In a blog post published on Thursday, Vladislav Hrčka wrote that the malware family, which ESET had called FontOnLake used custom and well-designed modules that could provider remote access, steal credentials and operate as a proxy server. Avast published details of the same malware on 25 August, while Lacework Labs published its research on 23 September. Both these firms called it HCRootkit.
He said one distinguishing characteristic of FontOnLake was that it was always accompanied by a rootkit that helped to conceal its presence. Additionally, the malware also used backdoors.
In order to fulfill these functions, FontOnLake used trojanised binaries for software like ssh [secure shell, used for remote access], cat [a standard Unix utility that reads files sequentially, writing them to standard output] and kill [a command-line utility to terminate processes].
Hrčka said the first known file from this family appeared on the Google-owned virus database VirusTotal in May 2020 and more followed.
"The location of the C&C [command and control] server and the countries from which the samples were uploaded to VirusTotal might indicate that its targets include south-east Asia," he wrote.
"We believe that FontOnLake’s operators are particularly cautious, since almost all samples seen use unique C&C servers with varying non-standard ports.
"None of the C&C servers used in samples uploaded to VirusTotal were active at the time of writing – which indicates that they could have been disabled due to the upload."
ESET had found three different backdoors "written in C++ and all use, albeit in slightly different ways, the same Asio library from Boost for asynchronous network and low-level I/O. Poco, Protobuf, and features from STL such as smart pointers are used as well".
Each of the backdoors had a common feature: each exfiltrated stolen credentials and its bash command history to its C&C.
Hrčka listed the following as comprising the functionality of the backdoors:
- "Exfiltrating the collected data;
- "Creating a bridge between a custom ssh server running locally and its C&C;
- "Manipulating files (for instance, upload/download, create/delete, directory listing, modify attributes, and so on);
- "Serving as a proxy; and
- "Executing arbitrary shell commands and Python scripts."
Two versions of the rootkit were found by ESET, and though both were based on the suterusu open-source project, they also had a number of custom techniques.
Tencent Security Response Centre also published research on the same malware; this is in Chinese.
A detailed white paper about FontOnLake has been published by ESET and can be downloaded here.