DePriMon: A New & Unique Way to Download Malware

The Lambert advanced persistent threat (APT) group -- sometimes called Longhorn -- is known by security researchers to specialize in attacks against European and Middle Eastern companies. It has, since 2011 or so, infiltrated government, financial, telecoms, energy, aviation, IT and educational targets. It is assumed to be state-sponsored.

The different forms of malware used by the group have been given color-coded names. ColoredLambert, for example, has been used by the group with this downloader and is linked to the Vault 7 leak of CIA capabilities.

ESET security researchers have found that the installation technique -- which they call DePriMon -- is unique and has not been previously seen. The actual method of infection is variable, but it apparently brings the second and third stages of the malware on board to disk during the initial stage. The second stage installs itself and loads the third stage using an encrypted, hardcoded path. This path is probably configured after the first stage of the attack occurs.

The second stage registers the third-stage malware DLL as a port monitor. Now, Administrator rights are required for creating this registry key. But the registered DLL will be loaded by spoolsv.exe at startup with SYSTEM privileges.

ESET found that a continual uninstallation trigger is operative during the infection process. As they put it, "The second stage checks regularly whether there is a file in the %system32% folder with the same name as the third stage DLL file but without the '.dll' extension. This file serves as an uninstallation trigger – should DePriMon find it, it removes both this file and its own components in a secure way by overwriting the binaries and then deleting them."

The third stage is tasked with getting the main payload from the malware operators. It does things in its own way.

For command and control (C&C) communication, it uses the Microsoft implementation of SSL/TLS, Secure Channel, instead of common APIs like WinHTTP or WinInet. The connection is initialized with a Windows socket and can continue with initialization of an authenticated Security Support Provider Interface (SSPI) session with the Negotiate/NTLM SSP. After that, DePriMon uses Schannel.

The malware uses the AES encryption with three different 256-bit keys for different purposes. One key decrypts strings within the malware, and the other two are used for the configuration file.

DePriMon is downloaded to memory and executed directly from there as a DLL using the reflective DLL loading technique. It is never stored on disk, which significantly adds to its overall stealth. The configuration file is extensive and contains several interesting elements -- its encryption seems to be properly implemented and so can protect the C&C communication effectively.

In summary, ESET call this "a powerful, flexible and persistent tool designed to download a payload and execute it, and to collect some basic information about the system and its user along the way."

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.