1. Executive Summary: The Regulus Threat

The ransomware landscape is heavily populated by established Ransomware-as-a-Service (RaaS) syndicates that constantly iterate their malware to evade detection. The Regulus variant is a highly destructive iteration operating under the notorious MedusaLocker family umbrella. Known for its rapid lateral movement and aggressive extortion tactics, Regulus represents a worst-case scenario for enterprise IT infrastructure.

Upon successfully breaching a corporate network, the Regulus variant rapidly traverses local drives, mapped network shares, and connected Storage Area Networks (SANs). It encrypts mission-critical data utilizing a complex, military-grade cryptographic algorithm (RSA+AES). It visibly alters the environment by appending a numerical extension to all impacted files, such as .regulus16, .regulus1, .regulus5, .regulus30, or .regulus100. A critical document originally named project_proposal.docx is instantly rendered useless as project_proposal.docx.regulus16.

Beyond file encryption, the Regulus variant operates on a strict double-extortion model. Threat actors quietly exfiltrate vast quantities of internal, confidential, and proprietary business data to their private servers before deploying the encryption locker. They threaten to release this data to the public or sell it to competitors if the ransom is not paid within a 72-hour window, transforming a technical IT outage into a severe, legally reportable data breach with massive compliance implications.

This exhaustive, enterprise-grade playbook provides IT administrators, network engineers, and incident responders with a clear, actionable roadmap. Based on the NIST Incident Response framework, this guide details how to execute immediate containment, hunt for the remaining payload, navigate the extortion threat, and execute a systemic recovery operation.

2. Threat Intelligence & Technical Specifications

To successfully counter a ransomware deployment, defenders must understand the adversary’s technical footprint. Regulus behaves identically to older MedusaLocker variants, relying heavily on exploiting weak perimeter security and prioritizing the destruction of backups.

3. Anatomy of a Regulus Attack (The Kill Chain)

A ransomware infection is never a singular event; it is the culmination of a sophisticated kill chain. Understanding this progression is vital for identifying the initial point of compromise and closing the security gaps in your architecture.

1. Initial Compromise: MedusaLocker affiliates frequently scan the internet for exposed Remote Desktop Protocol (RDP) ports (TCP 3389). They utilize brute-force tools to guess weak administrator passwords or purchase compromised credentials from Initial Access Brokers (IABs) on the dark web. They also heavily utilize phishing campaigns delivering weaponized Office documents.
2. Lateral Movement & Reconnaissance: Once inside, the malware remains dormant. The attackers manually explore the network, dumping credentials using tools like Mimikatz, and mapping the Active Directory structure to identify Domain Controllers and centralized file servers.
3. Data Exfiltration (The Extortion Setup): Before any files are locked, the attackers identify sensitive directories—HR records, financial ledgers, and proprietary source code. This data is silently pushed to a private external server. This guarantees their leverage even if you have perfect backups.
4. Execution & Backup Destruction: Upon acquiring Domain Admin privileges, the threat actors deploy the ransomware payload globally. The malware creates persistence mechanisms in the Registry, executes vssadmin.exe Delete Shadows /All /Quiet to wipe local backups, encrypts the data, and drops the RANSOM_NOTE.html files.

4. The Complete Ransom Note Analysis

During an active incident, the RANSOM_NOTE.html file is a vital piece of forensic evidence. It provides the unique personal ID required by incident responders and outlines the attackers’ psychological tactics. Below is the complete text of the Regulus HTML ransom note.

Your personal ID: – Key ID: – YOUR COMPANY NETWORK HAS BEEN PENETRATED ANY ATTEMPT TO RESTORE YOUR FILES WITH THIRD-PARTY SOFTWARE WILL PERMANENTLY CORRUPT IT. DO NOT MODIFY ENCRYPTED FILES. DO NOT RENAME ENCRYPTED FILES. We gathered highly confidential/personal data. These data are currently stored on a private server. This server will be immediately destroyed after your payment. If you decide to not pay, we will release your data to public or re-seller. So you can expect your data to be publicly available in the near future.. We only seek money and our goal is not to damage your reputation or prevent your business from running. You will can send us 2-3 non-important files and we will decrypt it for free to prove we are able to give your files back. Contact us for price and get decryption software. Email: [email protected] [email protected] (To contact us, please create a Proton or Hotmail account. Don’t use Gmail, as your messages will end up in spam.) Tor chat address:[.onion address provided in note] ATTENTION! THIS INFORMATION IS INTENDED FOR THOSE WHO WANT TO SEEK HELP FROM UNKNOWN OR UNVERIFIED SERVICES. NO ONE ELSE CAN HELP YOU. ONLY WE, WHOSE CONTACT INFORMATION IS PROVIDED ABOVE, CAN SEND YOU A DECRYPTOR AND DECRYPT YOUR FILES. THERE ARE NO PROGRAMS OR PEOPLE ON THE INTERNET READY TO HELP YOU. THERE ARE NO CHEAP, WORKING DECRYPTORS. IF YOU ASK FOR HELP OTHER THAN THE CONTACT INFORMATION PROVIDED IN THIS MESSAGE, YOU WILL PERMANENTLY LOSE YOUR MONEY AND FILES. BE CAREFUL AND DO NOT ATTEMPT TO RECOVER YOUR SYSTEM ANYWHERE ELSE. IF YOU INITIALLY CONTACTED US USING THE INFORMATION PROVIDED IN THIS EMAIL, RECEIVED A PRICE YOU WERE NOT SATISFIED WITH, AND FOUND A CHEAPER OPTION ELSEWHERE, THINK CAREFULLY BEFORE PAYING – YOU ARE DEFINITELY BEING SCAMMED!!! IF YOU DON’T CONTACT US WITHIN 72 HOURS, PRICE WILL BE HIGHER.

5. Phase 1: Detection & Threat Hunting

Upon discovering the .regulus extension, rapid network-wide detection is required to identify all compromised assets and locate the initial payload. Security teams must sweep the environment to root out the executables.

Actionable PowerShell Threat Hunt

Deploy this PowerShell script via your centralized management console (e.g., SCCM, PDQ Deploy, or EDR Live Response) to audit endpoints for Regulus indicators, including the destruction of shadow copies and the presence of the ransom notes:

# ==============================================================================
# Decryptors.org Incident Response Script: Regulus / MedusaLocker Audit
# Target: Windows Endpoints (Run as Administrator)
# ==============================================================================

Write-Host "Starting Regulus Network Audit..." -ForegroundColor Cyan

# 1. Check for encrypted file extensions in critical user directories
$testPaths = @("$env:USERPROFILE\Documents", "$env:USERPROFILE\Desktop", "C:\Data")
$infectionFound = $false

foreach ($path in $testPaths) {
    if (Test-Path $path) {
        # Using wildcard to catch .regulus16, .regulus1, etc.
        $encryptedFiles = Get-ChildItem -Path $path -Filter "*.regulus*" -Recurse -ErrorAction SilentlyContinue
        if ($encryptedFiles.Count -gt 0) {
            Write-Warning "[!] CRITICAL: Encrypted .regulus files detected in $path"
            $infectionFound = $true
        }
    }
}

# 2. Check for the presence of the specific ransom note
$ransomNote = Get-ChildItem -Path C:\ -Filter "RANSOM_NOTE.html" -Recurse -Depth 3 -ErrorAction SilentlyContinue

if ($ransomNote) {
    Write-Warning "[!] CRITICAL: Regulus ransom note (RANSOM_NOTE.html) discovered."
    $infectionFound = $true
}

# 3. Check for Known Registry Persistence (Common in MedusaLocker variants)
$registryPath = "HKCU:\SOFTWARE\PAIDMEMES"
If (Test-Path $registryPath) {
    Write-Warning "[!] CRITICAL: MedusaLocker persistence key found at $registryPath"
}

# 4. Check for suspicious VSS (Volume Shadow Copy) deletion events
try {
    $vssEvents = Get-WinEvent -LogName System -MaxEvents 200 -ErrorAction Stop | 
                 Where-Object {$_.Id -eq 7036 -and $_.Message -match "Volume Shadow Copy"}
    
    if ($vssEvents) {
        Write-Warning "[!] Warning: Shadow Copy service modifications detected in recent logs."
        Write-Warning "Threat actors likely executed VSS deletion commands to destroy local backups."
    }
} catch {
    Write-Output "[i] Could not parse Event Logs or no logs found."
}

if ($infectionFound) {
    Write-Warning ">>> IMMEDIATE ENDPOINT ISOLATION REQUIRED. DO NOT REBOOT. <<<"
} else {
    Write-Output "[i] No immediate signs of Regulus infection on this endpoint."
}

7. Phase 3: Cryptographic Triage & Recovery Strategies

Once absolute containment is verified and the initial access vector (such as a compromised firewall appliance or a weak RDP password) has been securely patched, the organization can move to the recovery phase. This must be executed methodically to prevent reinfection.

Method 1: The Gold Standard - Restoring from Immutable Backups

The only mathematically guaranteed method for overcoming a Regulus attack is a full architectural restoration from verified, immutable, or completely air-gapped backups.

• The "Clean Room" Rebuild: You cannot simply run an antivirus scan, delete the .regulus files, and assume the server is safe. Threat actors leave persistent, hidden backdoors (such as Cobalt Strike beacons, scheduled tasks, or disguised remote monitoring tools). Affected hard drives must be entirely formatted. Perform a bare-metal OS reinstallation on all impacted hosts.
• Active Directory Cleansing & Credential Rotation: Assume all Active Directory credentials, including Domain Admins, are compromised. Force a global password reset for all users and service accounts. Crucially, reset the krbtgt account password twice to invalidate any Golden Tickets the attackers may have forged. Audit AD for any recently created, unauthorized administrator accounts.
• Sequenced Restoration: Follow a strict cross-platform recovery map. Restore Domain Controllers first in an isolated, firewalled VLAN to establish clean DNS and authentication. Once identity management is stable, restore critical database servers, and finally, end-user file shares.

Method 2: Cryptographic Response & Handling Extortion

If your organization lacks immutable backups, the situation is dire. The Regulus variant utilizes robust cryptography, meaning simple brute-forcing of the key is mathematically impossible. However, specialized incident response firms sometimes uncover flaws in how specific threat actors implement their key generation.

If a legitimate decryptor becomes available through security research, or if an extortion payment is facilitated as a last resort to prevent the day-7 data leak, you must follow a strict, isolated automated decryption workflow:

1. Never decrypt on production hardware. The decryption tool provided by the threat actor may be bundled with secondary malware, data stealers, or logic bombs designed to trigger weeks later.
2. Clone the encrypted drives using professional forensic imaging software.
3. Mount the cloned drives on an isolated, air-gapped forensic workstation.
4. Run the decryption utility against the clone, never the original encrypted files.
5. Verify the file integrity of the decrypted data.
6. Scan the decrypted files with multiple next-generation EDR engines before moving them back to the newly rebuilt production environment.

8. Phase 4: Post-Incident Hardening & Architectural Resilience

Experiencing a Regulus attack is a catastrophic operational burden. The response to the infection must serve as a catalyst for a comprehensive enterprise security overhaul. The ultimate goal is to move your IT environment from a reactive, vulnerable posture to a proactive, resilient architecture.

• Eradicate Public RDP Exposure: Remote Desktop Protocol (RDP) should never face the public internet under any circumstances. This is the primary entry point for MedusaLocker affiliates. Access to the environment must require a VPN protected by strict multi-factor authentication (MFA), followed by a connection through a hardened jump-box or PAM (Privileged Access Management) solution.
• Implement Immutable Storage Vaults: A modern Veeam 3-2-1-1 backup structure is now mandatory, not optional. You must incorporate immutable repositories (such as Linux Hardened Repositories, AWS S3 with Object Lock, or Wasabi immutable buckets). Immutable storage guarantees that once data is written, it cannot be modified, encrypted, or deleted for a specified period—even by a threat actor who has successfully stolen Domain Admin credentials and executed the vssadmin.exe command.
• Deploy Endpoint Detection and Response (EDR): Legacy signature-based antivirus is blind to modern, fileless ransomware techniques. Deploy behavior-based EDR/XDR platforms (like CrowdStrike, SentinelOne, or Microsoft Defender for Endpoint) configured to automatically isolate network adapters on hosts that attempt mass file modifications or exhibit lateral movement behaviors.
• Enforce the Principle of Least Privilege (PoLP): Regular users should not have local administrator rights on their workstations. Furthermore, service accounts should be heavily restricted. If an attacker compromises a standard user account via a phishing email, they should not be able to traverse the network freely to reach critical servers.

The Regulus variant of MedusaLocker is a highly sophisticated, financially motivated threat employing ruthless psychological extortion. Navigating this crisis requires a cool head, adherence to strict incident response frameworks, and a methodical approach to containment. Should this playbook highlight gaps in your current defensive posture, treat it as a mandate to immediately overhaul your enterprise IT infrastructure.