Malicious email campaigns spreading the Dridex banking Trojan and the Locky ransomware have been inactive for over a week after the Necurs botnet that fueled them experienced an outage, Proofpoint researchers reveal.
Historically, the Dridex and Locky malicious campaigns have been some of the largest ever seen (with hundreds of millions of messages sent to potential victims), yet Proofpoint security researchers say that they essentially came to a stop on June 1. The culprit appears to be the disruption of the Necurs botnet, which was associated with the two pieces of malware before.
As it turns out, the botnet was playing a critical role in the distribution of malicious emails, which explains the sudden drop in Dridex and Locky infections. However, it wasn’t until the disruption occurred that security researchers were able to understand why the botnet was essential to these email campaigns.
Researchers at Anubis Networks, who have been monitoring the Necurs botnet for nearly a year through a sinkhole, previously estimated that it had tens of thousands of bots in its control. On June 1, however, after the botnet’s command and control center (C&C) was down for 24 hours, researchers saw nearly 1.1 million hosts attempting to connect to the sinkhole as they were looking for a new C&C.
Necurs was observed online a couple of years ago, when its rootkit functionality was used by the Gameover Trojan to prevent anti-malware programs from removing it from infected machines. Capable of disabling antivirus programs and of downloading additional malware onto the infected systems, the Necurs Trojan was being distributed by the Angler exploit kit in September 2014.
A peer-to-peer (P2P) hybrid botnet, Necurs was also seen leveraging a Domain Generation Algorithm (DGA) that enabled bots to search and find a new C&C when the active one goes down. In fact, this was the feature that allowed Anubis Networks to determine the exact size of the botnet when the active C&C server went down on June 1.
As it turns out, the botnet experienced short outages in the past as well, although none so long (previously, researchers saw over 650,000 hosts connecting to the sinkhole after the active C&C went down for several hours). What caused the current disruption and how long it would take for the botnet’s operators to regain control of their bots, it remains to be seen.
“The Necurs outage last week is our most obvious evidence to date of its use in the massive Locky and Dridex campaigns that we have been tracking this year. While this is not the first apparent Necurs outage we have seen, available data suggest that it involved a significant and ongoing failure of the C&C infrastructure behind the botnet,” Proofpoint researchers say.
One thing that appears certain, however, is that Dridex and Locky aren’t dead. According to Proofpoint, small malicious email campaigns have already started to emerge over the past few days, albeit they are powered by other, less efficient mechanisms. Necurs remains down for the time being, and it might take a while before Dridex and Locky infection campaigns return to their former glory.