A recently detected multi-stage malware has been updated daily since the beginning of the year, Avast security researchers reveal.
Initially discovered in August 2018 and dubbed Rietspoof, the malware’s first stage appears to be distributed through instant messaging clients such as Skype or Messenger. Victims are served a highly obfuscated Visual Basic Script with a hard-coded second stage in the form of a CAB file that expands into a digitally signed executable.
The initial VBS first checks if the logged user has Admin privileges, then deletes the CAB, runs the expanded executable file, and deletes the original script to cover its tracks. The threat also uses cmd /c to run commands from the command line, an apparent attempt to break behavior detection.
The expanded file is also executed if the current user is not Admin, but first a scheduled task Microsoft Windows DOM object helper is deleted, to create a new one pointing to the executable.
The newer version of the VBS creates a new LNK file in startup with the name WindowsUpdate.lnk and uses this to run an expanded PE file after startup, thus ensuring persistence.
Once deployed, the malware’s third stage uses a simple TCP protocol to communicate with its command and control (C&C) server. A newer version of the threat, however, also tries to leverage HTTP/HTTPS requests for C&C communication.
A homegrown protocol is used to retrieve Stage 4 from a hard-coded address. Stage 3 would include basic bot capabilities, but Stage 4 was designed as a downloader only, the researchers say. Additional stages might be involved in the attack chain, but haven’t been uncovered yet.
The security researchers have observed two versions of the third stage of Rietspoof, packing capabilities such as download/upload of files, start of processes, or initiation of a self-destruct function. The stage appears under rapid development, sometimes with two different branches running at once.
The malware’s C&C server has basic geofencing implemented, as it would only deliver the next stage (Stage 4) if the victim’s IP address is from the United States.
“This new malware, Rietspoof, has had a significant increase in its activity during January 2019. During this time, the developer has used several valid certificates to sign related files. Also, the payloads went through development, namely changing the implementation of the Stage 3 communication protocol several times,” Avast says.
The motives and modus operandi of the actors behind Rietspoof remain unknown, and Avast isn’t sure that the entire infection chain was uncovered. Despite its bot capabilities, the malware appears mainly designed as a dropper, supposedly targeting users in the United States (though this could be only a testing phase).
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Ionut Arghire is an international correspondent for SecurityWeek.
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