dcry 2.0

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .dcry (annotated with 2.0 in ransom notes to distinguish it from the earlier, non-decryptable *.dcry)
  • Renaming Convention: Original name is untouched; the payload appends “.dcry” once per file (report.xlsx → report.xlsx.dcry). Unlike many families, it does not embed an e-mail or victim-ID token between base-name and extension, which simplifies mass identification via simple wild-cards (*\*.dcry in Windows or find . -name "*.dcry" on Unix-like systems).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First widespread sightings in underground forums during early October 2017; peak infection wave registered by C2 sinkholes and CERTs 15 – 30 October 2017. Campaigns resurface sporadically in small clusters using the same 2.0 strain rather than re-branding, making archival detection signatures still relevant.

3. Primary Attack Vectors

  • Remote Desktop Protocol (RDP) brute-force is the dominant ingress:
    • Attackers scan for open port 3389/TCP, then launch credential-stuffing lists.
    • Once access is gained, manual deployment of the .exe (dcry.exe or winhost.exe) is executed.
  • Leveraged software weakness (not EternalBlue):
    • CVE-2017-0144 (SMBv1) is not exploited by dcry 2.0; it piggy-backs on plain Windows shares only after user sessions are already hijacked via RDP.
  • Secondary door: Spear-phishing e-mail with malicious ZIP dropper has been observed in ~15 % of samples (filename lure: Payment_Proof-3819.zip → Payment_Proof.exe). These droppers typically download the same RDP-configured payload from a Pastebin pastie (“raw” GitHub links have also been used).

Remediation & Recovery Strategies:

1. Prevention

  1. Disable RDP on perimeter or restrict to VPN-only access; enforce account lockout (3–5 attempts before delay).
  2. Use complex, unique passwords for every local administrator and domain account.
  3. Deprecate SMBv1 entirely using Group Policy (if not already done for other malware), even though dcry does not exploit it—removes noisy lateral movement surface.
  4. Enable Windows Credential Guard (Windows 10/11) or equivalent on Server 2016+; this defeats “mimikatz-style” credential harvesting that precedes RDP brute-force.
  5. Apply application whitelisting (AppLocker | WDAC) so only signed binaries can execute in strategic directories (C:\Windows, user profile temp paths).
  6. Central logging: forward Security Event IDs 4625 (failed logons), 4624 (successful logons) to a SIEM and set real-time rules for 50+ failures within 5 min.

2. Removal – Step-by-Step

(Best done from Safe Mode + Networking or via a trusted boot disk):

  1. Isolate: Pull the network cable / disable Wi-Fi immediately.
  2. Identify persistence:
    • Schedule Tasks → At1.job, MicrosoftDefender, or WinHost Update pointing to %AppData%\winhost.exe.
    • Run keys → HKCU\Software\Microsoft\Windows\CurrentVersion\Run\WinHost = "%AppData%\winhost.exe -min.
  3. Terminate malicious process: Use Task Manager, Process Explorer, or wmic process where name="winhost.exe" delete.
  4. Delete binaries: 
   del /f /q "%APPDATA%\winhost.exe"
   del /f /q "%TEMP%\nrstrt.exe"
   rmdir /q /s "C:\Users\Public\dcry"
  1. Purge scheduled tasks: 
   schtasks /delete /tn "WinHost Update" /f
  1. Acronis, Kaspersky, and Microsoft have published static & behavioral signatures: update on-demand scanner, run full scan; verify removal (hash logs should no longer see 2C5BCA22… of the infector).

3. File Decryption & Recovery

  • Recovery Feasibility: YES—dcry 2.0 used a flawed cryptographic routine (single, hard-coded AES-256 key derived from a Predictable PRNG seed). Researchers at BleepingComputer + Emsisoft released a free decryptor shortly after the public key schedule was broken.
  • Tools / Patches:
    • Official decryptor: Emsisoft Decryptor for Dcry 2.0 (June 2018); supports both ransomware payload hashes 1.3.3.7 and 1.3.3.9.
    • How-to decrypt:
    a) Copy affected drives to offline media (data integrity backup).
    b) Run decryptor as Administrator → browse to original file + its .dcry twin → key material is brute-forced locally (takes 1–2 min for 4 GB dataset).
    c) Tool preserves file timestamps and ACLs; verify first 50 files before bulk re-imaging.
  • Patch level: Post-remediation, apply the CVE-2017-0144 MS17-010 patch anyway to eliminate any residual ransomware toolbelt (most dcry operators later pivot to dual-load).

4. Other Critical Information

  • Unique Characteristics:
    • dcry 2.0 does NOT delete Windows Shadow Copies (vssadmin list shadows should remain intact)—another reason why decryptor-less recovery via Volume Shadow Copy Restore is often possible.
    • No automated network share encryption; encrypts only folders under %USERPROFILE%, mapped drives (by drive-letter), and fixed disks (it deliberately stops propagation earlier than competitors).
  • Broader Impact & Notability:
    • While the campaign returned a relatively low monetary return (~0.6 BTC per wallet cluster), the widespread re-use of the decrypted master key was instrumental in breaking two minor clone variants (nemesis and mysteryware) that shared 68 % bytecode.
    • The ​decryptor’s PoC code served as training material for universities and CERT exercises, making dcry 2.0 a textbook case study in both flawed implementation and community swift remediation.

Bottom line: For dcry 2.0, the key is fast RDP hygiene and the free Emsisoft decryptor. After remediation, treat every exposed endpoint as suspect and enforce least-privilege everywhere—this simple family never needed nation-state exploits, yet wreaked mid-sized business havoc entirely through reused weak credentials.