Security Arsenal’s monitoring of the MEDUSALOCKER leak site identified four new victim postings on September 12, 2026: two manufacturing firms (one in China, one in the UAE), a US construction company, and an Indian hospitality property. The affiliates likely compromised these organizations weeks earlier via perimeter exploitation — VMware vCenter path traversal (CVE-2026-59310), Cisco FMC hard-coded passwords (CVE-2026-20316), or Check Point VPN gateway authentication bypass (CVE-2026-50751) — then spent 7-21 days on reconnaissance, credential harvesting, lateral movement via PsExec and SMB shares, exfiltration staging with Rclone and WinSCP, and finally mass encryption with AES+RSA hybrid ransomware that appends randomized extensions and drops HOW_TO_RECOVER_DATA.html ransom notes.
The article provides SIGMA detection rules for shadow copy deletion (vssadmin delete shadows, bcdedit recoveryenabled no) and PsExec lateral movement. Those are useful for finding attacks already in progress. But detection after the scumbag is already inside your network, already moving laterally, already staging exfiltration tools — that’s playing defense when you’re already behind.
The Upstream Intervention: Block the Payload Before It Lands
Every step in this attack chain after the initial perimeter breach involves writing files to the Windows file system. The ransomware payload has to be written to disk before it can execute. The lateral movement tools (PsExec, Mimikatz, Rclone) have to be copied to remote machines via SMB administrative shares. The encryption binary has to write encrypted versions of your files and delete the originals.
FileSure Defend operates at the Windows kernel level via a filter driver that intercepts file system operations before they complete. You define a rule: unauthorized programs cannot write executable files to this system. The payload from the compromised VPN gateway or RMM tool tries to write an .exe, .dll, or .bat file to disk — it’s not an authorized program — the write operation is blocked at the kernel before the file is created.
The malware never lands. It never executes. The encryption phase never happens because the attack stopped at the delivery stage.
Here’s the specific FileSure rule configuration that would have prevented payload delivery:
File name filter: *.exe, *.dll, *.bat, *.ps1, *.vbs, *.scr
Operations: Create, Write
Programs (deny all except): Authorized deployment tools, Windows Update, your software distribution system
Drive type: Hard drives, Network drives
Result: Block
Outlook, Edge, Chrome, ConnectWise ScreenConnect, any remote access tool — none of them are authorized to write executables to disk. When the MEDUSALOCKER affiliate’s initial access payload tries to land, the write is blocked. The attack ends there.
The Fallback: Detect and Stop Bulk Encryption
If an attacker somehow bypasses the executable write block (say, via a living-off-the-land technique using only built-in Windows binaries), FileSure’s threshold-based detection catches the encryption behavior itself.
Ransomware that has landed on a machine begins mass-encrypting files — modifying hundreds of files per minute, renaming them with randomized extensions. A threshold rule detects this bulk modification pattern and blocks further encryption:
File name filter: * (all files)
Operations: Write, Rename/Move
Drive type: Hard drives, Network drives, Removable drives
Threshold: 20 matches within 60 minutes
Result: Block subsequent operations after threshold
Normal file save operations for typical users are well below 20 per hour. Ransomware encryption crosses that threshold within seconds. Subsequent write and rename operations are blocked. Damage is contained to the files modified before the threshold fired — usually a few dozen files, not thousands.
The article notes that MEDUSALOCKER affiliates routinely delete Volume Shadow Copies via vssadmin and disable recovery via bcdedit before encryption. Those commands don’t execute in a vacuum — they’re launched by a process that had to be written to disk first. Block the initial payload write, and the shadow copy deletion never happens either.
Most security is whack-a-mole: new attack, new signature, new patch, scumbag moves to the next vector. Controlling file operations at the kernel level is more durable than chasing attack signatures. MEDUSALOCKER’s playbook works because organizations allow any program to write any file anywhere. Change that assumption, and the playbook breaks.
Start a free 21-day trial of FileSure Defend at bystorm.com — one server, ten workstations, fully functional. Install it, test the threshold rule with our harmless ransomware simulator, and watch it block the encryption in real time.
Category: Ransomware
Tags: medusalocker, ransomware, perimeter exploitation, lateral movement, kernel filter driver, file system security, manufacturing sector
