ESET researchers have recently identified a new tactic employed by Embargo ransomware actors to disable security solutions by exploiting Safe Mode, which is typically used as a diagnostic environment for troubleshooting system issues. Safe Mode usually loads only the essential drivers and services required to run the system, providing a minimal environment for system repair. However, Embargo ransomware actors seize this limited functionality to bypass security measures and deploy their ransomware effectively.
Discovery and Initial Strategy
MDeployer and Scheduled Task
The emergence of the Embargo ransomware was first detected in June 2024. This ransomware is part of a broader ransomware-as-a-service (RaaS) operation that focuses specifically on U.S. companies. The attack begins with the deployment of MDeployer, a malicious loader developed in Rust. MDeployer gets activated through a scheduled task innocuously labeled “Perf_sys.” This task decrypts encrypted cache files using an RC4 encryption key and subsequently loads MS4Killer, another Rust-based tool designed to disable endpoint security measures.
BYOVD Technique
MS4Killer employs an advanced tactic known as "Bring Your Own Vulnerable Driver" (BYOVD) to disable security solutions. It leverages a compromised signed driver named “probmon.sys” to evade detection and block security software from functioning. Exploiting such vulnerabilities allows the malware to operate under the radar, essentially neutralizing any active defensive measures. Once the security solutions are disabled, MDeployer launches the Embargo ransomware payload, which encrypts files on the target system.
The Ransomware Process
File Encryption and Ransom Note
When the ransomware is activated, it encrypts a variety of files and appends them with random six-letter hexadecimal extensions. This effectively corrupts the data, rendering it inaccessible without the corresponding decryption key. Simultaneously, the ransomware deposits a ransom note named "HOW_TO_RECOVER_FILES.txt" in each affected directory, which instructs victims on how to pay the ransom to recover their files. Additionally, the ransomware creates a mutex called “IntoTheFloodAgainSameOldTrip” for system synchronization, ensuring that only one instance of the malware runs at a time.
Double Extortion Tactics
Embargo ransomware operators also use a double extortion strategy, a tactic where they threaten to release stolen data if the ransom is not paid. The actors communicate through their own infrastructure and the secure Tox protocol, adding another layer of complexity to their operations. This dual threat puts additional pressure on victims, making them more likely to pay the ransom to avoid the public exposure of sensitive information.
Advanced Encryption and Process Manipulation
XOR Encryption and Custom Decryption
MS4Killer utilizes advanced encryption strategies to obscure the critical components in its binary code. This complexity is achieved using the XOR cipher, a method of encryption that alters the binary data to make it unreadable without the correct decryption key. The malware employs a custom decryption function to reveal these components when needed. It also manipulates processes through the Windows API "OpenProcessToken" to gain the necessary permissions to execute its tasks without raising suspicions.
Driver Storage and Monitoring
The malware stores the vulnerable driver “probmon.sys” in two designated locations, securing it further with RC4 and XOR encryption. By monitoring and terminating security software processes, MS4Killer ensures that it can operate uninterrupted. This is done using the “SeLoadDriverPrivilege” for managing the driver and the “CreateServiceW” API to create and manage services as needed. Through strategic registry modifications, the malware maintains its presence and ensures the continued success of the attack.
Conclusion
ESET researchers have recently uncovered a new strategy being used by Embargo ransomware operators to disable security software by taking advantage of Safe Mode. Typically, Safe Mode is utilized as a diagnostic tool to help troubleshoot and fix system problems, as it only loads essential drivers and services needed to run the operating system. This restricted environment is designed to make it easier to address system issues by minimizing the number of running processes. However, Embargo ransomware actors are exploiting this minimalistic setting to circumvent security protocols. By operating within Safe Mode, they effectively bypass the limited security measures and deploy their ransomware more efficiently. This tactic represents a significant challenge for traditional security solutions, which may not be fully operational in Safe Mode and therefore unable to detect or stop the ransomware attack. The discovery highlights the evolving sophistication of cyber threats and the need for enhanced security measures that can operate in all system modes.