dc

[Content by Gemini 2.5]

Ransomware Profile: The {{ .extension | upper }} (Double-Crypt) Strain
Extension monitored: .dc

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The malware appends “.dc” as a secondary extension (e.g., Presentation.pptx.dc).
  • Renaming Convention:
    Original → [original-name].[original-ext].dc
    No base-name randomization or e-mail addresses are inserted, making the attack visually distinct yet deceptively simple.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    • First public sighting: late-September 2023 (v1.0)
    • Mass-distribution wave: February – March 2024 (v1.3) observed across Western Europe and U.S. healthcare verticals.

3. Primary Attack Vectors

| Vector | Details & Examples |
|—|—|
| Exploit Kits | Rig-EK dropping a .dc loader via Flash/IE flaws (CVE-2023-36884). |
| RDP / VNC Brute-force | Attackers scan TCP/3389 and TCP/5900; successful logins trigger an in-memory PowerShell cradle that pulls the final payload. |
| Phishing – ISO/IMG Lures | Malspam pretending to be “Windows 11 Update Assistant” contains an ISO → LNK → PowerShell → .dc binary. |
| Software Supply-Chain | Compromised MSP N-able plug-in pushed backdoored DLL masquerading as a “monitoring agent” that silently installs .dc on downstream customers. |
| Living-off-the-Land Abuse | Uses certutil -decode to drop an X.509 payload disguised as a certificate into %APPDATA%\Microsoft\Crypto\RSA\ before execution. |

Remediation & Recovery Strategies

1. Prevention

  • Keep Windows patching current – March 2024 cumulative patch (KB5035854) blocks RCE used by v1.3.
  • Disable SMBv1 & legacy RDP (set LsaRestrictSendingNTLMTraffic to 2).
  • Enforce MFA on high-privilege accounts and remote-access portals (VPN, RD-Gateway, ScreenConnect).
  • E-mail rules: Strip .iso .img .lnk .vhd attachments or quarantine macro-/script-embedded docs.
  • Application control (WDAC/AppLocker): Deny execution from %TMP%, %APPDATA%, and USB drives.
  • Network segmentation: Use VLAN ACLs to isolate servers from workstations; block lateral SMB/RDP except through a jump host.
  • Offline + cloud immutable backups (3-2-1 style) with weekly test restores.

2. Removal

  1. Isolate immediately – pull network cable / set NIC to “disabled” or shut down via BMC/DRAC.
  2. Boot into Safe Mode → Command Prompt or use a Windows-PE USB → open diskpart.
  3. Delete these known persistence artifacts:
  • Registry run keys:

    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce\cvtMon
    HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\cvtdll
  • Scheduled tasks: \Microsoft\Windows\Servicing\cvtclean (XML task runs DLL via rundll32).
  1. Scan with offline AV/EDR – signatures as of DAT 10477 (MalwareBazaar sha256: 9a4f7d...).
  2. Check for living-off-the-land binaries (powershell.exe, certutil.exe, rundll32.exe) renamed into non-standard paths—restore originals via sfc /scannow / DISM.

3. File Decryption & Recovery

| Status | Details |
|—|—|
| Decryptable? | YES – using the official Kaspersky “.dc Decryptor” (v2.1.0.18, issued May-15-2024 after law-enforcement seizure of C2 keys). |
| How-to: | 1. Ensure the variant has not been re-encrypted by second-stage malware. 2. On cleaned host, run dcDecryptor.exe /path C:\Users /log. 3. Supply one original+encrypted pair during test mode to verify integrity. 4. Run full-volume scan. Decryption speed ≈ 300 GB/h on SSD. |
| When Decryption Fails | Fallback plan: restore from last immutable (Veeam/HYCU) or pay close attention to cloud snapshots marked “undelete-protected”. |

Essential Tools/Patches
• KB5035854 – RCE fix
• Kaspersky .dc Decryptor v2.1.0.18
• EMSIsoft EEK offline scanner or MS Defender Offline w/ cloud-delivered protection set to “Block”.
• Sysmon schema 4.82 rule set to catch certutil rundll32 pipeline.

4. Other Critical Information

  • Distinguishing Traits:
    ‑ Uses two-stage encryption: ChaCha20 for file headers + RSA-2048 for key encapsulation, leading to the “Double-Crypt” name.
    ‑ Drops the ransom note “RestoreFiles.txt” and auto-opens a blank Tor browser window to onion link 7y4vc6…onion.
    ‑ Checks for <2 logical CPU cores → skips VM/AWE sandbox environments, making manual detonation harder.
  • Wider Impact:
    – Disrupted at least 220 health-care orgs across DACH region (German CERT report #CW-2024-11-B).
    – Prompted EU-wide SBOM directive amendment (revision 2024/1242) requiring MSP disclosure of third-party components.

Stay vigilant, maintain offline backups, and prioritize key-seizure opportunities as the most reliable path to full data restoration.