The Necurs botnet started 2017 with a four-month vacation, but ended the year sending tens of millions of spam emails daily as part of massive ransomware distribution campaigns.
Considered the largest spam botnet at the moment, Necurs was the main driver behind the ascension of the Locky ransomware (which in turn is associated with the Dridex banking Trojan) in 2016. As Necurs took a long vacation in the beginning of 2017, Locky was silent as well, but both resumed activity in April.
Over the course of 2017, however, the botnet was involved in the distribution of the Jaff, GlobeImposter, and Scarab ransomware families, as well as in ‘pump-and-dump’ schemes.
Over a 10-day period between December 19 and December 29, 2017, Necurs was once again involved in the distribution of ransomware, in addition to sending typical holiday-themed scam emails, data collected by AppRiver reveals.
The messages, AppRiver says, were distributing the Locky and GlobeImposter ransomware families and revealed the attackers’ preference to use malicious .vbs (Visual Basic Script) or .js (JavaScript) files located inside a .7z archive.
Consisting of between 5 and 6 million infected hosts and keeping around 1 or 2 million of them active at any given time, Necurs provides operators with remote access to the infected machines and can be used for various malicious activities, including malware downloads.
Starting on Dec. 19, the botnet was observed sending tens of millions of spam emails daily to distribute ransomware. It started at nearly 46 million emails on the first day (peaking at over 4.6 million messages per hour) and continued with over 47 million messages on Dec. 20 (peaking at 5.7 million per hour).
While the initial spam featured mainly .vbs files inside the .7z archive, .js files started appearing as well on the second day, and the traffic switched to .js files on Dec. 21-22, when it also started to taper off, at 36 million and 29 million messages per day, respectively. The botnet remained quiet from Dec. 23-25 and recommenced activity for only a couple of hours on Dec. 26.
“Hard to say why, however, I would hypothesize the operators may have been testing or monitoring the rate of infections and realized many workers are on vacation,” AppRiver’s David Pickett notes.
On Dec. 28-29, however, the botnet was highly active. It peaked at 6.5 million messages early morning on Dec. 28, but wasn’t active for long. On the next day, Necurs was observed sending nearly 59 million ransomware messages.
The malicious emails, the security researchers reveal, were masquerading as purchase orders and voicemails, but also claimed to contain images of interest to the intended victims.
Related: Necurs Returns With New Scarab Ransomware Campaign
Related: Necurs Botnet Distributing Locky Ransomware via Fake Invoices
Ionut Arghire is an international correspondent for SecurityWeek.
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