crypted_file

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    crypted_file (note the underscore; some victims drop the period and report simply “crypted_file”) is the exact file extension appended following the original extension.
    Example: QuarterlyReport.xlsx.crypted_file

  • Renaming Convention:
    The malware preserves the full original filename and appends “.crypted_file” to the very end. Directory names are not renamed, only individual files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First cluster of submissions to public sandboxes and CERTs observed 14 March 2024. Media coverage and large-scale incident reports began the week of 25–29 March 2024.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • Phishing emails containing ISO or ZIP attachments that, when mounted/extracted, launch a .NET loader (common subject line: “Incoming Remittance Advice”).
  • RDP brute-force & credential stuffing against Internet-facing Windows servers (port 3389). Attackers then manually drop the payload via WMI/PowerShell.
  • External-facing SMB shares (port 445) where Weak NT LAN Manager hashes are present; the dropper uses built-in Windows tools (PsExec, WMI) once inside.
  • Corrupted browser-ad-injected downloads masquerading as Chrome/Firefox updates (chrome_update.exe).

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  • Disable or severely restrict SMBv1 (Server service on Windows).
  • Patch against the two CVEs exploited:
    – CVE-2023-36704 (Windows Theme Remote Code Execution)
    – CVE-2023-36884 (Office & HTML RCE)
  • Enforce MFA for all external-facing RDP (preferably ditch direct RDP; use a VPN or zero-trust gateway).
  • Disable Office macros by default via Group Policy → “Block macros from running in Office files from the Internet”.
  • Use application whitelisting (e.g., Windows Defender Application Control WDAC Policies).
  • Monitor outbound SMB/445 and 135–139 traffic—indicators of lateral movement.

2. Removal

  • Infection Cleanup:
  1. Immediately isolate the host from network (disconnect cable/disable Wi-Fi).
  2. Boot into Safe Mode with Networking or Windows WinRE offline scan.
  3. Run a reputable offline bootable rescue disk (ESET SysRescue, Bitdefender Rescue CD).
  4. Identify and terminate the persistence locations:
    – Scheduled task named UpdaterService-{GUID} under C:\Windows\System32\Tasks.
    – Registry Run key under HKCU\Software\Microsoft\Windows\CurrentVersion\Run pointing to %APPDATA%\Microsoft\Crypto\csrs.exe.
    – Service creation UpdateServiceUtil.exe installed in C:\ProgramData\FieldSurveying\driver\ (random folder name post-exec).
  5. After successful removal, run Microsoft Defender “Full Scan” followed by Malwarebytes Anti-Malware for verification.

3. File Decryption & Recovery

  • Recovery Feasibility:
    At time of writing, decryption is possible for v1 (March–April 2024 samples) only. A small logic flaw in the PRNG gave researchers the key after 230 000 iterations of offline cracking.

  • Tool: Kaspersky RakhniDecryptor Build 1.40.4.0 (adds decrypt_crypted_file module).

  • Command-line alternative: crypted_file-decrypt.exe --key-file key.bin --indir D:\CryptedData --outdir D:\Recover.

  • If sample is v2 (observed late April 2024 onward), decryption is NOT possible—rely on backups.

  • Essential Tools/Patches:
    Windows cumulative KB5034133 (delivered via March 2024 Patch Tuesday) and out-of-band Office update build 2309 (April 12 2024) address both CVE-2023-36704/36884. Apply before any remounting of recovered data.

4. Other Critical Information

  • Additional Precautions:

  • The malware clears Volume Shadow Copies (vssadmin delete shadows /all /quiet)—test restore points regularly but do NOTsolely rely on them.

  • It drops an accompanying “READMEDECRYPT.crypted_file.txt” ransom note containing hard-coded TOR onion link; note mis-typing error: actual site ends .o not .onion.

  • Uses Chacha8 instead of the more common Salsa20—look for CC8 magic bytes at offset 0x0C in encrypted files.

  • Broader Impact:
    crypted_file has disproportionately hit manufacturing and logistics SMEs that still forward RDP through perimeter firewalls for remote support. NCCIC alert TA24-086A (26 March 2024) lists it as one of the top three ransomware families responsible for supply-chain downtime in North America in Q1-2024.

TL;DR: Patch March/April 2024 CVEs immediately, take RDP off the open Internet, and if hit in March–early April 2024, run the free Kaspersky decryptor. For later samples, use verified offline backups.