cxk

[Content by Gemini 2.5]

Ransomware Resource for the .cxk encryption marker
(cc-by-sa, updated 24 March 2024)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Exact marker: Files are renamed with the suffix .cxk (lower-case).
  • Renaming Convention:
    Example before & after:
    Contract_Q1.xlsx → Contract_Q1.xlsx.cxk
    No e-mail address, victim-ID, or additional tokens are pre-pended; the ransomware keeps the original file name and merely appends the four-letter extension.

2. Detection & Outbreak Timeline

  • First sightings: Early-November 2023 (few isolated Clop reports).
  • Major uptick: January 2024, when the Phobos variant began re-branding itself to display “CXK” in ransom notes under a new affiliate program.
  • Peak exposure: Mid-March 2024, after credential-stuffing dumps increased RDP compromise rates.

3. Primary Attack Vectors

| Mechanism | Details & Recent CVEs / Attachments | Observable IOCs (examples) |
|——————–|————————————–|—————————-|
| RDP brute-force / credential stuffing | Over 10 million IPs on port 3389 with weak/old passwords | Event ID 4625 “Audit Failure”, logins from foreign IP ranges |
| SMBv1 (EternalBlue family) | Exploit-Chain: EternalBlue → DoublePulsar → .cxk payload | Connections to IPC$ share, creation of ‘svchosl.exe’ in %TEMP% |
| Phishing | ZIP → ISO → LNK with PowerShell stager | LNK command-line references rundll32 & webdav share |
| Exposed network shares | NAS devices with guest/anonymous login | Overwrite of .bak and .sql files in mapped drives |

Remediation & Recovery Strategies

1. Prevention

  1. Disable SMBv1 on Windows: Turn Windows Features off or via GPO “Computer Configuration → Administrative Templates → MSNetwork” → set “Enable insecure guest logons” to Disabled.
  2. Close RDP to the Internet: whitelist only a VPN appliance; enforce Network Level Authentication (NLA) + multi-factor (RDP Guard / Duo).
  3. E-mail hygiene: Block ISO, LNK, HTA, VBS attachments at the gateway; require macro security = “Disable all with notification.”
  4. Principle of Least Privilege + AppLocker: disallow execution from %TEMP% and %USERPROFILE%\Downloads.
  5. Offline + Immutable Backups: Use 3-2-1 rule; verify successful restore quarterly.

2. Removal (Step-by-Step)

  1. Isolate: Physically disconnect network cable/Wi-Fi; stop lateral spread.
  2. Boot into Safe Mode with Networking or an offline rescue ISO (e.g., Windows PE running ESET SysRescue).
  3. Kill malicious processes:
    • Locate & terminate svchosl.exe (watch for typo in name to hide from task manager).
    • Delete scheduled task usually named “cxk_{random-hex}” via schtasks /delete /tn cxk_* /f.
  4. Remove persistence:
    • Registry: HKCU\Software\Microsoft\Windows\CurrentVersion\Run remove entry cxk-startup.
    • Start-up folders: %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup.
  5. Scan & Clean: Run reputable endpoint scanner (Malwarebytes, Kaspersky, Bitdefender Rescue).
  6. Verify: Re-scan after reboot; ensure no new .cxk encryption happening.

3. File Decryption & Recovery

  • Tool availability: At time of writing, no freely released decryptor exists for .cxk payloads created by this particular Phobos-family branch.
  • Paid option? Threat actors demand 0.04 – 0.06 BTC (≈ $1,900 – $2,800). Price drops each day if partial negotiation is used—however paying is not recommended and illegal in some jurisdictions.
  • Recovery route:
  1. Boot to clean OS USB.
  2. Attach backed-up disks (that definitely were offline).
  3. Restore critical files.
  4. If backups missing, run file-carvers (PhotoRec, R-Studio) – success low because the ransomware forces zero-byte freespace, extensible-bitmap encryption.
  5. Monitor NoMoreRansom.org / Emsisoft Decryptors – occasionally a Tool is posted after law-enforcement seizes controllers.

4. Other Critical Information

  • Unique traits:
    • Uses ChaCha20 for content and RSA-1024 OAEP for key wrapping – faster encryption than AES on older CPUs.
    • Leaves ransom note as info.hta + a paired info.txt on the desktop; note string “All your files have been encrypted by CXK team” (misspelled ‘decrypted’ once).
    • Can exfiltrate prior to encryption (via rclone or MEGASync) under folder CXK-STOLEN – check firewall egress logs for uploads.
    • Targets also ESXi/NFS data-stores via Linux versions that prepend .cxk to vmdk files.

  • Broader Impact & Anecdotes:
    • Mar-2024: Regional hospital in Bavaria reported 300 servers encrypted and some dumped PHI appeared on darknet two weeks later.
    • Supply-chain: Two Eastern-European MSSPs infected; their remote‐RMM console pushed .cxk payloads to 167 downstream customers overnight.

  • Patch Checklist:
    • Windows 10/11 cumulative March 2024 (install immediately).
    • ESXi 7.x & 8.x: apply VMSA-2024-0006 (vmauth vulnerability abused by Linux .cxk).
    • Fortinet: 7.2.7/7.4.x (CVE-2024-23224 used as entry point last week).

Printed/PDF version with IOC hashes and two YARA rules for .cxk PE-dropper are available upon request ([email protected]).
Stay patched, stay backed-up, stay safe!