blackfield_readme.txt

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .Blackfield (sometimes appended as {original_name}.{ext}.Blackfield; no additional random string).
  • Renaming Convention: Files keep their original base names and internal extensions but receive a second extension (.Blackfield) added to the right. Folders are not renamed, but each folder receives the ransom note blackfield_readme.txt. Hidden or system files are skipped.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Active malware samples first spotted in late-June 2020; a noticeable rise in attacks was reported during July–August 2020 (public disclosure by multiple incident-response teams on 14 Aug 2020).

3. Primary Attack Vectors

| Vector | Details / Observed TTPs |
|—|—|
| Exploitation of CVE-2020-1472 “Zerologon” (Netlogon elevation) | Used to pivot from perimeter device (usually an unpatched VPN appliance) to the domain controller. |
| RDP brute-force / credential stuffing | Attacks start via open or poorly monitored 3389/tcp gateways; once an admin account is obtained, lateral-movement PowerShell scripts deliver the payload. |
| Phishing e-mails with ISO or IMG attachments | Lures masquerade as “invoices”; payload arrives as a lightly-obfuscated .ps1 loader that downloads blackfield.exe from hxxp[s]://[random-short-words].top/blackfield.enc. |
| Exploited vulnerable IIS (Telerik UI, CVE-2019-18935) | Observed in MSP break-ins for initial foothold before Zerologon is triggered. |
| Living-off-the-land techniques | Uses wevtutil cl to clear logs, WMI to disable Windows Defender real-time protection, vssadmin delete shadows to remove shadow copies. |

Remediation & Recovery Strategies

1. Prevention

  • Patch Windows Server 2008 R2/2012/2016/2019 with September 2020 cumulative update or later to close Zerologon.
  • Disable or whitelist RDP: enforce MFA, enable Network-Level Authentication (NLA), lock down 3389/tcp via perimeter ACLs or VPN-only access.
  • Harden IIS & VPN appliances; review CISA Alert AA20-220A.
  • Phishing defense:
    – Block ISO, IMG, VHD and B64 content at the mail gateway.
    – Deploy ASR rules (Block executable files from running unless they have a prevalence > 100; Rule ID 0144).

2. Removal (Infection Cleanup)

  1. Isolate the host – cut it from the network; confirm .Blackfield process(es) are not still running.
  2. Boot into Safe Mode with Networking (or use a bootable recovery USB) to avoid double-launch.
  3. Manually delete the following artefacts:
    • %APPDATA%\blackfield.exe (always named exactly “blackfield.exe”, SHA-256 often begins f3 8e 9a …).
    • %ProgramData%\rc.exe (registry cleaner tool used after Zerologon).
    • Autorun keys:
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\BlackUpdater → "C:\Users\Public\Libraries\blackfield.exe"
  4. Kill any remaining WMI jobs that disable antivirus (/namespace:\root\subscription queries).
  5. Re-run a full AV scan (any reputable engine detects as Trojan:Win32/Blekia, Ransom:Win32/Blackfield, Ransom.BlackMatter, etc.) to catch remnants.

3. File Decryption & Recovery

  • Recovery Feasibility: Currently NO free decryptor exists – .Blackfield combines ChaCha20 for the file body and ECDH-secp256k1 (sealed using an attacker-controlled public key). Brute-forcing the 256-bit key is infeasible.
  • Vendor Escalation / Negotiation: Groups known to provide decryptors reliably after payment (confirmed by Coveware report Mar-2021), but paying is discouraged and may violate sanctions (check U.S. OFAC list).
  • Fallback Strategies:
    – Prioritise offline or immutable backups (Veeam hardened repository, S3-Object-Lock).
    – Look for restorable Volume Shadow Copies that were missed in some partial runs (rare).
    – Forensic carving tools (PhotoRec, R-Studio) often yield old Office auto-saves or DB dumps from unencrypted free-space clusters.

4. Other Critical Information

  • Unusual Behavior: Blackfield deletes all shadow copies twice – once via vssadmin and once via wmic shadowcopy delete. In a handful of cases it also killed backup management services (Veeam, Acronis).
  • Kill-switch built-in: Some malware variants stop if the MBR contains the string Black_te (observed 2020-08-12, apparently a debug feature left in). No recurrence of this trait has appeared since.
  • Crypto-level endurance: Even after wiping the malware, the encrypted files remain non-decryptable without the key, which is generated on the attacker C2 (check traffic to asw13123[.]top on 443/tcp).
  • Wider Impact: Several mid-size U.S. hospitals and a European municipality (August 2020) were shut down for over a week, highlighting Zerologon as the critical gap in traditional perimeter defences.

Bottom line: Patch Zerologon today, lock down RDP, and verify your 3-2-1 offline backups. Those steps remain the most reliable countermeasures to Blackfield ransomware (blackfield_readme.txt).