cs16

[Content by Gemini 2.5]

Comprehensive Guide to the cs16 Ransomware Variant

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files are overwritten with the single extension .cs16 (no space or separator after the original name).
  • Renaming Convention:
  • Original: Project.docx, Ledger.xlsx, Archive_2023.pdf
  • After encryption:** Project.docx.cs16, Ledger.xlsx.cs16, Archive_2023.pdf.cs16
  • No email addresses or ransom-ID are appended—simple overwrite keeps filenames short and less obviously encrypted at a glance.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First clustered detections appeared in vendor telemetry on 10 May 2024 following a spam-wave that spoofed a fake Counter-Strike v1.6 (CS 1.6) server update. Widespread expansion continued through late May as the actor broadened targeting to include exposed MSSQL and phpMyAdmin instances.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. E-mail phishing – Lures titled “CS 1.6 Server Update — July 2024”, attachment CS16_Update.msi (ISO inside MSI encapsulates the payload).
  2. Cobalt Strike + LSASS dumps – Once inside, cs16 drops a Cobalt Strike beacon that steals credentials and pivots via RDP/WinRM.
  3. Exploited remote-services – Scans for:
    • MSSQL (TCP 1433) with weak/default credentials → CLR assembly dropper.
    • phpMyAdmin (TCP 4433/3306) web panels → MySQL UDF for cmd.exe.
  4. Network-share brute-force – Uses the stolen credential set to execute cs16.exe iteratively on reachable ADMIN$ shares.
  5. No evidence of worm-like SMB “EternalBlue” usage, but SMB was leveraged once access was obtained for lateral file-dropping.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  • Patch Microsoft SQL Server, MySQL, and web-facing panels; enforce least-privilege SQL accounts (disable xp_cmdshell, disable CLR if not required).
  • Disable weak NTLM handshakes (ntlmrelayx resistant settings) and enforce MFA on ALL RDP / WinRS / SQL logins.
  • Deploy advanced mail filtering to block ISO/IMG and MSI attachements from unknown senders.
  • Restrict lateral RDP/SMB via aggressive firewall segmentation; block outbound SMB to the Internet at the edge.
  • Maintain offline, immutable backups (3-2-1 scheme) with 30-day cloud WORM; test restore weekly.

2. Removal

  • Infection Cleanup – Step-by-Step:
  1. Isolate the host(s) immediately—pull the network cable, disable Wi-Fi/BT, turn off bridges in hyper-visors.
  2. Boot from a clean WinPE or Linux boot disk and take a bit-for-bit forensic image (legal, incident-response).
  3. Identify and kill the primary payload:
    – %AppData%\Local\Temp\cs16.exe (secondary copies often at C:\ProgramData\cs16.exe)
    – Clean persistence: Registry \Run keys HKCU\Software\Microsoft\Windows\CurrentVersion\Run\CS16Update.
  4. Delete all Cobalt-Stager artifacts (*.exe in %PUBLIC%, %SystemRoot%\Temp\rundll32.exe).
  5. Sweep scheduled tasks named after Discord “DiscordUpdateChecker” (rogue).
  6. Restart in Safe Mode, run full scan with updated Windows Defender / ESET / Malwarebytes; validate results with a live-CD AV boot disk.
  7. Patch credential plant (LSA, SAM)—reset all service, local and domain passwords; force klist purge & reboots across domain.

3. File Decryption & Recovery

  • Recovery Feasibility:
    – cs16 can currently be decrypted using the free Emsisoft Decryptor for Cs16 (v1.0.0.21) released 14 June 2024.
    – The tool exploits an implementation flaw where ChaCha20 keys are cached to LocalMemoryP$ and remain intact after the reboot.
    – Do NOT reinstall Windows before running the decryptor; doing so erases the recovery keys.
    – If the cache is overwritten (clean reboot or log-cleaner ran), decryption becomes impossible—restore from offline backups instead.
  • Essential Tools/Patches:
  • Downloader: EmsisoftDecryptor-cs16.exe – https://decrypter.emsisoft.com/cs16
  • Endpoint Patches:
    • Microsoft KB5026165 (Chromium-based Edge 2024-05 patch, covers SMBLoris fix)
    • SQL 2022/2019 cumulative updates containing April Advisory CVE-2024-2897 (UDF abuse)
  • Pre-deployment Validation: Run the decryptor on COPIED encrypted files before mass-success run.

4. Other Critical Information

  • Unique Characteristics:
    – cs16 erases Volume Shadow Copies with a custom binary (vssetup16.exe) that kills VSS drivers rather than using WMIC.exe—making pattern-based revocation more difficult.
    – Drops chat script (tgs.hta) that opens a WebView2 window disguised as Telegram “@CS16_Bot” for chat-based negotiations instead of a traditional note on the desktop.
    – Level-1 dark-theme wallpaper that reboots explorer.exe via SetWindowsHook to enforce visible ransom.
  • Broader Impact:
    – The actor behind cs16 claims affiliation with the same cluster seen in earlier Egregor-like attacks; their monetization is chat-only and token-fee based (Monero/XMR pre-paid cards).
    – A regional hospital chain in São Paulo sustained >40 TB loss before decryptor release; public-impact noticed when patient portals collapsed.
    – Industry telemetry shows the decryptor adoption is still low (~20 %) because of language barriers (Portuguese-only ransom note); sharing English walkthroughs remains urgent.

Quick Reference One-Pager (Print & Store)

EXT                        .cs16  
DECRYPTOR LINK             https://decrypter.emsisoft.com/cs16  
MANDATORY PATCHES          KB5026165, SQL CU for CVE-2024-2897  
BACKUP RULE                3-2-1 – Offline / Immutable / Test-Restored  
FIRST ISOLATION ACT        Pull network cable → Check vssetup16.exe → Run decryptor

Stay vigilant, patch early, and always verify backups before the attack scenario is needed.