Wisniewski, principal research scientist at global cyber security firm Sophos, said: "Any day that begins by trying to catch up on all the details about two criminal infrastructure takedowns with arrests is a good day. Both the Netwalker and Emotet groups have caused innumerable emotional and electronic damage."
Authorities in Europe, the US and Canada said on Thursday AEDT that the Emotet botnet had been disrupted while the FBI said the same day it had taken down the site of the Windows ransomware NetWalker on the dark web and also arrested a Canadian who was using the malware for attacking companies.
"The world will certainly be a safer place with Emotet temporarily neutralised, but that is just part of the issue so far," Wisniewski said. "It appears that the primary Emotet operators were not apprehended.
He said the NetWalker ransomware syndicate was causing plenty of damage on its own, and even though only few organisations were targeted, it was doing enormous damage.
"It appears the Canadian Mounties have apprehended at least one of the suspects, but it is unclear what role this individual played in the greater scheme," Wisniewski said.
"Netwalker ransomware was distributed via an affiliate program, so it may be only a corner dealer rather than a kingpin.
"No matter the fate of either malware gang, the cyber crime world abhors a vacuum. Where there is an opportunity to profit from mass infections and sell off the spoils to others or a carefully planned ransomware intrusion, there is always another deviant who dreams of filling the void and revelling in the profit.
"The takedowns are great news, and we should celebrate the ever-increasing pace of law enforcement actions against cyber criminals, but we must remain vigilant. We have only put a dent in their armour and they will either return or be replaced by others with ambition to do the same."
Sherrod DeGrippo, senior director of Threat Research and Detection at security outfit Proofpoint, said Emotet had been around for many years and Proofpoint had tracked TA542, the actor behind the botnet, since 2014, when reports of its signature payload, Emotet, emerged.
“It has since become known as one of the world’s most disruptive threats," he said. "What makes Emotet particularly dangerous for organisations is that it has been the primary foothold for the future deployment of other banking trojans.
"At this point, any mainstream banking trojan may lead to devastating ransomware attacks. Their campaign volume is typically large, as we usually observe hundreds of thousands of emails per day when Emotet is operating.
"At this stage, it’s difficult to tell what this global action will bring. Law enforcement events can have, and previously have had, variable impact on disrupting the technology and operators of these large-scale botnets.
“Considering this appears to be a law enforcement action on the backend infrastructure of the Emotet botnet, this really could be the end. Further to this, if the threat actors behind the botnet (TA542) were apprehended or even disrupted in some way, that could have a significant impact on the potential of future operations.”
Adolf Streda, malware analyst at anti-virus firm Avast, said: "The takedown of Emotet is a milestone in the fight against cyber crime. Emotet has been like a Swiss army knife, with functionalities to steal people’s passwords, steal money from their bank accounts, and also adding victim’s machines to botnets, to launch further phishing campaigns.
"It has been using strong obfuscation methods to avoid being captured by anti-virus solutions, and it has been offered by the original threat actors as malware-as-a-service to other cyber criminals.
"Having such a wide reach and many prevalent families linked to their infrastructure is why seeing it disarmed by the authorities is positive news for the world of cyber security."