After a period of quiet, the Necurs botnet is back in action. A number of security companies have reported a massive surge in botnet activity which started on June 21, 2016.

The Necurs botnet has previously been used to send out huge volumes of Dridex malware and Locky; a sophisticated ransomware variant that was first discovered in February 2016. It is too early to tell whether this is just a temporary spike in activity or whether the botnet will be sending emails at the levels seen before the recent lull.

Necurs botnet activity dropped off on May 31. The volume of malicious emails being sent using the botnet fell to as few as 3 million emails per day. However, the number of emails being sent surged on June 21, shooting up to around 80 million emails. 24 hours later the volume of malicious emails had doubled to 160 million. The surge in activity comes is linked to a massive spam email campaign that is delivering emails containing malicious attachments which install Locky ransomware.

It is unclear why there was a period of quiet. Security experts having been pondering this since the dramtic fall in activity on May 31.

The Necurs botnet is massive and is believed to contain approximately 1.7 million computers, spread over 7 separate botnets. It is clear that the botnet had not been taken down, although activity across all seven of the botnets stopped. In April and May of this year, spam email volume was regularly exceeding 150 million emails a day. Now the Necurs botnet appears to be back up to speed.

Around the same time as the pause in activity, Russia’s FSB security service conducted raids resulting in the arrests of approximately 50 hackers. The gang was using the Lurk Trojan to defraud banks and other targets in Russia. It is unclear whether some of those arrests resulted in a disruption to the botnet, or whether the pause was for some other reason. Numerous theories have been suggested for the three-week pause, including the sale or the botnet and issues the operators may have had with the C&C infrastructure. If the botnet has changed hands, a single organization would likely be in control as activity across all seven botnets resumed at the same time.

The resurrection of the Necurs botnet is bad news. According to Proofpoint, the resurrection of the botnet has been accompanied by a new Locky variant which has new capabilities. The latest form of Locky is better at evading detection and determining whether it is running in a sandbox. The new capabilities were detected by Proofpoint shortly before the Necurs botnet went dark.