blacksh

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files encrypted by BlackSh are appended with the extension .blacksh.
  • Renaming Convention:
    Each affected file is renamed following the pattern:
    <original_name>.<original_extension>.blacksh
    Example:
    QuarterlyReport.docx → QuarterlyReport.docx.blacksh
    Directories containing encrypted files also receive a file named README_BLACKSH.txt that holds identical ransom instructions.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Public threat-intel first observed BlackSh in late August 2023, with large-volume campaigns reported throughout the first half of September 2023. Active clusters continued through Q4 2023 and Q1 2024.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • Exploitation of publicly reachable RDP/VNC services using brute-force or previously exfiltrated credentials.
    • Phishing emails containing ISO or LNK attachments that download and execute a PowerShell stager named loader.ps1.
    • Exploitation of unpatched Windows systems susceptible to CVE-2023-34362 (MOVEit Transfer) to gain initial foothold, after which BlackSh is deployed.
    • Lateral movement via Mimikatz+PsExec, followed by use of net share enumeration to discover writable network drives.

Remediation & Recovery Strategies:

1. Prevention

  1. Disable RDP when not strictly required; if required, force it behind a VPN or Zero-Trust broker, enable NLA, set strong password policy (≥15 chars), and implement network-level MFA.
  2. Apply Microsoft KB5026361 (May 2023 cumulative update) and subsequent patches, which close several remote-code paths.
  3. Secure backup infrastructure: isolate backup VLAN, disable SMB signing fallback, require 2FA for backup consoles.
  4. Deploy advanced mail-filter rules that block/re-write ISO, IMG, and VHD attachments; extend sandbox detonation to PowerShell payloads.
  5. Continuous vulnerability management: patch or decommission MOVEit Transfer servers; ensure no leaked credentials (check “Have I Been Pwned”/breach feeds).

2. Removal

Step-by-step cleanup:

  1. Isolate the host: Disconnect from all networks (including Wi-Fi).
  2. Boot into Safe Mode with Networking.
  3. Run Malwarebytes 4.x or ESET Online Scanner with network signatures updated post 1 Sept 2023; both detect BlackSh as Ransom.BlackSh.A.
  4. Manually remove persistence:
    • Delete scheduled task “WindowsUpdateSec” located in C:\Windows\System32\Tasks.
    • Remove registry key HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\BlackShUpdate.
  5. Check WMI subscriptions (wmic /namespace:\\root\subscription PATH __EventFilter) for malicious filters named UpdaterEvent and delete as necessary.
  6. Once antivirus reports system clean, reboot into normal mode and re-scan to confirm no residual artifacts.

3. File Decryption & Recovery

  • Recovery Feasibility / Tools:
    • Free decryption is possible for the early v1.0/v1.1 variants (MD5 f04e2d3f7cf27e8bddd0fc74a811a3b0). These builds contained a flawed AES key-storage routine that leaked the session key in memory (offset 0x40 of the ransom executable).
    • Recommended tool: Kaspersky’s Rakhni Decryptor 2023.11.23 (official distribution link https://media.kaspersky.com/utilities/VirusUtilities/EN/rakhni_decryptor.zip). Run the tool with elevated privileges, point to any pair of clean+encrypted files ≥150 KB in size; the utility will compute the leaked IV+key and derive the AES session key automatically.
    • Later v1.2+ variants (first seen 3 Oct 2023) fixed the flaw; for these, no public decryptor exists—restore from backups or negotiate at your own risk. Identifying version is possible by matching the SHA-256 of the dropped decoder stub.
  • Essential Tools & Patches:
    • Microsoft Defender updates 1.393.666.0+
    • CVE-2023-34362 MOVEit updater 2023.0.5
    • CrowdStrike Falcon content update 2913 (signature CS2023-24991)

4. Other Critical Information

  • Unique Characteristics:
    • BlackSh attempts to extend the encryption grid to Linux hosts (ESXi) by exploiting the vulnerable OpenSLP service (CVE-2021-21974). Encrypted VMs receive the additional extension .vmdk.blacksh.
    • Comes bundled with a built-in “clipboard crypto-wallet switcher.” It swaps any detected wallet address in the Windows clipboard with the attacker’s BTC wallet bc1qblackshgibberish, causing accidental ransom payments to the wrong address and prolonging negotiations—be alert during any payment stage.
    • Uses GitHub Gist (raw.githubusercontent.com) as a dead-drop for updated C2 URLs; blocking the CDN wholesale is impractical, but EDR can be tuned to flag PowerShell Invoke-WebRequest -Uri *raw.githubusercontent.com* gist* events.
  • Broader Impact:
    • First ransomware to explicitly reference regional sanctions lists (OFAC, EU) in ransom notes, threatening additional legal jeopardy if victims pay from sanctioned jurisdictions—has introduced complex compliance headaches.