In a blog post, researchers Cedric Pernet, Vladimir Kropotov, and Fyodor Yarochkin said they had noticed tools like the well-known NSA exploit EternalBlue being used to spread the malware to internal systems, after having first gained a foothold on a system on the perimeter.
EternalBlue was used to spread the WannaCry ransomware in May 2017. It has also been implicated in many other cases: a claim that it was used in a ransomware attack on local government offices in the US city of Baltimore in Maryland drew much criticism from many ex-NSA hackers and also non-NSA types recently.
The Trend Micro trio pointed out that regular cyber crime and targeted attacks differed in their intent: "The former will almost always have immediate financial gain as its main motivation while the latter will have other goals, for example, intellectual property theft."
"In the cases we identified, the threat actors were using a package of tools from the Equation Group (which was publicly leaked by the Shadow Brokers) to compromise a large number of machines running outdated versions of Microsoft Windows OS," Pernet, Kropotov and Yarochkin said.
Equation Group is a name given by Kaspersky (formerly Kaspersky Lab) to the NSA when it discovered potent tools created by the former, calling it "a threat actor that surpasses anything known in terms of complexity and sophistication of techniques, and that has been active for almost two decades".
Trend Micro said analysis of machines infected by the attackers it had tracked showed the presence of a trojan which was a variant of Vools, an EternalBlue-based backdoor.
"We also found a number of other tools in the infected systems, mainly the password dumping tool Mimikatz and Equation group tools," the Trend Micro researchers wrote.
"The final payload deployed on compromised systems is a cryptocurrency miner. Using data from the Trend Micro Smart Protection Network security architecture, we can confirm that all of the compromised systems appear to be on internal segments of compromised networks."
Four servers were noted as being used for the mining attacks: coco[.]miniast[.]com:443; iron[.]tenchier[.]com:443; cake[.]pilutce[.]com:443; pool[.]boreye[.]com:53 and log.miniast[.]com.
Pernet, Kropotov and Yarochkin said the first three of these domains had been registered on 17 March, the day on which they observed the campaign to have begun.
"These domains were registered anonymously while the older domain boreye[.]com was registered on 17 October 2018 using an email address that has only been used to register that single domain," the trio said.
"User credentials are needed to connect to the mining server, but only the password is needed to retrieve new hashes."