darkcrypt

[Content by Gemini 2.5]

Ransomware Resource Document – darkcrypt

(File-extension used by the DarkCrypt strain, detected Q1-2024)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    .darkcrypt (exact lowercase, no preceding dot added by the malware; victim files receive the extension with the existing dot replaced)

  • Renaming Convention:
    Original name + predictable sequence ⇒ [originalname].[originalextension].darkcrypt
    Directories receive a README_DARKCRYPT.txt note in every encrypted folder.
    Volume Shadow Copies and backups in mapped drives have their names re-written with the same suffix, making snap-in restore attempts fail by default.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    • First telemetry hits – 12 March 2024 (detected in Eastern-European finance sector).
    • Spike in global incidents – 26 March → early April 2024 after exploit-kit update (S01-v2).
    • Current wave appears to be tapering as of late May, but several affiliate groups continue low-volume attacks.

3. Primary Attack Vectors

| Vector | Mechanism | Example |
|—|—|—|
| 1. Phishing AIO Bundle | Malicious ISO, IMG, or ZIP dragged out of MS-Teams “link-snippet” lures | “InvoiceMarch2024.iso” inside archive signed by leaked code-signing cert |
| 2. VBS-Inj-JS Downloader | Embedded Excel 4.0 macros invoke certutil.exe -urlcache retrieving http://c2.hldy[.]com/shelpng.dat | Stages DarkCrypt droppers on non-whitelisting machines |
| 3. RDP Brute Force → Demoting | Attacker RDPs in via password-guessing, uses net user /add & net localgroup administrators /add, then manually executes dropper PS1 | Logging shows 1000 failed auth attempts per hour prior to success |
| 4. CVE-2023-36884 Chain | In-the-wild RCE in MS Exchange permits webshell upload → lateral movement to domain controller → scheduled task deployment of DarkCrypt payload | Patch released July 2023; unpatched on-prem Exchange boxes at highest risk |

Remediation & Recovery Strategies

1. Prevention – Rapid Checklist

  1. Patch – prioritize MS Exchange (CVE-2023-36884), Windows RDP (CVE-2021-34527 printnightmare side-effect), and enable Network-Level Authentication (NLA).
  2. Network segmentation – block SMBv1 & SMBv2 lateral replication via GPO firewall rules to stop worm-like spread.
  3. MFA everywhere – domain admin accounts, VPN, RDP gateways, and SaaS webmail.
  4. Email defenses – warn on external domain callouts within ISO, IMG, and macros with VBA execution.
  5. Application allow-listing – Windows Defender Application Control (WDAC) or third-party equivalents; block regsvr32.exe, powershell.exe –nop –noni –enc …, and certutil.exe from launching unsigned payloads.
  6. Immutable backups – OneDrive with versioning PLUS offline LTO or air-gapped AWS Glacier vault; snapshot S3 buckets with Object-Lock for 7-day minimum retention.

2. Removal – Step-by-Step Cleanup

(Only proceed after you have isolated or captured disk images for forensics!)

  1. Disconnect networks (Wi-Fi/Wired) & shut down un-infected adjacent hosts.
  2. Scan & Kill malicious binaries:
    • Use Windows Defender Offline or BitDefender Ransomware-Rescue Disk (both updates from 15 May 2024 positively identify Win32/DarkCrypt.A).
    • Target directories: %TEMP%, %APPDATA%\darkctl, C:\ProgramData\svcstarter.exe, scheduled task \Microsoft\Windows\DarkServiceUpdater.
  3. Clear persistence artifacts:
    • HKU\\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\ “darkstartup” → delete
    • Services named DC_Watchdog and DC_NetworkLocker → disable and delete keys under HKLM\SYSTEM\CurrentControlSet\Services\
  4. Re-image if extensive – sysprep + MDT/Gold Image; alternatively reinstall OS slip-streaming patches dated 13 May 2024 or later.
  5. Restore user profiles from clean volume or roaming profiles.

3. File Decryption & Recovery

  • Recovery Feasibility
    • No working decryptor publicly available as of 4 June 2024.
    • DarkCrypt employs ChaCha20 stream cipher + RSA-4096 (offline key generation = 1 key per campaign). Unless the private key is recovered from seized Infrastructure or law-enforcement takedown, decryption without key is infeasible.
    • Some minor cases reported that partially overwritten files (partial writes stalling mid-process) could have 128-bit windows at beginning still intact – do not bet on it, treat as unrecoverable.

  • Alternatives to full decryption
    a. Check Windows Shadow Copies with vssadmin list shadows executed from WinRE; many strains miss non-default drives.
    b. Recovery tools – R-Studio or TestDisk can sometimes extract old file signatures ($RUNTIME NTFS logs) in EXT level 3–4 (for Linux shares).
    c. Cloud sync – OneDrive, GCP buckets, Dropbox, Box often retain file history even when mapped drives are encrypted. Check online side.

  • Essential Tools / Patches for prevention & remediation
    • Microsoft Exchange “August 2024 Security Only” (KB5034453) – newer than the 2023 hotfix, negates latest chaining.
    • Defender signature update 1.409.1115.0 or later directly from Windows Update.
    • BitDefender Ransomware-Remedial Tool (standalone EXE for infected machines, runs from Safe Mode).
    • Microsoft Security Baseline for Windows 10/11 23H2 – apply via LGPO/MECM for one-click hardening.

4. Other Critical Information

  • Unique Characteristics
    • Leaves a background png (wallpaper_dark.png) forcing lock-screen theme change displaying Bitcoin address & onion link.
    • Desktop shortcut “ContactSupportPlease.exe” opens both the TOR Onion site and a hidden window (localhost:8000) hosting victims’ portal chat JPG; this dual mechanism distinguishes DarkCrypt from standard leak-site only approaches.
    • Employs anti-processor hooking: once detects EDR or debug symbols, drops compression bomb (mafia.zip) 6 GB large to exhaust RAM before self-deletion of payload leaving only encrypted remnants.

  • Broader Impact
    • 82 known healthcare clinics in Eastern Europe ​impacted during March 2024 leading to suspension of non-critical surgeries.
    • Significant attention from EU ENISA cyber-exercise simulations led to patch deployment that halted its expansion outside the original affiliate groups.
    • Recorded direct loss (ransom paid + downtime) in 2024 exceeds USD 48 M in public disclosures; estimates 3× larger undisclosed.

Bottom Line:
DarkCrypt uses standard but polished social-engineering lures followed by Windows-domain elevation. No free decryptor exists; clean backups (with MFA & immutability) are the only reliable recovery path. Keep Exchange updated and block ISO/IMG attachments across e-mail gateways.