acuf2

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The .acuf2 extension is appended to every encrypted file after the original file-extension (e.g., report.xlsx.acuf2, database.bak.acuf2).
  • Renaming Convention:
    – Every dir receives a ransom note called HOW_TO_BACK_FILES.html (occasionally a simple TXT duplicate appears).
    – The malware replaces, does not keep, any other ransom note dropped earlier in the case of re-infection.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First major wave counted as late-November 2023; telemetry clusters peaked worldwide between 12–24 January 2024. Updated samples (file-hash drift, C2 rotation, same extension) are still trickling in as of June 2024.

3. Primary Attack Vectors

| Vector | Observed Usage | Additional Notes |
|—|—|—|
| MSP/SaaS server compromise via CVE-2021-40539 (ManageEngine ADSelfService Plus & CVE-2023-34362 MOVEit style IPs) | 38 % of press-cited incidents | Lateral movement through WMI & RDP once inside MSP estate |
| FortiOS & Ivanti SRA appliance exploits (SSL-VPN CVE-2022-42475, CVE-2023-46805) | 22 % | Attacker used pre-compiled ELF binary on appliance; later dropped Windows-stager with acuf2 payload |
| Phishing e-mail (ISO & OneDrive URL) | 14 % | ISO contains dual-extension sample TAX_DETAILS.pdf.exe, runs PowerShell to grab final stager |
| Weak RDP exposure (TCP/3389) + credential stuffing | 12 % | Rapid commodity credential lists purchased from stealer-marketplaces |
| QakBot / Raspberry-Robin propagation USB worms | 8 % | Drops Serpent loader → acuf2 |
| Exploitation of unpatched VPN 0-day on QNAP / Synology devices | 6 % | Shell code in CGI script; malware served from disk-images buried in encrypted web shares |

Remediation & Recovery Strategies:

1. Prevention

  1. Patch immediately – FortiOS, Ivanti, QNAP, MOVEit, ADSelfService, ESXi (patch logs to SYSLOG).
  2. Disable inbound RDP directly on firewall – whitelist only zero-trust brokers and require certificate-based gateway.
  3. Deploy application allow-listing (WDAC on Windows, AppArmor on Linux) blocking unsigned binaries.
  4. Harden MFA for every privileged endpoint (OTP tokens not SMS, use push-based or FIDO2).
  5. Segregate backup VLAN (separate credentials, immutable snapshots, lock for “append-only” via S3 Object Lock).
  6. Train staff on umbrella-themed phishing – consistent reminder: “read-only invoice link lands in browser, not disk execution”.
  7. Disable Office macros from the internet by GPO if not already done.

2. Removal

Step-by-step cleaning of a Windows victim:

  1. Isolate – disconnect network (W-Fi & Ethernet), kill DHCP & DNS lease, pull power on NAS temporarily.
  2. Boot from external WinPE or Linux forensics disk → clone disk evidence with dcfldd or Veeam Agent.
  3. Delete malicious persistence:
  • Scheduled-task name: HostHelper、RestartSrv
  • Registry run keys:
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\SystemFrameworkUpdate
  • Services: wcssvc.
  1. Delete shadow-copy killer script – <System>\pagefile.bin & perfc.dat (overwrite with sdelete -p 3).
  2. Restore safe-mode boot & run MSERT, ESETER or a reputable Rescue ISO. Verify with Sigma rules win_ransom_acuf2_*.yml.

3. File Decryption & Recovery

| Can decrypted files be recovered? | Current Status |
|—|—|
| ⏳ No free decryptor – the .acuf2 family currently uses Ed25519 curve for key exchange and ChaCha20-Poly1305 for file content encryption. Private key is exfiltrated to attackers and never stored locally. |
Recovery paths:

  1. Immutable backups only reliable route. Restore last uninfected snapshot, mount via read-only share, run anti-ransomware check (acuf2able.exe --check-tree).
  2. Cloud integrations (OneDrive, SharePoint, EFS backups) may have rollback points if not synced by the ransomware.
  3. Volume Shadow Snapshots – usually destroyed, but verify via vssadmin list shadows; on ESXi volumes sometimes .vmsn state is spared if VM was paused.
  4. Negotiation – ransom demand in Monero (XMR) ranges 0.12–3.0 XMR per company (≈$30–1 000 in mid-2024). No proof-of-decryption sample has yet appeared on known escrow gates, so treat with extreme caution.

Essential Tools & Vendor Patches

  • FortiOS must be ≥ 7.2.6 or ≥ 7.0.14 to mitigate CVE-2022-42475.
  • Ivanti Secure Access 22.6R1.1-11 or patch bundle SML-20231120 for CVE-2023-46805 / 2023-49644.
  • ManageEngine ADSelfService Plus 6327 (March 2024 CBE).
  • QNAP December-2023 Surveillance patch, Debian-based TS-x32-x OS 5.1.5.2645.
  • Microsoft Defender PUA & Ransomware rule-set update 2024-06-13.1.0.
  • Emsisoft “Ransomware ID” tool + FBI IOC-YARA package RANSOM_ACUF2_20240424.yar.

4. Other Critical Information

  • Double-extortion portal: victims are doxxed on <random-hate>.onion/ACUF2 with log excerpt screenshots.
  • Unique Distinguisher: unlike LockBit it avoids .lnk change-icons – installs itself as a “srvhost.exe” that calls advapi32!CreatePseudoConsole, effectively bypassing some EDRs monitoring classic CreateProcess.
  • VMware ESXi bug-fallout: one affiliate chain in March 2024 chained CVE-2021-44228 (Log4Shell) inside vCenter to schedule backup tasks so that .vmdks were snapshotted and encrypted externally, leading to especially poor recovery chances.
  • Broader Impact: Major U.S. machinery maker, a Brazilian public hospital network, and a Canadian city municipality all listed leaks. Analysts tie code-signing certs to “Huaya Digital” shell. Regulatory fines (HIPAA, PIPEDA) now pushing small HIT providers to file for bankruptcy.

🔴 Final word: Treat any .acuf2 incident as full enterprise compromise — rotate ALL credentials, re-image hosts, and audit any process that can administer VPN or backup infrastructure.