In July 2024, the ransomware group Embargo targeted US companies using the malicious loader MDeployer and EDR killer MS4Killer.
MDeployer deployed MS4Killer, which disabled security products, before executing the Embargo ransomware.
The ransomware encrypted files with a random six-letter extension and dropped a ransom note, while Embargo, operating as a RaaS provider, used double extortion to pressure victims into paying.
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Embargo ransomware group has deployed new Rust-based tools, MDeployer and MS4Killer, to target US companies, where MDeployer acts as a loader, decrypting and executing MS4Killer. This EDR killer abuses a vulnerable driver to disable security solutions.
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Both tools are actively developed and tailored to each victim’s environment. Embargo’s use of Rust for its ransomware payloads and sophisticated operations suggests its status as a well-resourced and dangerous threat actor.
The DLL variant of MDeployer, when executed with admin privileges, leverages Safe Mode to evade security solutions, which modifies system settings to boot into Safe Mode and disables Windows Defender, then creates a persistence service to execute the loader post-reboot.
Once in Safe Mode, it disables selected security tools by renaming their directories.
After deploying the Embargo ransomware payload, it cleans up Safe Mode configurations. It reboots the system back to normal mode, which is commonly employed by ransomware groups, and exploits Safe Mode’s limited functionality to bypass security measures.
The MDeployer ransomware loader employs a multi-stage attack process. It begins by targeting specific security solutions, leveraging a BAT script to reboot the system into Safe Mode, and renaming the installed security software’s directory.
If errors occur during payload decryption, ransomware execution, or MS4Killer execution, the loader logs them in the fail.txt file and creates the stop.exe file.
After successful execution, the loader performs a cleanup routine, terminating MS4Killer, deleting payloads and the vulnerable driver, and creating stop.exe. If stop.exe already exists, the loader deletes its PE file before rebooting the system.
According to ESET researchers, the loader used by the Embargo threat group is a persistent threat that is executed by a scheduled task created by an elevated system user.
The loader is likely delivered by a tool like WinRM-fs. It is still under active development, as evidenced by inconsistencies and bugs in the code, which uses a vulnerable driver, MS4Killer, to terminate security product processes.
MS4Killer is based on an existing proof-of-concept tool and has been extended with additional features such as scanning for running processes, hardcoded process names, and encrypted strings.
MS4Killer, a ransomware affiliate tool, leverages a vulnerable minifilter driver, probmon.sys, to disable security solutions, which embeds the driver as an encrypted blob and loads it into the system after decrypting and dropping.
The tool then compares running processes against an embedded list of security software process names to identify and terminate those that pose a threat.
Analysis reveals that only a subset of process names is actively monitored, suggesting that MS4Killer is tailored to target specific security solutions in compromised environments.
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