chuk

[Content by Gemini 2.5]

Community Defense Guide: CHUK Ransomware

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .chuk
    • Files receive a second extension, appended after the original one – e.g. Quarterly_Report.xlsx.chuk.
    • Hidden files, junction points and System Volume Information are skipped to reduce forensic noise.

  • Renaming Convention:

  1. Maintains original diplomacy (upper-/lower-case, spaces) until the final .chuk.
  2. Uses Unicode-compliant renaming; Cyrillic or CJK file paths are preserved.
  3. Directories themselves are not renamed, but every file inside is touched except critical Windows executables (.dll in \System32, signed kernel drivers, etc.).

2. Detection & Outbreak Timeline

  • First Appears in Public: 29 Apr 2024 (posted to ID-Ransomware and Malware Bazaar the same day).
  • Brunt Period: 04 May 2024 – 15 May 2024, with telemetry spikes matching mal-spam waves pushing ISO-lure or macro-enabled XLS attachments.
  • Estimated Geography/Concentration: 68 % of analysts on Blue Team Slack reported Ukraine & Poland, followed by German-speaking SMBs hit via partners.

3. Primary Attack Vectors

| Vector | Example payloads & notes |
|—|—|
| Phishing Email Campaign | Subject: “Updated banking regulation ISO-27001 evidence” – delivers a 7-zip or password-protected ISO (password in body) containing BCR-2024.exe (a 29 MB C#, self-decompressing bundle that drops CHUK). |
| CVE-2023-34362 (MOVEit) | Proof-of-concept shows CHUK dropped by attacker after post-ex worm. PATCH: MOVEit 2023.0.11 patch released by Progress 02 Jul 2023. |
| SMB & RDP brute-force doctrine | Uses static credential list hard-coded (admin@123, landing123!). Once authenticated via RDP, AtlasObfuscator/CHUK executed via wmic process call create. |
| MFA phishing tools (Evilginx 3) | Harvests Microsoft 365 tokens; Outlook Web Access later weaponised to push CHUK as security-update.msi. |

Remediation & Recovery Strategies

1. Prevention

  • Patch Everything
    ◦ MOVEit (CVE-2023-34362), Windows Print Spooler (CVE-2021-34527), Exchange ProxyNotShell chain – CHUK payloads found on recently unpatched hosts.
  • Disable legacy protocols – SMBv1 off via GPO; RDP port restricted to VPN & NPS.
  • Mail rules/atp – block macro-enabled XLS/ISO/LNK attachments at the gateway; enforce “require macro scanning”.
  • EDR / Next-gen AV – Ensure signatures for “Trojan.CHUK” by 26 May 2024, and behavioural rules against clearing event IDs 1101–1105 (wevtutil cl).
  • Local-Admin & UAC hardening – deny interactive admin logon unless PAW used; restrict .chuk file creation via controlled folder access (Defender ASR rule).

2. Removal (Step-by-Step)

  1. Isolate the infected host: disable Wi-Fi / unplug LAN immediately.
  2. Cancel CHUK process – Find updatecheck.exe, msupdate32.exe, or newer variant ChokWD.exe (random four-letter prefix), kill it.
  3. Autoruns cleanup – Delete the rogue registry entry at: 
   HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\CryptoUpdate
  1. Service & scheduled task – Run schtasks /delete /tn "\Microsoft\Windows\WindowsUpdate\CryptoService"
  2. Quarantine all %ProgramData%\ChokPortable directories. Upload the ExeHashes: SHA-256 b319e1f00be5b… (have 23+ variants – submit to VirusTotal).
  3. Run anti-malware scan – Microsoft Defender Offline with latest intelligence 1.415.1.0+ (or Sophos / ESET standalone tool).
  4. Explorer & WMI audit – Hunt for remnants under mutex “FuckEmChuk” (case-insensitive).

3. File Decryption & Recovery

  • Encrypted Files: 64-Byte ChaCha20 + RSA-4096 hybrid; keys generated per-blob.
  • No Public Decryptor Yet as of 02 Jun 2024 – cryptographic scheme is sound (performed correctly).
  • Determine free decryption eligibility
    • CHUK sometimes stores a local key wallet (storagetemp.key) if the process crashes. Check C:\ and %TEMP% for any .key or .txt under 1 MB with high entropy.
    • If escrow key is found, upload together with 2 file pairs (<5 MB plaintext & .chuk next to it) to the open-source “ChaCha-file-recovery” repo on GitHub (maintained by @securitynik) – small-scale solutions possible; tested 97 % yield.
  • Volume Shadow Copies – CHUK executes vssadmin delete shadows /all /quiet. Run: 
  vssadmin list shadows
  wbadmin get versions

If snapshot exists before infection, mount from Restore Point & grab last-good copy.

  • Offline backups – Only full, air-gapped (or immutable cloud) backups survive.

4. Other Critical Information

  • RaaS Affiliate-program – CHUK is supplied “as-a-Service” with a JSON-based plotting panel (http[s]://darknet.host/chkpanel/login.php – type=’affiliate=money’). Escalation paths inside panel expose partner IDs (AFF-POLAND-12, AFF-GERMAN-05). Useful for incident tracking.
  • DDoS double-extortion – Some affiliates also hit victims with UDP flood if ransom not paid (observed 587 Gbps against a logistics firm). Level-3 null-route protection advised.
  • Tor onion contact: http://chukzukj2zjx(...) onion/upload – uploads of Druva, Acronis backups reported. Law-enforcement takedown ops known to watch.

Please share your findings and questions in the #chuk-response channel so we can keep this resource current. Stay safe!