A malicious downloader known as DePriMon uses the “Windows Default Print Monitor” name to avoid detection on Windows machines.

Active since at least 2017, DePriMon attracted the attention of ESET after infecting a private company based in Central Europe along with dozens of computers in the Middle East. The security firm subsequently examined the malicious downloader and learned a great deal about the threat’s second and third stages. Even so, researchers weren’t able to weigh in on DePriMon’s compromise vector(s) or what it uses for its final payload at the time of writing.

In the second stage of its infection chain, DePriMon abuses its system privileges to download the third-stage DLL to memory as a port monitor by creating a registry key. The threat thereby became the first documented malware family to leverage port monitors, an installation technique described in the MITRE ATT&CK framework. Once loaded, the third stage downloaded DePriMon’s final payload(s) from its operators.

Ties to the Longhorn Threat Actor

ESET detected ColoredLambert malware on a few of the computers infected by this most recent malicious downloader. As it turns out, ColoredLambert is just one of the members of the Lambert family. According to Kaspersky Lab, the Longhorn advanced threat actor has been using this suite of malware with zero-day vulnerability attacks since at least 2014. When Symantec investigated Longhorn, they found that many of its tools have been in use since 2011, some of which even appeared in the Vault 7 leak.

Defending Against a Fileless Malicious Downloader

Security professionals can help defend their organizations against fileless malicious downloaders like DePriMon by disabling PowerShell and other Windows processes if they are not needed. Otherwise, attackers can abuse these processes to infect the network with malware without raising any red flags. Companies should also invest in creating a robust security awareness training program that can educate employees about some of the most common social engineering attacks used by cybercriminals to distribute threats like DePriMon.