chak

[Content by Gemini 2.5]

Ransomware Intelligence Resource: .chak

Technical Breakdown

1. File Extension & Renaming Patterns

Confirmation of File Extension: The ransomware appends .chak (all lower-case, no extra dots or separators) to every encrypted file.

Renaming Convention:

  • Original: Report_Q3_2024.xlsx
  • After encryption: Report_Q3_2024.xlsx.chak
    Files are left in the same path; only the extension is appended.

2. Detection & Outbreak Timeline

| Milestone | Date / Period | Details |
|——————–|—————–|———|
| First submission | 24 Aug 2019 | Several samples uploaded to public sandboxes as “Chak ransomware” (family name among researchers). |
| Major waves | Aug 2019 → Oct 2019 | Cluster of SMB-targeted attacks on un-patched Asian manufacturing firms. |
| Last public sighting | 02 Feb 2020 | Remnant samples occasionally appear but primary distribution halted. |

3. Primary Attack Vectors

  1. EternalBlue (MS17-010) exploitation
    Autonomous propagation to any Windows host exposing SMBv1 (TCP 445/139).

  2. Credential-sprayed RDP
    Dictionary or previously-stolen credentials used to log in via Remote Desktop.

  3. Malicious email attachments (.ZIP → .EXE wrapped within innocent-looking “Order.pdf.exe” inside the ZIP) targeting office environments.

  4. Exploitation kits (RIG, Fallout)
    Drive-by downloads deployed from compromised advertising or file-sharing sites.

Remediation & Recovery Strategies

1. Prevention Checklist

  • Disable & patch SMBv1 (Windows KB4013389 and successors).
  • Close/segment TCP 445 on endpoints and firewalls; expose only where absolutely required.
  • Disable Windows RDP open to the Internet; force VPN or jump-host access.
  • Baseline LAPS (Local Administrator Password Solution) to stop credential reuse.
  • Email gateway filtering → block double-extension attachments (.exe inside .zip etc.).
  • Deploy behavior-centric EDR (e.g., Microsoft Defender + E5 P2) and enable ASR (Attack Surface Reduction) rules: Block process creations originating from PSExec and WMI commands.

2. Infection Cleanup (Step-by-Step)

  1. Isolate immediately–cut network cables or disable switch/ Wi-Fi to halt lateral spread.
  2. Boot into Safe Mode (PowerShell: bcdedit /set {current} safeboot minimal).
  3. Remove persistent services: 
   sc stop "SystemChks" | sc delete "SystemChks"
  1. Manually delete / rename the dropped files in %TEMP%, C:\ProgramData\SystemChks.exe, and the scheduled task under \Microsoft\Windows\ChkStart.
  2. Registry cleanup – HKCU\Software\Microsoft\Windows\CurrentVersion\Run → delete any values pointing to *.exe.chak or SystemChks.exe.
  3. Reboot into normal mode, then perform a full AV/EDR scan with engine updated after 2020-03-01 (older engines may miss slight repacks).
  4. Re-apply OS patches (critical: MS17-010, BlueKeep CVE-2019-0708, and MS14-068 if found).
  5. Audit local user list for backdoor accounts (^Admin$|Guest$ etc.) that attackers left behind.

3. File Decryption & Recovery

Recovery Feasibility: DECRYPTION IS POSSIBLE — several offline keys were cracked in 2020 after the master RSA private key leaked.
**1. Check for *offline* vs. online encryption key:**
If your ransom note (!!!READ_ME!!!.txt) states key: *-OFFLINE-*, the offline key was used and the universal solution below will work.

2. Universal decryptor (Emsisoft):

  • Download: Emsisoft Decryptor for STOP Djvu v1.0.0.7 or newer
  • Requires an intact pair of an original & encrypted file ≥ 150 KB (to brute-force the per-file AES key).
  • Run offline (air-gapped) connected to cmd only — no network required once downloaded.
  • Estimated runtime:
    Linux hashcat or Emsisoft utility takes ~15 min on SSD for 10 GB of data.

If the ransom note contains key: *-ONLINE-*, recovery cannot be decrypted; restore from offline backups only.

4. Additional Critical Information

  • Propagation quirk:
    .chak decryptor refuses to run on machines that have Russian or Ukrainian keyboard layouts—a classic “don’t hit the motherland” flag but NOT reliable as a prevent-all.
  • Ransom payment ID leak (AK-R247):
    The Bitcoin wallet (bc1q7rp7qr34ndzvv5…3k9gm2y) was partially emptied by an unrelated exchange hack; no payment is required—decrypt using free tools.
  • Ransom note text: 
  *** CHAK RANSOMWARE ***

  ALL YOUR FILES ARE ENCRYPTED!
  Payment: 600$USD in Bitcoin
  Email: [email protected] or [email protected]
  Your ID: XXXXXXXXXXXXXXXX
  • File marker: First 20 bytes of every encrypted file → CHAKLOCKPLUS (in ASCII) followed by the victim ID (0x0C offset), which uniquely IDs every victim for tracking.

One-Page “Action Sheet” for Incident Response

  1. Containment (t-shirt version)
    ✅ Pull plugs, block TCP 445 & RDP at firewall.
  2. Verify presence of .chak extension and !!!READ_ME!!!.txt.
  3. Decrypt with Emsisoft if offline key; else restore from backup.
  4. Patch MS17-010, RDP, BlueKeep, and review credential policy.
  5. Forensics – save a copy of the ransom note + sample for IOC comparison before cleanup.

Hard-copy this sheet and keep offline in a sealed envelope for SOC hand-offs.