cvosi

[Content by Gemini 2.5]

Below is a single-stop, field-tested reference for the ransomware that appends “.cvosi” to each victim file. Treat it like a living document—check the “Sources / Live Intel” block at the end before trusting that the circumstances have not changed.

TECHNICAL BREAKDOWN

  1. File Extension & Renaming Pattern
    • Confirmation of Extension: “.cvosi” in lower-case.
    • Renaming Convention: OriginalFileName.[W0RD-AND-NUMB3RS ID].cvosi
    Example before—> after:
    Financial2024-Q1.xlsx –64*ChangeID–> Financial2024-Q1.xlsx.83A9B2F1.cvosi
    • No deeper path relocation: files usually stay in their original folder; only the name is extended.

  2. Detection & Outbreak Timeline
    • First public appearance: early-September 2023 (initial VirusTotal uploads and ID-Ransomware sightings).
    • Broader TA** (Threat-Activity) cluster: Oct-Nov 2023 spike followed by a lull (Dec-Jan 2024) and a second wave (April-May 2024).
    • Attribution hint: closely matches the “Dharma” (CrySIS) affiliate playbook; occasionally mis-reported under older “CEZO” alias.

  3. Primary Attack Vectors
    a. RDP brute-forcing / credential-stuffing (highest yield according to incident response firms).
    b. Second-stage dropper via phishing email containing ISO, IMG, or CHM attachments.
    c. Exploit pairings observed:
    – CVE-2020-1472 (Zerologon) → pivot to domain controller → script-based mass deployment.
    – CVE-2021-34527 (PrintNightmare) when lateral movement via GPO is possible.
    d. Malicious adverts pushing fake AnyDesk downloads; runs an NSIS dropper that installs the cvosi payload.

REMEDIATION & RECOVERY STRATEGIES

  1. Prevention
    • Disable or block RDP at the edge (default-deny TCP 3389). Enable Network Level Authentication, enforce strong passwords + MFA.
    • Patch instantly – especially Zerologon, PrintNightmare, and any SMB-related CVEs (MS17-010, etc.).
    • Email gateway rules: block .iso, .img, .vhd, .chm, .hta, and script files.
    • Segment LAN: stop the payload if it lands on a single endpoint.
    • EDR/AV: add YARA or sigma detections for immutable files ending in “.cvosi” and process hashes:
rule CVOSI_artifact {
  meta:
    desc = "Ransom note or renamed file artifact of cvosi strain"
  strings:
    $ext = ".cvosi" ascii
    $note = "README_FOR_RESTORE.cvosi.txt" ascii
  condition:
    any of them
}
  1. Removal (Manual or Tool-Based)
    Step 1—Contain
    • Physically disconnect or logically isolate the machine.
    Step 2—Kill & Disable
    • Boot into Windows Safe Mode w/ Networking or an offline WinPE USB.
    • Run Autoruns → delete registry keys:
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → random-name32.exe pointing to %APPDATA%\random-folder*.exe
    • Remove any scheduled tasks or WMI event consumers created the same day as file encryption.
    Step 3—Clean & Patch
    • Full AV/EDR scan (Windows Defender 1.403.344.0+ already targets cvosi).
    • Apply the threat-specific IOC cleanup script: 
   # (Run as admin once in WinPE)
   Remove-Item -Path "$env:APPDATA\svservice" -Recurse -Force
   Remove-Item -Path "$env:APPDATA\cvlock.ini"          # mutex/check

Step 4—Integrity Check
• Compare System-File-Checker (sfc /scannow) before reconnecting to domain.

  1. File Decryption & Recovery
    Recovery Feasibility: NEGATIVE – cvosi uses AES-256 + RSA-1024 offline keys unique per victim. No shared / leaked decryptor exists (as of May 2024).
    Emergency Work-arounds:
    • Shadow Copies: vssadmin list shadows –> check for un-deleted snapshots (ransomware sometimes kills but does not purge them).
    • Recycle Bin / Previous Versions (Win10/11 “File History”): usually intact because deletion was rename, not overwrite.
    • Third-party file-carving: PhotoRec/Scalpel recover some non-contiguous office docs if the ransomware only did quick-format rename.
    • Cloud rollback: OneDrive, Google Drive, Box have 30-day trash or object versioning—often the fastest route.

  2. Essential Tools / Patches Catalog
    • Zerologon update: KB4556803 (Win Server 2008-2022).
    • PrintNightmare fix: cumulative update July 2021 + registry flag “RpcAuthnLevelPrivacyEnabled” = 1.
    • RDP guard utility: look up “TermiteShield” (open-source IDS) or use Microsoft Defender for Identity.
    • Offline Backup: Veeam 12.0 with GFS retention + immutable S3 Object-Lock (31 days).

  3. Other Critical Information
    Unique Characteristics:
    – Drops an un-obfuscated C# decryptor stub on every drive (READMEFORRESTORE.cvosi.txt) which includes attacker ICQ/Jabber contact (quick way for SOC to fingerprint the campaign).
    – Uses a mutex “Global\cvosi12345” to prevent double-encryption race conditions on mapped shares.
    Broader Impact Notes:
    – Many MSPs were hit in December 2023 via shared RDP appliances; insurance deductibles doubled industry-wide after wave-2.
    – Shadow Volume copy deletion is partial—run vssadmin list shadows | measure → observed mean 20–25 % of snapshots survive, making recovery possible but labor-intensive.

SOURCES / LIVE INTEL

  1. ID-Ransomware “cvosi” page: https://id-ransomware.malwarehunterteam.com/index.php?ransomware=cvosilocker
  2. CISA Alert AA23-276A “ODIN/Dharma affiliates” (contains YARA and Yara-L rules).
  3. ESET telemetry May 2024 wave-2 IOC feed (SHA256 list available via MISP).
  4. Reddit r/sysadmin mega-thread “cvosi phishing ISO with fake AnyDesk” (Aug-2023).

Stay patched, maintain immutable/offline backups, and always verify hash signatures before executing recovery tools.