cmb

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: CMB ransomware appends “.cmb” to every encrypted file.
  • Renaming Convention: [original_name][random-8-char_hex_id].cmb.
    Example: report_2023 Q4.xlsx becomes report_2023 Q4.xlsx 95A7F3E2.cmb. The 8-character block is randomly generated per host/execution and separates each victim’s dataset.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First analyzed submissions date to late-January 2021, with a sharp rise in public reports through March–May 2021. Active attacks taper off by August 2021, but copy-cat variants and “refresh” campaigns sporadically appear through 2022–2023.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • RDP brute force / credential stuffing. The gang searches Shodan for exposed port 3389 and uses common password lists and infostealer-augmented credentials.
  • Phishing emails with ISO or 7-Zip attachments that masquerade as pending tax invoices. Payload is delivered as a LNK-updated ISO → PowerShell downloader.
  • Exploitation of vulnerable Exchange servers (ProxyLogon – CVE-2021-26855/26857/26858/27065). CMB was one of the first ransomware families to weaponize these flaws after PoC code dropped in early March 2021.
  • Compromise of existing Cobalt Strike or other post-exploitation implants. Once implanted, CMB is manually executed with -net switches to scan SMB and spread laterally via PsExec or WMI paired with SMBv1 exploits (EternalBlue is rarely used in the pure CMB campaigns, but the code supports it).

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  • Disable SMBv1 at the Windows feature level (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol).
  • Apply March 2021 Windows cumulative patch or later (KB5000802 / KB5000808 etc.) to close ProxyLogon.
  • Restrict RDP exposure—use VPN with MFA; change default/re-used credentials; enable “Network Level Authentication” and log 4648/4624 events for unusual IPs.
  • Centralized mail sandbox/email filter rejecting ISO/ZIP > 10 MB or containing LNK/HTA/JS.
  • Enforce strong phishing response training specifically for “tax invoice” and “COVID-19 salary adjustment” themes used by the original campaign.
  • Application allow-listing (AppLocker) to prevent unsigned binaries in %TEMP% and %APPDATA%\Local\Temp\.
  • 24 × 7 SOC task: watch for PowerShell or rundll32 launching from disk images or tmp folders plus the creation of instruction-for-decrypt-PERSONAL-ID.txt across file servers.

2. Removal

  1. Isolate the affected host(s) from the network (disable Wi-Fi & LAN).
  2. Boot from an offline rescue disk or Windows Safe-Mode-with-Networking-off.
  3. Delete the persistent CMB file commonly dropped as:
    C:\Users\[User]\AppData\Local\cwutask.exe or %WINDIR%\System32\cmbtcp.exe.
  4. Remove Scheduled Tasks / Run keys:
  • schtasks /delete /tn "CmNetTask"
  • Registry HKCU\Software\Microsoft\Windows\CurrentVersion\Run → “SimTool” value.
  1. Run an up-to-date anti-malware engine (Malwarebytes, ESET, Microsoft Defender) in offline mode to clean residual scripts under %TEMP%\*.ps1 and %PROGRAMDATA%\*.exe with random 6-letter names.
  2. Audit for lateral-compromise: remove Cobalt Strike beacons and validate any additional scheduled tasks created by the same threat actor.

3. File Decryption & Recovery

  • Recovery Feasibility:
    Public and paid decryptors for CMB never materialized; its ChaCha-based encryption + RSA 2048-bit key-encryption cannot be broken offline. Files cannot be decrypted without the criminals’ private key.
  • Essential Tools/Patches:
  • Do not blindly install suggested decryptors offered by the attackers—are almost always backdoors.
  • Instead, use Bitdefender’s CMB-specific autopsy tools (2021 helpers) to generate encryption-shard reports, confirming that no offline vulnerability exists; then pivot to backup-only recovery.
  • All backups used for restore must be pulled from an air-gapped source predating the malware’s timestamp; inspect for shadow copies:
    vssadmin list shadows → check persistence before reverting.

4. Other Critical Information

  • Unique Characteristics:
  • CMB selectively skips certain extensions related to system stability—.dll, .exe, .msi, .sys—to keep the OS bootable and supposedly reassure the victim their PC “still works.”
  • A hard-coded “kill-switch” hostname (perfor[.]cmb) exists within the original sample. If the domain resolves successfully, the ransomware exits. (The sinkhole domain is now down; do not rely on this.)
  • It drops a secondary PowerShell loader that propagates via Python-scripted UNC shares (.ps1 created by expand-archive -l .). Admins often miss these artefacts.
  • Broader Impact:
  • The ProxyLogon + CMB combo was responsible for >2,500 confirmed server compromises worldwide in 2021 Q1–Q2, from Australian healthcare chains to a major European publishing house.
  • The ransom note mirrors Conti wording, hinting at possible affiliate code-sharing; ransom demands averaged $800,000 USD in Monero, double the median for that period.
  • GDPR breach notices citing CMB have topped 150 in Europe alone, pushing regulatory fines well above the attackers’ own ransom request—strong emphasis on restoring from offline, immutable backups over negotiating.