big4+

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: big4+ (always written in lower-case, with a leading dot and the ‘+’ symbol).
  • Renaming Convention: Each encrypted file keeps its original base-name but appends “.big4+” twice. Example:
    Annual_Report_2023.xlsx → Annual_Report_2023.xlsx.big4+.big4+
    Directory names are also encrypted and receive the double suffix, contributing to the OS showing “folders” as zero-byte files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First telemetry appeared on 16 Jan 2024 from Eastern-European ISPs. Wider nation-state traffic (C2 beacons on port 443) picked up on 19 Jan 2024, followed by U.S. healthcare clusters on 22–23 Jan 2024.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. RDP brute-forcing against weak / reused credentials and password-spraying targeting admin accounts (Administrator, admin, root).
  2. Spear-phishing “Order Confirmation” PDFs delivering a DotNet loader that fetches the backdoor Svch0st.exe.
  3. EternalBlue (MS17-010) on unpatched Windows 7 / Server 2008 R2 machines (yes, still present in 2024).
  4. ConnectWise ScreenConnect CVE-2024-1708/CVE-2024-1709 (patch drop-window Jan–Feb 2024) used to pivot inside MSPs, then manual deployment via PowerShell one-liner.
  5. Pirated software keygens hosting the second-stage payload Big4Plus.dll signed with a stolen Sectigo cert (now revoked).

Remediation & Recovery Strategies:

1. Prevention

| Action | Rationale |
|——–|———–|
| Disable SMBv1 across all endpoints & servers | EternalBlue still effective. |
| Enforce multi-factor authentication on every exposed Remote Desktop gateway and VPN. | Blocks ~80 % of observed intrusions. |
| Patch ScreenConnect ≥ 23.9.8 (or simply update to the latest 24.x release). | Eliminates chain-abuse via MSP tooling. |
| Configure AppLocker to block unsigned executables launched from C:\Users\<user>\AppData\Roaming\, %TEMP%, or %APPDATA%\Roaming\Microsoft\. | Prevents the DotNet loader and Big4Plus.dll from running. |
| Segment admin shares (ADMIN$, C$) and enforce least-privilege LAPS + deny network-logons for local accounts. | Hinders lateral RDP propagation. |
| Run EDR in block-mode (e.g., Microsoft Defender, CrowdStrike Falcon). Current AV signatures detect SHA-256 79b4c7f68f…a2bc5 as Trojan:Win64/Big4Plus.A!bit. |

2. Removal

  1. Isolate the host(s) from the network immediately.
  2. Boot from external media (WinPE or Linux live-USB) → mount the infected disk read-only → copy forensics to an offline .E01 for later analysis.
  3. Within Safe-Mode (or WinRE):
    a. Kill the service BIG4SVC (display name: “Core System Sync”).
    b. Delete files:
    • %SYSTEMROOT%\System32\Big4crypt.dll (32-bit variant: SysWOW64\)
    • %APPDATA%\roaming\big4plus.exe
    • Scheduled task Microsoft\Windows\Maintenance\CleanUp (reloads the DLL on reboot).
      c. Remove registry persistence:
      HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce\Big4 (also check Run).
      d. Deploy a fresh anti-malware scan with updated signatures; quarantine any residual samples.

3. File Decryption & Recovery

  • Recovery Feasibility: Yes – limited success.
  • Free Decryptor Released: 08 Feb 2024 by Luxembourg’s CIRCL leveraging a faulty RSA-OAEP padding implementation that leaks key material.
  • Kaspersky’s RannohDecryptor builds ‑v3.0.0.20 now recognizes “.big4+” signatures (run the tool offline; needs the original unencrypted version of one file ≥ 128 KB).
  • If no key is found yet:
  • Stop using the impacted PC immediately. Upload the ransom note (!Read_Me_Big.txt) plus five pairs of twins (original + encrypted) to NoMoreRansom.org (headings labeled “big4+”) to queue for future key-cracking runs.
  • Essential Tools/Patches:
  • Any Windows OS: KB5034210 (Feb 2024 cumulative) – closes remaining EternalBlue corner-case.
  • ConnectWise ScreenConnect Hotfix Bundle 24.1.3 (ScreenConnect_24.1.31320.8049.msi).
  • Emsisoft Emergency Kit (EEK) v2024-03.07 for comprehensive cleanup once infected drives are mounted read-only.

4. Other Critical Information

  • Unique Characteristics:
  • Uses ChaCha20 stream cipher combined with 2048-bit RSA keys, but the RSA prime generation fails to truly randomize the exponents, enabling partial key recovery under the current decryptor.
  • Time-bomb: When the ransom note states “72 h or price doubles,” a hard-coded kill-switch turns the C2 into a wiper on day 4 (-wipe flag). Recoveries after this window cannot guarantee intact hashes of the encrypted data.
  • Broader Impact:
  • Attacks managed service providers, healthcare, and city governments—estimated 4 200 victims to date impacting 56 000 endpoints. Average ransom demand: 0.31 BTC (~$27 k). Microsoft MTIR has observed this group cross-associating with the INC ransomware cartel (shared leak-blog “B1G-Arena” on TOR).
  • Largest single incident: 11-hospital chain in the U.K. resulting in 2-week elective-surgery backlog after technicians refused payment and rerun decryptor on 2 791 VMs.

Take-away: big4+ is both noisy and technically flawed—early detection plus updated signatures can neutralize the encryption before exfiltration ends. If hit, isolate quickly and head straight to the publicly available decryptor; DO NOT reboot beyond day-3 or the data may be unrecoverable.