A state-sponsored espionage campaign is leveraging new custom malware to monitor incoming remote desktop protocol (RDP) connections and infect connecting clients with a backdoor, according to a warning from security firm Bitdefender.
The campaign has been operational since the beginning of 2022 and appears aligned with the interest of China-based threat actors, the company said in a blog post documenting the malware activity.
Bearing the hallmarks of a state-sponsored group, Bitdefender said the espionage campaign stands out with two custom tools written in the Go programming language — the Logutil backdoor and the RDStealer malware.
The threat actor behind these attacks, Bitdefender says, has been active since at least 2020, initially relying on off-the-shelf malware such as AsyncRat and Cobalt Strike.
In late 2021, the APT actor shifted to custom malware like RDStealer, which can capture clipboard contents, log keystrokes, and harvest data from the infected machines.
What makes RDStealer special, however, is the capability to monitor incoming RDP connections and infect the connecting clients that have client drive mapping (CDM) enabled.
A virtual channel that allows for data transfers between RDP servers and clients, CDM displays the local drives of the client machine during the remote desktop session. CDM is typically always enabled on clients, with the configuration managed on the server-side.
On an infected machine, RDStealer continuously monitors for RDP connections with CDM enabled and, if one is detected, it notifies the command-and-control (C&C) server, starts exfiltrating data from the connecting client, and deploys the Logutil backdoor on it.
The backdoor leverages multiple DLL sideloading techniques to evade detection, including by abusing the Windows Management Instrumentation service (Winmgmt).
On an infected system, Logutil establishes persistence, communicates with the C&C directly or through a proxy server on the same network, and executes commands retrieved from the C&C, to load DLL libraries, execute commands, download/upload files, and list folders.