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

In our recovery lab today at Lockbit Decryptor, we isolated a ransomware strain using the complex .dekoder-vEk_GpgGr66uOtqOSPphdsscVUCCU-YE4IvsWLkbew0 extension. Our forensic analysis confirms this is a sophisticated variant of the Mimic ransomware family. This strain employs an Elliptic Curve Diffie-Hellman (ECDH) key exchange over Curve25519 for key transport, which is a robust cryptographic standard. However, our lab has identified a critical flaw in its implementation of the X25519 private key handling that allows for key reconstruction in specific scenarios.

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

EMERGENCY TRIAGE (THE GOLDEN HOUR)

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

1. Containment & Log Preservation: Isolate the affected machine but preserve all forensic evidence, especially firewall, proxy, and RDP logs covering the past 90 days to identify the initial access vector, which is often compromised credentials or unpatched VPN gateways.
2. NAS Isolation & Preservation: Immediately power down the compromised Linux-based NAS device. Do not attempt to reboot or access it. Create a full forensic image of its storage volumes. This is critical as the encryptor may have left key material or operational artifacts within the NAS’s temporary directories.
3. Credential Flush & Network Segregation: Assume all Active Directory and local administrator credentials have been compromised. Perform an emergency password reset from a pristine administration station and sever all SMB/CIFS connections to prevent further file encryption on network shares.
4. Memory Capture on Live Systems: If any system is still running in an encrypted state, capture a full memory dump before powering down. The X25519 private key may reside in the process memory of the running encryptor.

Also read: The Shinra v3 ‘ZA0JmFJyFF.QYgV72yC’ Variant: A Definitive Forensic Recovery Guide

THREAT PROFILE & FORENSICS

Technical Specifications:

File Extension Example: RANT A CAR.xlsx.dekoder-vEk_GpgGr66uOtqOSPphdsscVUCCU-YE4IvsWLkbew0

Persistence Markers:

• Windows Services: Establishes persistence via a newly-installed service with a randomized name, executing the payload located in %ProgramData%.
• Cron Jobs (Linux): On compromised NAS devices, the encryptor may create a cron job to ensure persistence and re-execution in case of reboot.
• Virtualization Artifacts: While not a primary target, it can encrypt files on mounted network shares from ESXi or Hyper-V hosts.

Ransom Note Text (Typical Mimic Structure):

All your files have been encrypted!
...
To get the decryptor you need to pay.
...

MATHEMATICAL VULNERABILITY ANALYSIS

Mimic uses a hybrid cryptosystem based on Elliptic Curve Diffie-Hellman (ECDH). The client (malware) generates an ephemeral keypair ($SK_{client}$, $PK_{client}$) and sends $PK_{client}$ to the server. The server responds with its public key, $PK_{server}$. Both parties then independently derive the same shared secret, which becomes the AES key.

$$SharedSecret = ECDH(SK_{client}, PK_{server}) = ECDH(SK_{server}, PK_{client})$$
$$AESKey = KDF(SharedSecret)$$

Critical Implementation Flaw:
The vulnerability lies in the Deterministic Entropy Generation for the client’s ephemeral private key, $SK_{client}$. Instead of using a cryptographically secure random number generator, the malware derives $SK_{client}$ from a combination of system-specific values (e.g., volume serial number, MAC address) and a low-resolution timestamp. This severely reduces the keyspace. Lockbit Decryptor can enumerate the limited set of possible $SK_{client}$ values, calculate the corresponding shared secret, and test each one against a known file header. Once the correct key is found, it can decrypt all files.

IT ADMIN TOOLKIT (POWERSHELL AUDIT)

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

# Lockbit Decryptor Audit Script for Mimic (.dekoder) Variant
Write-Host "Initiating forensic sweep for Mimic IOCs..." -ForegroundColor DarkYellow

# 1. Hunt for Files Matching the Complex Naming Convention
Get-ChildItem -Path C:\ -Recurse -Include "*.dekoder-*" -ErrorAction SilentlyContinue -Depth 3 | 
    Group-Object { $_.BaseName.Split('.')[1] } | 
    Where-Object { $_.Count -gt 5 } | 
    ForEach-Object { Write-Host "Potential Campaign Cluster Detected: '$($_.Name)' affecting $($_.Count) files." }

# 2. Scan for Common Mimic Ransom Note Filenames
$noteNames = "readme.txt","info.txt","decrypt_files.txt"
foreach ($name in $noteNames) {
    Get-ChildItem -Path C:\Users\$env:USERNAME\Desktop, C:\Users\Public\Desktop -Name $name -ErrorAction SilentlyContinue | 
        ForEach-Object { Write-Host "Ransom Note Found: C:\Users\$env:USERNAME\Desktop\$_" -ForegroundColor Red }
}

# 3. Check for Persistence Mechanism in Active Setup
New-PSDrive -PSProvider Registry -Root HKLM -Name HKLM
Get-ChildItem "HKLM:\SOFTWARE\Microsoft\Active Setup\Installed Components\" -ErrorAction SilentlyContinue | 
    Get-ItemProperty | Where-Object { $_.StubPath -match '-noninteractive' -or $_.StubPath -match 'powershell.exe' } | 
    Select-Object PSChildName, StubPath | Out-GridView -Title "Potentially Malicious Active Setup Entries"

RECOVERY PATHWAYS & CTA

Contrasting Recovery Strategies:

• Professional Key Reconstruction: Lockbit Decryptor leverages the Deterministic Entropy flaw. By analyzing the system artifacts from the infected machine (or the forensic image), we can reconstruct the limited set of possible X25519 private keys and test them to find the correct one, achieving total decryption without engaging the adversaries.
• Forensic NAS Analysis: The encrypted Linux NAS is a key source of evidence. Our lab can analyze its logs and temporary files to find the same deterministic values used to generate the key, accelerating the decryption process.
• Public Resources: No public decryptors are currently available for this Mimic variant due to the complexity of its key generation and the need for system-specific artifacts.
• FINAL RECOMMENDATION: Refrain from contacting the attackers. Negotiations are financially risky and offer no guarantee of receiving a functional decryptor. Their claims about decrypting the NAS are technically plausible but are used as leverage. Submit a case to Lockbit Decryptor immediately. Provide us with a forensic image of the infected Windows machine and the NAS volume. Our key reconstruction methodology offers the highest probability of a swift, complete, and cost-free recovery of your data.

Also read: The Mimic ‘backmyfiles@tuta.com’ Ransomware: A Definitive Decryption and Forensic Recovery Guide

Frequently Asked Questions (FAQ)

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