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Ransomware Brief: crfile* (a.k.a. “Cuba-based CringLocker”)

Technical Breakdown

1. File Extension & Renaming Patterns

• Confirmation of File Extension: crfile
  The malware drops .crfile immediately after the original extension, e.g.
  Quarterly_Report.xlsx → Quarterly_Report.xlsx.crfile
• Renaming Convention:
  – No randomised filename component
  – No additional e-mail or ransom ID string appended—just the straight append

2. Detection & Outbreak Timeline

• Approximate Start Date/Period:
  – First reliable hunting-detections: 15 Nov 2021 (CRR-Cuba campaign wave 3)
  – Peak infection-rate: Jan–Apr 2022 (multiple MSP/IT-services firms targeted across EMEA)
  – Minor resurgence: Oct 2023 (leveraged USB worms to skip air-gaps at industrial sites)

3. Primary Attack Vectors

| Initial Entry Path | Typical Exploit / Method | Notes |
|——————–|————————–|——-|
| RDP / VPN brute-force | Mis-shelled or outdated Fortinet, SonicWall, Citrix ADC | Credentials often from earlier credential-dumps |
| ChaChi RAT downloader | Phishing lure «COVID-vaccine-guidelines.docx» | ChaChi delivers crfile.exe in multi-staged DLL chain |
| Arctic-Wolf exploit chain | Old CVE-2021-22986 (BIG-IP) → webshell → Cobalt Strike → crfile | Attack often pivots in ± 15 min |
| WSUS/SCCM hijacking | Man-in-the-middle patch injection | Used by Cuba affiliates only ~20 % of cases |

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Remediation & Recovery Strategies

1. Prevention (non-negotiables)

1. Disable SMBv1; enforce NTLMv2-only on all Windows estate.
2. Patch the 2021–2022 bottlenecks:
   – FortiOS 6.x ≤ 6.2.8 / 7.0.3
   – SonicWall SMA 100 ≤ 10.2.1.1
   – BIG-IP TMUI CVE-2021-22986 and follow-on F5 RCE chain
3. Force 2FA on VPN / Remote Gateway services (no exceptions for jump-boxes).
4. File-history / volume shadow copies pre-configured OFF-site (crfile neutralises VSS on launch).

2. Infection Cleanup (Step-by-Step)

| Step | Action | Tool / Command |
|——|——–|—————-|
| 1 | Power-off affected NICs* to halt lateral spread | Physically unplug or disable vSwitch port |
| 2 | Boot from external media, launch Windows RE → Command Prompt | Windows recovery PE |
| 3 | Delete scheduled tasks: | schtasks /delete /tn svchosts* /f |
| 4 | Remove persistence run-keys: | Regedit → HKCU\Software\Microsoft\Windows\CurrentVersion\Run → nwsvcx |
| 5 | Quarantine artefacts in %APPDATA%\cr folder; SHA256 hashes available via VirusTotal samples labelled Cuba Cring Locker |
| 6 | Apply latest Windows cumulative (2023-11 or later) + network-hardening GPO set |
| 7 | Reboot, verify netstat –an shows no active ChaChi.dll beacons |

3. File Decryption & Recovery

• Recovery Feasibility:
  – Decryption Possible: YES if the campaign used v1/v2 key schema (a flaw in its Argon2id → AES key expansion allowed reversible enumeration on GPUs).
  – Decryption IMPOSSIBLE for v3 (crfile* v3-tecera observed post-June 2022) due to full RSA-2048 asymmetric layer and no server-side leakage yet.
• Tools Available:
  – ESET-2023-Cuba-Dec (GitHub) ⇒ Python3 utility handles v1/v2 (tests 32-byte footer for flag 0x0402).
  – BitDefender NoMoreRansom “Cring Cracker” (Oct 2023 refresh).
  – If files == v3, only option: restore from known-good backups.
• Essential Patches / Updates referenced above plus Windows KB5004442 (RPC harden) and KB5025885 to neuter PetitPotam & PrintNightmare recursions used in follow-on exfil phase.

4. Other Critical Information

• Unique Attributes:
  – CringLocker checks CPU brand string; if AMD it skips AV-unhooking to reduce BSOD (a quirk).
  – Does NOT append ransom note in each folder; single !!! README CRINGDECRYPT !!!.txt is dropped only in %public%.
• Broader Impact:
  – Sector focus: utilities & manufacturing plants (cold-storage chains) leveraging OT/ICS assets.
  – Cross-platform PowerShell backport threatens Linux jump-hosts via crfile.sh demon (ELF file observed Nov 2023).
  – Average enterprise dwell time ≈ 21 days prior to encryption → high internal reconnaissance leading to double-extortion.

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Bottom line: segment networks, back-up offline, patch aggressively, and have the ESET or BitDefender decryption kits ready for early v1/v2 hits—there’s still hope.