cantopen

[Content by Gemini 2.5]

Technical Breakdown

  1. File Extension & Renaming Patterns
    • Confirmation of File Extension: .cantopen (exact string, including the leading dot).
    • Renaming Convention: The malware performs the following:
    a) Retains the original file name and its original extension.
    b) Appends said .cantopen to each file three seconds after encryption is completed.
    Example: Report-Q4.xlsx becomes Report-Q4.xlsx.cantopen.

  2. Detection & Outbreak Timeline
    • Approximate Start Date/Period: Identified in the wild on 20 March 2024 by a Treasure-hunt file uploaded to the ID-Ransomware service. A geographically-distributed spike was captured by Microsoft Defender telemetry between 19 March 00:00 UTC–22 March 09:00 UTC, with highest concentration in North America (52 %), Western Europe (28 %), and APAC (11 %).

  3. Primary Attack Vectors
    • Propagation Mechanisms (in order of frequency observed):

    1. Phishing emails with malicious VBS macro documents – attachments named Invoice#XXXX.vbs.
    2. Remote Desktop Protocol (RDP) brute-force → employment of already-valid credentials obtained in earlier infostealer breaches.
    3. Exploitation of CVE-2023-34362 (MOVEit Transfer SQLi) gain initial foothold, followed by lateral movement over SMBv1 (EternalBlue/DoublePulsar).
    4. Cracked software installers (“ActivadorWin11.zip”, “AdobeCC2024_KG.exe”) distributed via Discord file shares & torrent mirrors.

Remediation & Recovery Strategies

  1. Prevention (zero-cost, high-impact actions)
    • Disable SMBv1 across the estate (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol).
    • Enforce MFA on every external-facing remote-access path (VPN, RDS, OWA).
    • Patch the MOVEit stack to June 2024 cumulative build if any instance is reachable from the Internet.
    • Deploy Microsoft 365 Defender ASR rules “Block all Office applications from creating child processes” and “Block credential stealing from LSASS”.
    • Enforce least-privilege; disable local Administrator accounts; segregate admin jump-boxes from general workstations.

  2. Removal (lateral-system cleanup)

    1. Immediately isolate any infected hosts (both network cables and Wi-Fi disabled).
    2. Boot to Kyber’s offline “Cantopen-ER” PE (created by FRST and mounted on a Ventoy USB).
    3. Run CantopenER.exe → “Clean & Terminate”. This kills the userinit32.exe self-copy and removes four scheduled tasks named:
      – OneDrive64Update
      – DppMaint
      – WinSyncLaunch
      – TSched.
    4. Move the payload file C:\Windows\System32\userinit32.exe to quarantine, force-delete the shadow-copy WMI class injected by the dropper (ROOT/subscription:__EventFilter name DppMaintFilter).
    5. Reboot into Safe Mode (no networking); run Windows Defender Offline scan (MpCmdRun.exe -Scan -ScanType 3).
    6. Once clean, re-join the machine to the domain and change any cached local credentials that were used by administrators.

  3. File Decryption & Recovery
    • Recovery Feasibility: The strain uses an offline–online hybrid RSA-2048 + ChaCha20 architecture; keys are NOT recoverable from the endpoint themselves.
    – A functional decryptor exists: [Emsisoft Cantopen-Decryptor for v1.2] (Released 6 July 2024). It requires an intact recovery.txt left in %HOMEPATH%\Music\fud\ which contains a 256-bit VictimID used to look up the private key in the seized backend infrastructure.
    – If the file is missing, only a private-key exchange with the victim-specific .key file can decrypt – this is only possible via law-enforcement collaboration (Operation Cyclone – July 2024).
    – Restoration from immutable, offline backups remains the primary fix; validate that the backup repository itself is segmented (air-gapped or WORM).

  4. Other Critical Information
    • Unique Characteristics:
    – In-memory delay timer of 3 000 ms post-encryption deliberately confuses EDR rollback rules that snapshot on file-write events.
    – Generates a double ransom note: traditional README_CANTOPEN.txt on every volume plus e-mailable HTML page: _ME_RECOVER_ME_[RANDOM-HEX].html.
    – Targets network printers with random 2 MB STL/PDF payload jobs, reportedly to stall last-attempt “Print-to-PDF” bypasses.
    • Broader Impact:
    – 257 North-American municipalities reported disruption to planning-permit e-Government portals, leading to a temporary suspension of all municipal inspections from 23 Mar–15 Apr 2024.
    – Healthcare vertical suffered 38 confirmed intrusions via MOVEit CVSS-10 exploit, impacting radiology-PACS servers; Department of Health and Human Services (HHS) issued sector-wide alert OCR-2024-05.