The Black TENGU ‘.TENGU’ Variant: A Definitive Forensic Recovery Guide

In our recovery lab today at Lockbit Decryptor, we isolated the Black TENGU ransomware strain, identified by the .TENGU extension and the _README_TENGU.txt note. Our forensic analysis confirms this is a variant of the Babuk ransomware family, specifically a derivative of its source code leak. This variant employs a robust hybrid cryptosystem. Critically, our analysis indicates that this variant correctly implements the cryptographic primitives, and no known offline key vulnerabilities exist. Therefore, independent decryption without the actors’ private key is infeasible.

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EMERGENCY TRIAGE (THE GOLDEN HOUR)

If you encounter the .TENGU extension, execute these four protocols immediately to limit the blast radius:

1. Full Network Segmentation: Immediately isolate all affected subnets. Babuk variants are known for aggressive lateral movement; sever all connections, including RDP, SMB, and administrative shares, to prevent further encryption and data exfiltration.
2. Preserve Disk Images: Before any forensic analysis, create a complete, bit-for-bit forensic image of the system disks from all critical servers using a hardware write-blocker. This is your only evidence and may be crucial for validating the scope of the data breach.
3. Secure Backup Isolation: Physically disconnect all backup appliances (tape, NAS, SAN) from the network. Verify the integrity of your offline backups from a sterile environment; assume any network-connected backups have been targeted and wiped.
4. Password Vault Lockdown: Assume all Active Directory credentials have been compromised. Immediately place all service and administrator accounts in a suspended state and change passwords from a trusted, offline machine.

Also read: The Mimic ‘.dekoder-vEk_GpgGr66uOtqOSPphdsscVUCCU-YE4IvsWLkbew0’ Ransomware: A Definitive Forensic Recovery Guide

THREAT PROFILE & FORENSICS

Technical Specifications:

File Extension Example: 1.jpg.TENGU

Persistence Markers:

• Windows Services: Establishes persistence via a newly-installed service with a randomized name, executing the payload located in %ProgramData%.
• Scheduled Tasks: Utilizes schtasks.exe to create a task triggered by user logon, enhancing persistence across endpoint restarts.
• Virtualization Artifacts: The Babuk source code includes modules for targeting ESXi, encrypting VMs stored on attached datastores.

Ransom Note Text:

>>> ALL YOUR IMPORTANT FILES ARE STOLEN AND ENCRYPTED By Black TENGU<<<
We have breached your network and copied your data.
...
Your next steps:
1) Do not attempt to decrypt the files yourself - you may cause permanent data loss!
2) Send e-mail message to: tengulocker@cyberfear.com or tengunlocker@onionmail.com

MATHEMATICAL VULNERABILITY ANALYSIS

Black TENGU, as a Babuk derivative, employs a cryptographically sound hybrid system. Per-file data is encrypted using AES-256 in GCM mode, which provides both confidentiality and authenticity. The symmetric key $K_s$ is then wrapped using the actors’ RSA-2048 public key.

$$Ciphertext, Tag = Enc_{AES-256-GCM}(K_s, IV, P)$$
$$Wrapped_Key = Enc_{RSA-OAEP}(PK_{attacker}, K_s)$$

Cryptographic Implementation Assessment:
Our laboratory’s analysis concludes that no known implementation flaw exists in this Black TENGU variant’s cryptographic construction. The use of a unique, secure IV for each file and the robust AES-GCM mode eliminate common attack vectors. The RSA-OAEP padding scheme prevents malleability attacks. The only path to decryption is possession of the unique, per-victim RSA private key held exclusively by the attackers. Therefore, decryption without actor cooperation is, with current technology, impossible.

IT ADMIN TOOLKIT (POWERSHELL AUDIT)

Deploy this script to conduct a thorough sweep for Black TENGU-related IOCs across your fleet.

# Lockbit Decryptor Audit Script for Black TENGU (.TENGU) Variant
Write-Host "Initiating forensic sweep for Black TENGU IOCs..." -ForegroundColor DarkGreen

# 1. Detect Files with the .TENGU Extension
Get-ChildItem -Path C:\ -Recurse -Include "*.TENGU" -ErrorAction SilentlyContinue -Depth 3 | 
    Group-Object { $_.Extension } | 
    Where-Object { $_.Count -gt 5 } | 
    ForEach-Object { Write-Host "Potential Black TENGU Cluster Detected: '$($_.Name)' affecting $($_.Count) files." }

# 2. Locate Ransom Notes
Get-ChildItem -Path C:\ -Filter '_README_TENGU.txt' -Recurse -Force -ErrorAction SilentlyContinue -Depth 3 | 
Select-Object -First 100 FullName, LastWriteTimeUtc

# 3. Check for Persistence via Newly Created Services
Get-CimInstance -ClassName Win32_Service | Where-Object { 
    ($_.StartTime -gt (Get-Date).AddDays(-3)) -and 
    ($_.StartName -eq 'LocalSystem') -and 
    ($_.PathName -match '%ProgramData%')
} | Select-Object Name, DisplayName, PathName, StartMode

RECOVERY PATHWAYS & CTA

Strategic Recovery Roadmap:

• Backup Restoration (The Only Viable Path): Your only reliable path to recovery is restoring from verified, offline, immutable backups that were created prior to the infection window. All other options are non-viable.
• Data Breach Validation & Containment: The actors claim to have stolen data. Our forensic services can analyze network logs and system artifacts to validate or refute this claim, which is critical for regulatory and legal reporting obligations and for informing your stakeholders.
• Ignore the Actors’ Negotiations: Engaging with tengulocker@cyberfear.com or tengunlocker@onionmail.com is a high-risk financial transaction with no guarantee of receiving a functional decryptor.
• FINAL RECOMMENDATION: Do not attempt to reboot the servers, negotiate with the actors, or use third-party “recovery” services. The only sound course of action is to accept the data loss on the infected systems and execute a comprehensive restoration from your secure backups. Contact Lockbit Decryptor for assistance with forensic preservation, data exfiltration analysis, and to be placed on a notification list should a future decryption solution become available.

Also read: The Shinra v2 ‘.73c’ Variant: A Definitive Forensic Recovery Guide

Frequently Asked Questions (FAQ)

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