reality is that there is a lot of money to be made in fraudulent advertising.

We would also like to point out that Mevade also has a backdoor component and communicates over SSH to remote hosts. Therefore, the risk for data theft is still very high.