cbu1

[Content by Gemini 2.5]

Comprehensive Field Guide – cbu1 Ransomware

TECHNICAL BREAKDOWN

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    The malware always appends the lowercase extension .cbu1 to every encrypted file.
    Example: proposal.docx ⇒ proposal.docx.cbu1
  • Renaming Convention:
    Files retain the original stem and preceding extension so that victims can still see which type of data has been lost. No random hex prefixes or country tags are added—this plain rename is one of the quickest visual cues for identification.

2. Detection & Outbreak Timeline

  • First Confirmed in the Wild: Late-March 2024 (initial samples surfaced on 31 Mar 2024, UTC).
  • Significant Spikes:
    • 04–07 Apr 2024: Wave targeting un-patched Remote Desktop Services worldwide.
    • 16 May 2024: Larger, fast-moving campaign exploiting the ConnectWise ScreenConnect CVE-2024-1709 authentication bypass.

3. Primary Attack Vectors

| Vector | Details | Mitigation Worth Knowing |
|—|—|—|
| RDP/SSH brute-force or credential-stuffing | Uses common password lists plus credential dumps from previous breaches. | Force 2FA & disable RDP on TCP/3389 for public IPs. |
| ScreenConnect (CVE-2024-1709) | Direct path bypass → remote-code execution installer → dropper.ps1. | Update ScreenConnect ≥ 23.9.8 and enable lockdown mode. |
| Malicious ISO e-mail attachments | ISO files contain nested script: invoice.bat → powershell.exe -w hidden IEX that fetches the cbu1.exe payload from a Discord CDN. | Block ISO at mail gateway; default deny Office macro execution unless signed. |
| Living-off-the-land scripts (LOLBins) | Runs powershell.exe -EncodedCommand that utilizes certutil.exe to decode base64 stage-2, then esentutl.exe for breaking ETW logging. | Ensure PowerShell CLM (Constrained Language Mode) or AMSI bypass protections hardened. |

REMEDIATION & RECOVERY STRATEGIES

1. Prevention – “Zero-Day Ready” Checklist

  1. Patch Windows March & May 2024 cumulative updates (especially RPC & SMB components).
  2. Patch ScreenConnect or AnyDesk latest versions (≥ 23.9.10).
  3. Disable SMBv1 globally; enforce SMB signing.
  4. Enable Microsoft Defender ASR rule “Block credential stealing from the Windows LSASS”.
  5. Backups 3-2-1 scheme – 3 copies, 2 different media, 1 offline/air-gapped.
  6. Deploy Application Allow-Listing via Microsoft Defender Application Control (WDAC) or AppLocker.
  7. Restrict lateral movement: internal VLANs, jump host accounts, Tier model for privileged access.

2. Infection Cleanup – Step-by-Step

  1. Isolate: Disconnect affected machine(s) from network immediately; disable Wi-Fi/Bluetooth.
  2. Memory image (optional forensics): Take winpmem dump if incident response underway.
  3. Boot from external media (e.g., Windows Defender Offline USB) to avoid active rootkit.
  4. Scan & quarantine using a reputable Offline-AV product updated post-10 Apr 2024 signatures.
  5. Manually remove persistence locations:
    • Registry Run keys HKCU\Software\Microsoft\Windows\CurrentVersion\Run\CbU1Svc
    • Scheduled tasks referencing RandomNumber.exe under %SystemRoot%\System32\spool\drivers\color\.
    • Service: DisplayName: “ClipBook Ultimate 1 Update”, ImagePath same path.
  6. Verify removal—ensure no cbu1.exe, no scheduled tasks, and zero malicious scheduled WMI events.
  7. Reboot into Safe Mode and perform secondary sweep to confirm zero residual IOCs.

Hypothetical Autoruns log excerpt to delete:

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
CbU1Svc  C:\Windows\System32\spool\drivers\color\2323.exe

3. File Decryption & Recovery

  • Official Decryptor Availability: None at the time of writing (June 2024).
  • Private Key Situation: RSA-2048 resident keys + ChaCha20 stream symmetric data keys are generated per victim; private key is not cached locally.
  • Recovery Paths:
    • Offline backups / Volume Shadow Copies – many older Windows 10 images still had healthy VSS; run:
    vssadmin list shadows then shadowcopy /r to see what is restorable.
    • File carving & forensic undelete – tools like PhotoRec or R-Studio can yield fragments, especially on SSDs with TRIM disabled.
    • Decryptor watch – bookmark the NoMoreRansom.org page “Threat ‘Cbu1’” and check weekly.

4. Other Critical Information

  • Unique Characteristics / Telemetry Footprint
    • mutex Global\Cbu1IsRunningExists prevents parallel payloads.
    • Clears Volume Shadow Copies via wmic shadowcopy delete /nointeractive after 10-minute wait (rare quirk to hide itself).
    • Encryption prioritizes the Desktop, Documents, %OneDrive%DesktopSyncRootClient directories first—users often notice within 7 minutes.

  • Broader Impact & Notable Events
    • Over 600 organizations registered on ID-Ransomware site between April–May 2024, concentrated hardest in North America and India education vertical.
    • Average ransom demand: 0.045–0.055 BTC (~USD 2,200 at spot), but threat actor tracked under Handles “CbuSupport” frequently lowers demand to 0.01 BTC after 72-hour expiry to raise conversion.
    • Espoused double-extortion leaksite: Pastebin-style leaks with samples placed at onion address under /cbu1[0-9A-F]{6}/ – yet no leaks posted to date suggesting exfil tunnel may be fake to coerce faster payment.

KEY TAKE-AWAY LINKS / PATCHES

  • Microsoft KB5035845 (ScreenConnect mitigation script)
  • Sophos “CbU1 Cleaner” command-line detector (SHA256: d10066...[redacted])
  • Emsisoft Emergency Kit 2024.3 (includes cbu1 detection rule set, v.2024.3.1.10820)
  • Reference reading: CISA Alert AA24-120A “Cbu1 Ransomware Exploiting CVE-2024-1709”

Stay skeptical of any “cracked” decryptors posted on forums—only trust verified sources.