There is a new gang known as Embargo ransomware that specializes in ransomware-as-a-service (RaaS). According to a study by ESET researchers published Wednesday, the Embargo ransomware group is a relatively young and undeveloped ransomware gang. It uses a custom Rust-based toolkit, with one variant utilizing the Windows Safe Mode feature to disable security processes.

ESET researchers say that the Embargo ransomware group is developing custom Rust-based tools to defeat the cybersecurity defenses put in place by companies and governments. There is a new toolkit that was discovered in July 2024 during an attack on US companies by ransomware and is made up of a loader and an EDR killer, MDeployer, and MS4Killer, respectively, which can also be accessed and downloaded online. There are several ways in which MS4Killer can be utilized. 

For instance, it can be compiled according to each victim's environment, targeting only specific security solutions. As it appears that both tools were developed together, there is some overlap in functionality between them. Several of the programs that were developed as part of the group, including MDeployer, MS4Killer, and Embargo's ransomware payload, are written in Rust, thus suggesting that the language is one that the developers use most often. It is claimed that the group has committed ten acts of cybercrime on its dark web leak site, including a non-bank lender from Australia, a police department from South Carolina, and a community hospital from Idaho. 

An interview conducted in June with a self-proclaimed representative of Embargo said that the group specializes in ransomware-as-a-service, with affiliates taking an extortion payment of up to 80%. It is believed that the toolkit discovered by Eset consists of two primary components: MDeployer, which is designed to deploy Embargo's ransomware and other malicious payloads, and MS4Killer, which is built to exploit vulnerable drivers to disable endpoint detection and response systems. 

In both MDeployment and MS4Killer, Rust is used as the programming language. Because of its memory protection features as well as its low-level capabilities, it can be used to create malware that is both effective and resilient. A study conducted by Eset reported that Embargo can target both Windows and Linux systems with Rust. It was in May 2024, one month after the first observation of Embargo in the ESET telemetry in June 2024 that Embargo was publicly observed for the first time. There are several reasons why the group has drawn attention besides the fact that it successfully breached high-profile targets as well as the language it used for its ransomware payload that piqued people's curiosity. 

As part of its development, Embargo chose Rust, which is a cross-platform programming language that provided the potential to develop ransomware that targets both Windows and Linux platforms. The Embargo group follows in the footsteps of BlackCat and Hive as yet another group developing ransomware payloads using Rust programming language. It is clear from Embargo's mode of operation that it is a well-resourced group considering its modus operandi. This system also allows victims to communicate with it via Tox, which results in the communication being managed by the system itself. It is a group that uses double extortion to force victims to pay him and then publishes the stolen information on its leaked website too. 

It is the MDeployer that Embargo uses mainly to install malicious loads on victims' computers within the compromised network to destroy them. An application for this purpose is designed to make it easier to execute ransomware and encrypt files. Two payloads are executed, MS4Killer and Embargo ransomware. Additionally, two encrypted files, a.cache, and b.cache, which were dropped by an unknown stage in the previous step, are decrypted and delivered to the victim. 

If the ransomware finishes encrypting the system, the MDeployer terminates the MS4Killer process, deletes all the decrypted payload files and the driver file dropped by MS4Killer, and finally restarts the computer. Besides the fact that MDeployer can run as a DLL file with administrative privileges, it has also the ability to reboot the victim's system into a Safe Mode if it is executed with administrator access. This is because major cybersecurity defenses aren't switched on in Safe Mode, which allows threat actors to continue operating undetected. The initial intrusion vector is unknown, however, once MDeployer has installed itself on the victim machine, it decrypts MS4Killer from the encrypted file "b.cache" and drops the file "praxisbackup.exe" into the system. 

In every single case observed by ESET, the MDeployer used the same hardcoded RC4 key to decrypt both files from "a.cache" and dropped and executed them as "pay.exe." MDeployer decrypted both files using the same hardcoded RC4 key. It has been reported that MS4Killer allegedly builds upon the S4Killer proof-of-concept tool available on GitHub and drops the vulnerable mini-filter drive problem.sys version 3.0.0.4 as part of what is known as the "Bring Your Own Vulnerable Driver" idea (BYOVD), which is a technique developed to deal with driver vulnerabilities in general. The researchers wrote in their paper that MS4Killer exploits this vulnerability to obtain kernel-level code execution and interacts with security software to carry out its malicious purposes. 

The Embargo's version of MS4Killer differs from the original MS4Killer in that Embargo has hardcoded a list of the processes to be killed into its binary. It has also encrypted the embedded driver blob which is an RC4 hash. Using cloud-based techniques, ESET researchers describe how MS4Killer runs in an endless loop and constantly seeks out processes that need to be terminated.   

MDeployer, a component of the Embargo ransomware attack chain, meticulously logs any errors encountered during its operations in a file named “fail.txt.” Upon completion of the attack — whether by successful ransomware deployment or an error in loader execution halting the attack — the MDeployer initiates a cleanup routine. This process includes terminating the MS4Killer loop and deleting specific files such as praxisbackup.exe, pay.exe, and a vulnerable driver. 

Additionally, it generates a control file named “stop.exe,” which certain MDeployer versions reference to prevent re-execution and, consequently, double encryption. Embargo, developed in Rust, appends each encrypted file with a unique, randomly generated six-character extension combining letters and numbers, such as “.b58eeb.” It also drops a ransom note titled “HOW_TO_RECOVER_FILES.txt” in each affected directory. The group has established its secure infrastructure for covert communication with victims but provides the option to negotiate through Tox chat as well. 

Although still developing, Embargo shows signs of ambition, borrowing techniques from established ransomware-as-a-service (RaaS) groups. These include implementing the "bring your vulnerable driver" (BYOVD) strategy, exploiting Safe Mode, and leveraging the adaptable Rust programming language. ESET's analysis highlights Embargo’s indicators of compromise (IoCs) and its tactics, techniques, and procedures (TTPs), offering guidance to help organizations defend against this emerging threat.