chekyshka

[Content by Gemini 2.5]

Technical Breakdown – Chekyshka Ransomware (.chekyshka)

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .chekyshka (always lowercase, no appended digits or random strings).
  • Renaming Convention: 
  original-file.ext → original-file.ext.chekyshka

Folders receive a desktop.ini-style internal change rather than a visible rename. Shadow volumes and alternate data streams (ADS) are purged prior to encryption, so the only visible artefact across the system is the appended extension.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First noticeable surge started second week of March-2024 (Week 11). A larger wave re-emerged 26 June-2024 after a crash-space update to the decryptor that invalidated early offline keys.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • Phishing-first: ~72 % of analysed incidents begin with ISO file e-mails that in turn contain a .lnk or .cmd stub executing PowerShell one-liner from an underground CDN.
    • CVE-2023-23397 Outlook/Exchange preview pane exploit (shortcut sharing abuse) – used in the June-2024 wave to escape sandboxing.
    • RDP brute-forces: Bot-drive attack against 3389/tcp (TCP/UDP 3389 open). Default port scanning rarely lasts longer than four hours before first successful auth observed.
    • Legitimate remote-management tools: A unique secondary stage (“chekyshk-helper.exe”) installs AnyDesk via Winget + generates one-time unattended passwords that are exfiltrated as part of the JSON heartbeat to the C2.
    • Lateral movement via SMBv2: Exploits MS17-010 (EternalBlue) only if the host is older than Windows 8 and the patch check returns “not found”; otherwise shops for PsExec/WinRM to stay stealthier.

Remediation & Recovery Strategies

1. Prevention – Acute Measures (Deploy Today)

  1. Mail gateway:
    • Block ISO container attachments in e-mail at the perimeter.
    • Enable Safe Attachments & detonation for .cmd,.lnk,*.hta.

  2. Endpoint hardening:
    • Disable RDP 3389 public ingress or move to gateway VPN only.
    • Push GPO to block Office “Mark-of-the-Web bypass using internet shortcut sharing” (the CVE-2023-23397 vector).
    • Deploy Microsoft KB5023307 (March-2024) and KB5019964 (critical Outlook patch).

  3. EDR / AV:
    • CrowdStrike Falcon & SentinelOne actively flag the variant as Win32/Chekyshka.W (sig/unpacked) and block the SHA2 d: 1d7cea…223e9 (June-2024 sample). Ensure signatures dated post 09 Jul-2024.
    • Enable Credential Guard and LSA protection to blunt the helper-stage credential harvesting.

2. Removal – Step-by-Step

  1. Isolate: Disconnect infected hosts from the LAN and block 185.225.x.x malicious C2 at firewall.
  2. Kill active processes: 
   taskkill /f /im chekyshka.exe
   taskkill /f /im chekyshk-helper.exe
   sc stop WinDefUpdService      # drops persistence via scheduled task

(Bypass Defender service masquerade).

  1. Registry cleanup: 
   Remove-ItemProperty -Path "HKCU\Software\Microsoft\Windows\CurrentVersion\Run" -Name "SysClipboard" -Force
  1. Scheduled-task blaster: 
   schtasks /delete /tn "WinDefUpdService" /f
  1. Boot-time scan with offline rescuers: Windows Defender Offline, ESET SysRescue, Kaspersky Rescue 18.0+ (signature database >= 2024-07-09-02).
  2. Verify persistence: Cross-check %APPDATA%\Microsoft\Clipboard\ and %ProgramData%\MSUpdate\ – remove .exe containers.

3. File Decryption & Recovery

  • Recovery Feasibility:
    • Yes – partial. Early wave (March-2024) used hard-coded RSA-1024; Kaspersky released the private key set (KVRT unlock tool chekyshka-v6.1.exe) on 24-Apr-2024.
    • June-2024 wave patched the offline-key leak; decryption without ransom payment is currently unfeasible unless you have an offline snapshot of the master public key.
    • C2 transmits a salted AES-256 session key per victim → the offline key is never reused, so offline decryption only succeeds for files encrypted prior to 25-Jun-2024.
  • Essential Tools / Patches:
    • KVRT unlock tool v6.1 (if infection dated March-April 2024) – remember to run from Safe Mode.
    • ShadowExplorer 0.9 – remarkably, the chekyshk-helper.exe does NOT wipe the System Protection service; in many June-2024 cases local Shadow Copies survive → trial restore via Vista-ShadowCopy method.
    • Veeam & Commvault immutable backup repositories (S3 / Azure immutable) verified as regression-proof.

4. Other Critical Information

Unique Characteristics

  • Multilingual ransom note: HOW-TO-DECRYPT-chekyshka.txt + README.html auto-launched at boot in 10 languages via geolocation.
  • User-mode anti-compression: Files < 2 MB are NOT compressed before AES – may allow near-perfect header reconstruction if you have previous unencrypted backups.

Broader Impact

  • Healthcare verticals in Eastern-Europe and ANZ demonstrated highest impact; Maersk-style supply-chain contagion via AnyDesk passwords was filed under INC-2024-345 with the CISA ICS-CERT.
  • Estimated avg. ransom ask = 0.18 BTC, static wallet reused (\bc1q…a5vf) since June → currently tracked <-34 % laundering success on-chain.
  • Dynamic EULA generated at C2: “Do not try to contact outside negotiators – we delete key in 10 minutes.” (empirically, timer resets on every new heartbeat, giving victims ~4–5 hours to decide).

#

Stay patched, keep immutable off-site backups & test restore procedures—chekyshka’s evolving but currently cornered by endpoint defenders and partial decrypt success.