Medusa Ransomware Deploys Malicious Driver to Evade Security

26th March 2025 | Advisory, Blog

As ransomware threats go, the Medusa ransomware group has a lengthy and notorious history of successfully infiltrating organizations and deploying a double-extortion strategy to pressure victims into paying. This double-extortion approach involves first encrypting your sensitive data and then threatening to publicly release it, causing embarrassment, loss of customers, and significant harm to your company’s reputation and goodwill.

In a recent development, the Medusa ransomware group obtained an expired software signing certificate, enabling them to create a malicious software driver known as ABYSSWORKER. Because this malware is digitally signed, users can unknowingly install it without receiving typical security warnings about unsigned software. This signed driver also helps the malware evade many security solutions designed to detect unsigned threats. Once installed, ABYSSWORKER specifically disables multiple anti-malware solutions, clearing the path for successful ransomware attacks.

Understanding the ABYSSWORKER Driver

The ABYSSWORKER driver is a malicious component designed to mimic legitimate software, specifically the CrowdStrike Falcon driver (“CSAgent.sys”). By utilizing a revoked code signing certificate, this driver gains unauthorized access to system kernels, effectively bypassing multiple security measures.

Mechanism of Action

Once deployed, ABYSSWORKER performs several critical functions:

Process Protection: It adds its process ID to a list of globally protected processes, making it resistant to termination.
I/O Control Handling: The driver listens for incoming device I/O control requests and dispatches them to appropriate handlers based on I/O control codes.
EDR Disruption: It can manipulate files and processes to terminate or permanently disable Endpoint Detection and Response (EDR) systems, effectively blinding security measures.

Implications for Cybersecurity

The use of such sophisticated techniques by the Medusa ransomware group poses significant challenges:

Enhanced Stealth: By disabling security tools, attackers can operate undetected, increasing the potential damage.
Kernel-Level Access: Gaining kernel access allows attackers to perform privileged actions, making remediation more complex.

Recommendations for Organizations

To mitigate the risks associated with this advanced threat:

Train and Test Users with Phishing Simulations: Malicious drivers like ABYSSWORKER are often delivered via phishing emails claiming that a device driver is outdated and must be updated for optimal performance. By regularly training users to recognize suspicious emails and testing their ability to spot phishing attempts, you empower them to confidently delete these fraudulent messages without interacting with malicious links or attachments.
Regularly Update Systems: Ensure all operating systems, software, and firmware are patched and up to date to close known vulnerabilities.
Remove Administrative Rights: Even signed software and driver installations typically require administrator privileges. By restricting admin rights from most users, your organization can effectively prevent many of these malicious installations and reduce the risk of successful attacks.
(Advanced) Network Segmentation: Divide networks to restrict lateral movement and contain potential breaches.
Monitor for Anomalies: Deploy advanced monitoring tools to detect unusual activities indicative of such sophisticated attacks.

By adopting these proactive security measures, organizations can better defend against evolving threats like ABYSSWORKER drivers from the Medusa ransomware gang.

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