With Coinhive's shutdown in March 2019, however, cybercriminals have resorted to more standard tactics, infecting Windows and Linux servers and finding ways to allow their malware to persist for longer periods.
Two research analyses released this week by Trend Micro and Guardicore highlight the techniques used by cybercriminals' attempts to maintain their control of compromised servers. In a September 16 report, security firm Trend Micro found that the Skidmap Linux malware uses a variety of techniques — from the complex, such as loading malicious kernel modules, to the simple, such as inserting SSH keys into the authorized-keys file. Meanwhile, the 2-year-old Smominru botnet has used a variety of techniques to hold its ground on the more than 4,900 networks and 90,000 servers a month the malware infects every month.
On compromised Windows servers, Smominru schedules tasks using scripts, exploits the SQL service, creates administrative users, and installs a remote access trojan, says Ophir Harpaz, a security researcher at Guardicore Labs.
"They are just shooting in as many directions as possible," she says, adding that the attackers are not just worried about defenders. "These threat actors are trying to eliminate each other, so the more ways you find to be persistence on the machine, the less chance you have of losing access to the compromised host."
Because such easy-to-use services are no longer the rule, adding persistence to cryptomining malware has become a priority for cybercriminals, Augusto Remillano II and Jakub Urbanec, threat analysts at Trend Micro, stated in their analysis of Skidmap. The researchers found that the attackers, like the Smominru group, have taken a kitchen-sink approach, using techniques from Linux kernel module (LKM) rootkits to overwriting the rm binary for the Linux command used to remove software.
"Its use of LKM rootkits — given their capability to overwrite or modify parts of the kernel — makes it harder to clean compared to other malware," the researchers stated in the analysis. "In addition, Skidmap has multiple ways to access affected machines, which allow it to reinfect systems that have been restored or cleaned up."
In its research into Smominru, Guardicore's team found the credentials for a command-and-control server that collected information on the botnet, finding information and credentials for compromised servers. The security firm monitored the information on the server over time to study the infection patterns. About a quarter of the systems had been infected more than once, showing that the focus on persistence pays off for the attackers.
And the focus becomes more important because the vast majority of computers on the Internet are difficult to infect. For every organization serious about security, there are a few individuals or small businesses that have a few machines that have not been updated or secured, says Daniel Goldberg, a security researcher with Guardicore.
"Many groups compete to be on the same small number of machines — it's still hundreds of thousands of machines, but only a fraction on what is on the Internet," he says. "There is this whole undergrowth of forgotten servers lying around on the Internet and available for hacking."
The company estimates that the botnet has control of an average of 6,500 processing cores per day to use for cryptomining.
Therefore, any attempt at persistence can pay off for attackers. The Linux cryptominer Skidmap, for example, even adds an SSH key to the authorized keys file on Linux, allowing for a simple but effective form of a backdoor. This often works because companies do not monitor those files, Kevin Bocek, vice president of security strategy and threat intelligence at security firm Venafi, said in a statement.
"We see these tactics used so effectively to target critical infrastructure because security teams rarely have oversight of SSH keys that control access," he said. "These keys don't expire — creating an encrypted backdoor that attackers can use until they're detected."
While the recent attack focus on both Windows and Linux, some experts see Linux servers to be an increasingly valuable target for attackers focused on cryptomining.
"Over the last several months, we've seen more evidence that suggests that attackers are continuing to increase their focus on Linux as a vehicle to obtain access to compute and bandwidth resources," Casey Ellis, chief technology officer and founder of crowdsourced security firm Bugcrowd, said in a statement.
For server administrators, the best defense is to keep your servers updated and patched and monitor their performance. If the company often uses open source repositories, they should be verified. And all systems should enforce the concepts of least privilege, only allowing the level of access necessary to the user.
While cryptomining is often not considered as significant a threat as, say, ransomware, allowing attackers to steal processing power can have a significant impact on a company, Trend Micro's researchers said.
"Cryptocurrency-mining threats don't just affect a server or workstation's performance — they could also translate to higher expenses and even disrupt businesses especially if they are used to run mission-critical operations," they stated in their analysis. "Given Linux's use in many enterprise environments, its users, particularly administrators, should always adopt best practices."
Check out The Edge, Dark Reading's new section for features, threat data, and in-depth perspectives. Today's top story: "Poll Results: Maybe Not Burned Out, But Definitely 'Well Done'Veteran technology journalist of more than 20 years. Former research engineer. Written for more than two dozen publications, including CNET News.com, Dark Reading, MIT's Technology Review, Popular Science, and Wired News. Five awards for journalism, including Best Deadline ... View Full Bio