1. Executive Summary: The Stealth of HMDL

The ransomware ecosystem is characterized by its loud, disruptive nature—threat actors typically want you to know immediately that your files are locked by visibly appending ominous file extensions. However, a highly sophisticated new variant known as the HMDL Ransomware takes the opposite approach. Discovered during recent telemetry analysis by security researchers, HMDL focuses on deep stealth and complex cryptographic authentication.

Upon breaching an enterprise network, the HMDL variant encrypts mission-critical files but does not append any file extension. A critical document originally named Q3_Financials.xlsx remains named Q3_Financials.xlsx. The only visible indicator of a compromise is the inability to open the files, system instability, and the sudden appearance of a text file titled !!README_HMDL!!.txt deposited across the network shares.

This “no-extension” tactic is highly destructive because it effortlessly bypasses rudimentary File Server Resource Manager (FSRM) anti-ransomware screens that rely on blocking known bad extensions. Furthermore, HMDL utilizes ChaCha20-Poly1305 (AEAD)—a highly advanced Authenticated Encryption with Associated Data cipher. The Poly1305 Message Authentication Code (MAC) guarantees that if an IT administrator attempts to modify, rename, or “repair” the encrypted file with a third-party tool, the authentication tag breaks, rendering the file permanently unrecoverable—even if the correct decryption key is later obtained.

The attackers demand a relatively calculated ransom of 0.05 BTC and communicate exclusively via unique, victim-specific ProtonMail addresses. This exhaustive playbook provides IT administrators and incident responders with a clear, actionable roadmap to detect the stealthy HMDL payload, navigate the strict AEAD cryptographic constraints, and execute a systemic recovery operation.

2. Threat Intelligence & Technical Specifications

Understanding the cryptographic architecture of HMDL is critical. The malware operates as a standard Windows executable but utilizes advanced cryptographic libraries that complicate standard recovery efforts.

3. Anatomy of an HMDL Attack (The Kill Chain)

How does a stealthy ransomware variant like HMDL infiltrate a secure network? The kill chain relies on bypassing traditional perimeter defenses through social engineering and exploiting endpoint trust.

1. Initial Compromise: HMDL affiliates rely heavily on spear-phishing campaigns. They deliver malicious payloads disguised as legitimate business documents (e.g., invoices, shipping manifests) that utilize macro-enabled Office documents or obfuscated JavaScript to bypass email gateways.
2. Payload Execution & Evasion: Once the payload executes on a workstation, it attempts to escalate privileges. The malware deliberately avoids changing file extensions to delay detection by network administrators and automated FSRM scripts monitoring for anomalous file renaming events.
3. Defense Destruction: The malware executes commands (often detected by Kaspersky as Trojan.Win32.DelShad) to silently delete local Volume Shadow Copies (VSS) and disable Windows Startup Repair using vssadmin.exe and bcdedit.exe. This ensures local system rollbacks are impossible.
4. Encryption & Key Wrapping: HMDL generates a unique ChaCha20 symmetric key for the machine. It encrypts the files, calculates the Poly1305 MAC, and embeds both into the file header. It then takes the symmetric key and encrypts (seals) it using the attacker’s embedded RSA-2048 public key.

4. The Ransom Note & Poly1305 Authentication Warning

The HMDL ransom note is highly technical and explicitly outlines the cryptographic trap waiting for users who attempt to manipulate their files. Below is the complete, unaltered text of the !!README_HMDL!!.txt file.

— HMDL RANSOMWARE — Your files have been encrypted with ChaCha20-Poly1305 (AEAD). The symmetric key used was sealed with RSA-2048-OAEP and is embedded inside every encrypted file’s header. Victim ID: 4d583fa35e51a2ae To recover your data: 1. Send 0.05 BTC to: [bitcoin wallet address] 2. Email [email protected] with your Victim ID. 3. You will receive a decryptor + your private key. Do NOT rename, move, or modify encrypted files – the Poly1305 tag will break and recovery becomes impossible. — END —

5. Phase 1: Detection & Threat Hunting

Because HMDL does not alter file extensions, standard PowerShell scripts looking for *.locked or *.hmdl will fail. Security teams must hunt for the specific ransom note and cross-reference it with recent mass file modification events.

Actionable PowerShell Threat Hunt

Deploy this PowerShell script via your centralized management console (e.g., EDR Live Response, SCCM) to audit endpoints for HMDL indicators based on note drops and shadow copy deletions:

# ==============================================================================
# Decryptors.org Incident Response Script: HMDL Stealth Audit
# Target: Windows Endpoints (Run as Administrator)
# ==============================================================================

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

$infectionFound = $false

# 1. Check for the presence of the specific HMDL ransom note
$noteSearchPaths = @("$env:USERPROFILE\Desktop", "$env:USERPROFILE\Documents", "C:\Data")

foreach ($path in $noteSearchPaths) {
    if (Test-Path $path) {
        $ransomNote = Get-ChildItem -Path $path -Filter "!!README_HMDL!!.txt" -Recurse -ErrorAction SilentlyContinue
        if ($ransomNote.Count -gt 0) {
            Write-Warning "[!] CRITICAL: HMDL ransom note (!!README_HMDL!!.txt) discovered in $path"
            $infectionFound = $true
        }
    }
}

# 2. Check Event Logs for VSS Deletion (A primary indicator of Ransomware Execution)
try {
    # Searching System logs for Service Control Manager events relating to VSS stopping unexpectedly
    $vssEvents = Get-WinEvent -LogName System -MaxEvents 200 -ErrorAction Stop | 
                 Where-Object {$_.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: vssadmin.exe Delete Shadows /All /Quiet"
    }
} catch {
    Write-Output "[i] Could not parse Event Logs or no VSS manipulation found."
}

# 3. Detect Mass File Modifications (Since extensions aren't changed)
# Check if more than 50 files in Documents were modified in the last 2 hours
$twoHoursAgo = (Get-Date).AddHours(-2)
$modifiedFiles = Get-ChildItem -Path "$env:USERPROFILE\Documents" -File -Recurse -ErrorAction SilentlyContinue | Where-Object { $_.LastWriteTime -ge $twoHoursAgo }

if ($modifiedFiles.Count -gt 50) {
    Write-Warning "[!] Alert: Anomalous mass file modification detected ($($modifiedFiles.Count) files modified in last 2 hours)."
    Write-Warning "This is a secondary indicator of active stealth encryption."
}

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

7. Phase 3: Cryptographic Forensic Triage & Recovery

Once absolute containment is verified and the initial access vector has been patched, the organization can transition to the recovery phase.

Method 1: The Cryptographic Reality of AEAD

The combination of ChaCha20-Poly1305 and RSA-2048-OAEP is mathematically unbreakable with modern computing power. The symmetric key required to decrypt your files is locked inside the file header, and it can only be unlocked by the attacker's private RSA key. Brute-forcing this is impossible.

If you lack immutable backups and must recover the data, you will likely need to engage the threat actors. However, never negotiate alone. Professional incident response firms handle the communication to lower the ransom significantly, ensure the safe transfer of decryption tools, and verify that the provided decryptor doesn't contain secondary malware or logic bombs.

Method 2: Restoring from Immutable Backups

If you have uncompromised, immutable backups (e.g., disconnected external storage, or cloud backups with Object Lock):

• The "Clean Room" Rebuild: Because ransomware often drops persistent backdoors (like Cobalt Strike beacons), you cannot simply delete the encrypted files and assume the server is clean. You must wipe the infected machines entirely and perform bare-metal OS reinstallations.
• Credential Rotation: Assume all Active Directory credentials are compromised. Force a global password reset for all users and service accounts. Reset the krbtgt account password twice to invalidate any forged Golden Tickets.
• Sequenced Restoration: Safely migrate your data back from your offline backups into the newly rebuilt, clean environment.

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

Surviving an HMDL attack is a grueling operational challenge. The response to the infection must serve as a catalyst for a comprehensive enterprise security overhaul.

• Deploy Next-Gen EDR (Endpoint Detection and Response): Because HMDL does not change file extensions, legacy antivirus relying on static signatures and extension blocking will fail. You must deploy behavior-based EDR platforms (like CrowdStrike or Microsoft Defender for Endpoint) that monitor for rapid entropy changes in files (the hallmark of AEAD encryption) and automatically kill the offending process.
• Implement Immutable Storage Vaults: A modern Veeam 3-2-1-1 backup structure is strictly mandatory. You must incorporate immutable repositories. Immutable storage guarantees that once backup data is written, it cannot be modified, encrypted, or deleted for a specified retention period—even if an attacker deletes local shadow copies via vssadmin.
• Harden Email Security: Since HMDL relies on phishing, implement strict DMARC, SPF, and DKIM policies. Deploy email sandboxing to detonate suspicious attachments in a virtual machine before they reach user inboxes. Disable Microsoft Office macros globally via Group Policy.
• Enforce the Principle of Least Privilege (PoLP): Regular users must not have local administrator rights on their workstations. If a user triggers a payload, the lack of admin rights will severely limit the malware's ability to delete Volume Shadow Copies or traverse the network.

The HMDL ransomware represents a highly sophisticated, stealth-focused threat employing ruthless cryptographic authentication. Navigating this crisis requires a cool head, absolute adherence to file preservation, 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.