Medusa Ransomware Disables Anti-Malware Tools with Stolen Certificates
Cybercriminals are exploiting custom and compromised drivers to disable endpoint detection and response (EDR) systems, facilitating undetected malicious activity. Elastic Security Labs (ESL) has identified a financially motivated campaign deploying MEDUSA ransomware, utilizing a loader paired with a revoked certificate-signed driver named AbyssWorker. This driver, originating from a Chinese vendor, is designed to neutralize EDR solutions.
As per ESL’s investigation, shared with Hackread.com, this tactic blinds security tools and allows malicious actors to operate freely, increasing the success rate of their attacks.
The AbyssWorker driver, originating from a Chinese vendor, is a key component in a campaign that installs itself on victim machines and systematically targets and silences various EDR solutions.
“This EDR-killer driver was recently reported by ConnectWise in another campaign, using a different certificate and IO control codes, at which time some of its capabilities were discussed. In 2022, Google Cloud Mandiant disclosed a malicious driver called POORTRY, which we believe is the earliest mention of this driver,” researchers noted in the blog post.
The actual filename of the malicious driver is identified as smuol.sys (a 64-bit Windows PE driver). It cleverly mimics a legitimate CrowdStrike Falcon driver, probably to blend into legitimate system processes. ESL identified multiple samples on VirusTotal dating from August 2024 to February 2025, all signed with revoked certificates from various Chinese companies, including Foshan Gaoming Kedeyu Insulation Materials Co., Ltd and FEI XIAO, among others. These certificates, while widely used across various malware campaigns, are not specific to AbyssWorker.
Upon initialization, AbyssWorker establishes a device and symbolic link, registering callbacks for major functions. A critical defence evasion mechanism involves stripping existing handles to its client process from other processes, preventing external manipulation. It also registers callbacks to deny access to handles of protected processes and threads.
The driver’s core functionality resides in its DeviceIoControl handlers, which execute a range of operations based on I/O control codes. These operations include file manipulation, process and driver termination, and API loading. A password is required to enable the driver’s malicious capabilities. For file operations, AbyssWorker uses I/O Request Packets (IRPs), bypassing standard APIs.
AbyssWorker can remove notification callbacks, replace driver major functions, detach mini-filter devices, terminate processes and threads, and restore hooked NTFS and PNP driver functions. Notably, it can trigger a system reboot using the undocumented HalReturnToFirmware function. These capabilities directly support MEDUSA ransomware’s ability to operate without security interference.
A key obfuscation technique AbyssWorker employs is calling “constant-returning functions” throughout the binary to complicate static analysis. However, Elastic deemed it inefficient, as they are easy to identify and declared it “an inefficient obfuscation scheme.”
Nevertheless, AbyssWorker represents a significant threat, demonstrating the increasing sophistication of kernel-level malware designed to disable security infrastructure. ESL has provided a client implementation example, offering researchers a means to further explore and experiment with this malware. To further aid in detection, Elastic Security has released YARA rules, available on their GitHub repository, enabling security professionals to identify instances of AbyssWorker within their environments.
Thomas Richards, Principal Consultant, Network and Red Team Practice Director at Black Duck, a Burlington, Massachusetts-based provider of application security solutions, commented on the latest development, stating,
“The Medusa malware is living up to its name, finding new ways to infect hosts even after one method has been blocked. Using a batch file to disable system services is a short-term ploy as it can be detected and blocked. Security teams should be on alert for any systems that have a time change and review end-user permissions to prevent the user from stopping the time service.“
Top/Featured Image by WaveGenerics from Pixabay
