The latest botnet takedown was the result of a coordinated operation involving international police and private tech companies across 35 countries.
The operation was conducted successfully after researchers successfully broke the domain generation algorithm (DGA) implemented by the Necurs malware, which helped it remain resilient for a long time.
DGA is basically a technique to unpredictably generate new domain names at regular intervals, helping malware authors to continuously switch the location of C&C servers and maintain undisrupted digital communication with the infected machines.
"We were then able to accurately predict over six million unique domains that would be created in the next 25 months. Microsoft reported these domains to their respective registries in countries around the world so the websites can be blocked and thus prevented from becoming part of the Necurs infrastructure," Microsoft said.
Additionally, with the help of court orders, Microsoft has also obtained control over the U.S.-based infrastructure Necurs uses to distribute malware and infect victim computers.
"By taking control of existing websites and inhibiting the ability to register new ones, we have significantly disrupted the botnet."
First detected in 2012, Necurs is one of the world's most prolific spam botnet that infects systems with banking malware, cryptojacking malware and ransomware, and then further abuses them to send out massive amounts of spam emails to new victims.
To avoid detection and maintain persistence on targeted computers, Necurs utilizes its kernel-mode rootkit that disables a large number of security applications, including Windows Firewall.
Necurs was noticed mainly in 2017 when it started spreading Dridex and Locky ransomware at the rate of 5 million emails per hour to computers across the globe.
"From 2016 to 2019, it was the most prominent method to deliver spam and malware by criminals and was responsible for 90% of the malware spread by email worldwide," researchers at BitSight said in a separate report published today.
"During 58 days of investigation, for example, we observed that one Necurs-infected computer sent a total of 3.8 million spam emails to over 40.6 million potential victims," Microsoft said.
In some cases, the attackers even started blackmailing victims for a ransom claiming that they have knowledge of their extramarital affairs and threatened to send proof to the victim's spouse, family, friends, and co-workers.
According to the latest stats published by researchers, India, Indonesia, Turkey, Vietnam, Mexico, Thailand, Iran, the Philippines, and Brazil are the top countries that have been hit by the Necurs malware.