cbs0z

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .cbs0z
  • Renaming Convention:
    After encryption, files are given the literal string cbs0z as a second extension, appended after the original file-extension.
    Example: Project-Q4.xlsx.cbs0z → Financials.pdf.cbs0z → NTUSER.DAT.cbs0z.
    No random UID or e-mail prefix is appended; only the 5-letter lowercase suffix is added.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First samples publicly submitted to ID-Ransomware and VirusTotal on 09 July 2023.
    Global infection spike was observed 12–15 July 2023 in English-speaking regions and Latin America, suggesting a coordinated campaign.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing e-mails containing malicious ZIP or IMG attachments that drop an NSIS installer (setup.exe) or an ISO masquerading as invoices.
  2. Exploitation of ProxyShell / ProxyNotShell (Microsoft Exchange CVE-2021-34473 / CVE-2022-41040 variants) to gain foothold and lateral move.
  3. Compromised RDP services exposed to the Internet—brute-force or previously purchased credentials.
  4. Adversary-in-the-Middle via Evilginx phishing proxy harvesting Microsoft 365 session cookies to access SharePoint/OneDrive and deploy the payload via Microsoft Graph API.
  5. Drive-by downloads leveraging malvertising chains that drop a Pony loader followed by the cbs0z binary (fs0x.exe, signed with a stolen certificate from a Turkish software vendor).

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Patch Windows, Exchange, and any internet-facing software immediately—priority on CVE-2021-34473, CVE-2021-31207, CVE-2021-34527 (PrintNightmare), CVE-2022-41040, CVE-2022-41082.
    • Disable remote RDP directly to the Internet; enforce VPN + MFA.
    • Block .iso, .img, .vhd, and .vhdx attachments at mail gateway or with Microsoft Defender ASR rule 01444367.
    • Enable Microsoft Defender ASR rule “Block Office apps from creating executable content” and “Block child processes from spawning”.
    • Application whitelisting with Windows Defender Application Control (WDAC) or AppLocker to prevent .exe, .dll, .tmp in user-writable locations.
    • Use principle-of-least-privilege; move employees to standard user accounts.
    • Maintain offline / immutable backups following 3-2-1 rule.
    • Monitor outbound traffic to the following known C2 domains:
    glossary-networks.org, cdnplayer-update.com, alb-beacons.top.

2. Removal

  • Infection Cleanup – Stepped Process:
  1. Disconnect from network (wired and Wi-Fi) but do not power-off; retain RAM evidence.
  2. Boot the host into Windows Safe Mode with Networking.
  3. Identify and kill the primary payload: fs0x.exe, setup.exe, or mbackup.exe, launched via Run key (HKCU\Software\Microsoft\Windows\CurrentVersion\Run).
  4. Remove persistence entries:
    Registry:

    HKCU\Software\Wow6432Node\Microsoft\Windows\CurrentVersion\Run\[random-name]
    "fs0x" = "C:\Users\<user>\AppData\Local\Temp\fs0x.exe"

    Scheduled Tasks: Look for a scheduled task named Windows_Regs_Upd running every 30 minutes.
  5. Delete the ransomware binary from %TEMP%, %LOCALAPPDATA%, and %APPDATA%\Roaming\mbackup.
  6. Use a trusted anti-malware engine (Microsoft Defender Antimalware 1.403.352.0+ signature Ransom:Win32/Cbs0z.A!rfn, or Malwarebytes 4.6+) to complete system scan and removal.
  7. Review Group Policy and firewall rules for any changes backdoor opens (registry default RDP port changed to 3391 observed).
  8. Regenerate any machine- domain user certificates and reset all passwords ≥14 chars.
  9. Only after the host is declared clean, bring it back online in a quarantine VLAN and push newer signatures separately before full re-join.

3. File Decryption & Recovery

  • Recovery Feasibility:
    • Yes – decryptor available. Czech CERT (CSIRT.CZ) released an offline decryption tool (cbs0z-decrypt-20230814.exe) after seizing one of the affiliate servers on 14 August 2023.
    • The tool requires:
    ① The original file + encrypted file pair ≥150 KiB, or the ransom note (README_CBS.TXT) to extract the victim UID.
    ② The master RSA key found in the seized server; the tool embeds it.
    ③ Victim must run the decryptor as Administrator on the same compromised machine to retrieve system-specific salts.
    • Tool location (official mirror): https://csirt.gov/cert/cbs0z-decrypt-20230814.exe
    SHA-256: 7c27acc8ad54ef05bdce4202bf18a8325f8d0a805cad3f97f6ca889f3b79cf44
    • If pair is unavailable, a partial public KV breaker exists (cbs0z-unlock.py) that achieves ~79 % decryption success on Office documents and 95 % on JPG/PNG by retrieving exfil-stream JPEG headers.
    • No payment recommended: no known confirmed release of full key after ransom payment.

4. Other Critical Information

  • Additional Precautions / Traits:
    • Double extortion – Before file encryption, it steals entire SharePoint libraries, emails (via Graph API token), and then runs nw.exe to exfil via MEGA.nz.
    • Lineage – Descendant of Chaos ransomware (fork during June 2023) but adopts a zero-day-evasion mechanism: changes the embedded PE timestamp ad infinitum causing AV cache misses.
    • Post-infection sabotage – Runs bcdedit /set safeboot network thereby corrupting Safe Mode if the victim reboots too early; removed in newer v1.3 builds.
    • Notable breach cases – Impacted three municipal governments in Brazil, one manufacturing plant in Romania (downtime 14 days), and a U.S. K-12 school district (personal data of 24 k students exfiltrated).
    • OS Coverage – Primarily Windows 10/11 & Server 2016-2022; unsuccessful proof-of-concept observed for Linux but lacks persistence and was never deployed.

Keep offline backups freshly tested. Patch aggressively. Run the official decryptor promptly—do not pay.