The risk actors behind the Medusa ransomware-as-a-service (RaaS) operation have been noticed utilizing a malicious driver dubbed ABYSSWORKER as a part of a convey your individual susceptible driver (BYOVD) assault designed to disable anti-malware instruments.
Elastic Safety Labs stated it noticed a Medusa ransomware assault that delivered the encryptor by the use of a loader packed utilizing a packer-as-a-service (PaaS) referred to as HeartCrypt.
“This loader was deployed alongside a revoked certificate-signed driver from a Chinese language vendor we named ABYSSWORKER, which it installs on the sufferer machine after which makes use of to focus on and silence completely different EDR distributors,” the corporate said in a report.
The motive force in query, “smuol.sys,” mimics a reliable CrowdStrike Falcon driver (“CSAgent.sys”). Dozens of ABYSSWORKER artifacts have been detected on the VirusTotal platform relationship from August 8, 2024, to February 25, 2025. All of the recognized samples are signed utilizing probably stolen, revoked certificates from Chinese language corporations.
The truth that the malware can be signed provides it a veneer of belief and permits it to bypass safety methods with out attracting any consideration. It is price noting that the endpoint detection and response (EDR)-killing driver was previously documented by ConnectWise in January 2025 beneath the identify “nbwdv.sys.”
As soon as initialized and launched, ABYSSWORKER is designed so as to add the method ID to a listing of world protected processes and hear for incoming machine I/O management requests, that are then dispatched to acceptable handlers based mostly on I/O management code.
“These handlers cowl a variety of operations, from file manipulation to course of and driver termination, offering a complete toolset that can be utilized to terminate or completely disable EDR methods,” Elastic stated.
The checklist of a number of the I/O management codes is under –
- 0x222080 – Allow the driving force by sending a password “7N6bCAoECbItsUR5-h4Rp2nkQxybfKb0F-wgbJGHGh20pWUuN1-ZxfXdiOYps6HTp0X”
- 0x2220c0 – Load essential kernel APIs
- 0x222184 – Copy file
- 0x222180 – Delete file
- 0x222408 – Kill system threads by module identify
- 0x222400 – Take away notification callbacks by module identify
- 0x2220c0 – Load API
- 0x222144 – Terminate course of by their course of ID
- 0x222140 – Terminate thread by their thread ID
- 0x222084 – Disable malware
- 0x222664 – Reboot the machine
Of specific curiosity is 0x222400, which can be utilized to blind safety merchandise by looking out and eradicating all registered notification callbacks, an method additionally adopted by different EDR-killing instruments like EDRSandBlast and RealBlindingEDR.
The findings comply with a report from Venak Safety about how risk actors are exploiting a legitimate-but-vulnerable kernel driver related to Examine Level’s ZoneAlarm antivirus software program as a part of a BYOVD assault designed to realize elevated privilege and disable Home windows security measures like Reminiscence Integrity.
The privileged entry was then abused by the risk actors to ascertain a Distant Desktop Protocol (RDP) connection to the contaminated methods, facilitating persistent entry. The loophole has since been plugged by Examine Level.
“As vsdatant.sys operates with high-level kernel privileges, attackers had been in a position to exploit its vulnerabilities, bypassing safety protections and antivirus software program, and gaining full management of the contaminated machines,” the corporate said.
“As soon as these defenses had been bypassed, attackers had full entry to the underlying system, the attackers had been in a position to entry delicate info akin to person passwords and different saved credentials. This knowledge was then exfiltrated, opening the door for additional exploitation.”
The event comes because the RansomHub (aka Greenbottle and Cyclops) ransomware operation has been attributed to the usage of a beforehand undocumented multi-function backdoor codenamed Betruger by no less than one in every of its associates.
The implant comes with options usually related to malware deployed as a precursor to ransomware, akin to screenshotting, keylogging, community scanning, privilege escalation, credential dumping, and knowledge exfiltration to a distant server.
“The performance of Betruger signifies that it could have been developed with a view to reduce the variety of new instruments dropped on a focused community whereas a ransomware assault is being ready,” Broadcom-owned Symantec said, describing it as one thing of a departure from different customized instruments developed by ransomware teams for knowledge exfiltration.
“Using customized malware aside from encrypting payloads is comparatively uncommon in ransomware assaults. Most attackers depend on reliable instruments, dwelling off the land, and publicly obtainable malware akin to Mimikatz and Cobalt Strike.”
