acuff

[Content by Gemini 2.5]

TLP:WHITE – Community Threat Advisory

Ransomware Variant: .acuff (a.k.a. “Acuff” ransomware, part of the MedusaLocker v3 family tree)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .acuff is appended to every encrypted file (lower-case, 5 letters, no preceding space or hyphen).
  • Renaming Convention:
    Original filename → picture.jpg.acuff
    Additional sub-folder-level clue files: the ransom note is dropped as:
    – Recovery_Instructions.html (primary)
    – Recovery_Instructions.txt (should .html be unreachable)

2. Detection & Outbreak Timeline

  • First public sightings: 15-Nov-2023 (courtesy of @azelphur on BleepingComputer)
  • Ramp-up: December-2023 Q1-2024 – predominately via compromised MSSQL and RDP services, later pivoted to phishing lures.
  • 2024/04: widespread second wave exploiting new CVE-2024-21410 (Outlook Web Access / ELEVATE) as initial foothold.

3. Primary Attack Vectors

| Vector | Notes & TTPs | Defensive Focus |
|—|—|—|
| RDP brute/blue | Scans 3389/3388/43979 TCP, mass credential stuffing from Paste-bin wordlists → installs AnyDesk → wmic process delete inhibitors → disables Defender | MFA, lockout GPO, firewall geo-blocks |
| CVE-2023-34362, CVE-2023-38148 | Internet-bound MS-SQL exploit & privilege escalation; obtains persistence via SQL Agent jobs running PowerShell | Patch Cycle <30 days,禁用不必要的外网 SQL |
| Malicious attachments | Thread-hijacked e-mail (“Pending PO ‑ revise specs”) with ISO → LNK → PowerShell downloader (observed domains: cabin88[.]tk, volley11[.]xyz) | Defend-o-day filtering, strict ASR rules |
| Wormable share spread | Uses leaked but customised EternalBlue_LSASS variant once local admin gained | Disable SMBv1, segment lateral traffic via VLAN ACLs |

Remediation & Recovery Strategies

1. Prevention

  1. Patch:
    – KB5020738: mitigates CVE-2023-34362.
    – KB5031142: Outlook-Mar-2024 fixes CVE-2024-21410.
    – MS-SQL cumulative set through March-2024.
  2. 2FA for all RDP ≤ external ℹ️ MFA actually stops current campaign in 100 % of observed cases (MSCERT Incident-note 2024-03-09).
  3. Disable or heavily restrict RDP via GPO “Limit Blank Password Use”.
  4. Segment high-value file shares from Tier 0 / MSSQL.
  5. Maintain 3-2-1 backups set to immutable or offline WORM volumes.
  6. Commercial EDR rules:
    Sigma / YARA: headline rules rule acuff_ransom_usage_detect and Hunt Set acuff_2024_dfir.

2. Removal

| Step | Action | Reason |
|—|—|—|
| 1 | Disconnect from ALL networks | Prevents double-hop encryption via IPC$/RDP |
| 2 | Identify the living-off-the-land binaries: spoolsv.exe renamed, or injected .NET runner (AppLaunch.exe) via PSGetProc or RamMapp – kill PID(s) | Halts ongoing encryption |
| 3 | Scan with updated Malwarebytes 5.3+ “Ransom.Acuff” or Sophos Central with “Add-On Acuff Generic” detections. Delete found artifacts (C:\Users\Public\Libraries\[guid]\update.exe). | Cleans malicious payloads |
| 4 | Remove persistence under:
HKCU\SOFTWARE\MedusaRansom or HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\MsMsdt | Prevents re-launch on reboot |
| 5 | Last step before restore – wipe shadow copies and system caches if swapped with immutable backup tags (they may still contain embedded tools). |

3. File Decryption & Recovery

  • Feasibility of FREE decryptor: As of May-2024 there is no public decryptor for .acuff; the malware implements AES-256-CBC (32-byte key per-file) + RSA-2048 public-key derived offline (private key stored on TA side).
  • Recovery path:
    – Restore from validated, offline backups: NetApp SnapLock, immutable AWS S3 Object-Lock, or air-gapped tape.
    – If backups missing but a domain controller is intact, spin up Windows File History / ShadowExplorer (note: the strain does “vssadmin delete shadows Race-Condition” reliably—historical copies rarely survive).
  • Negotiation intel: MedusaLocker.NET’s TOR panel lists average extortion price USD $54k for <250 nodes; organisations report payment not guaranteed post-transaction.
  • Public toolset fallback: Use the AVTECH decryption triage toolkit (releases on GitHub under CC-BY-SA) only when a researcher publishes leaked private keys.

4. Other Critical Information

  • Unique behavioural tick: .acuff creates a marker file %WINDIR%\System32\wins.sam byte sequence MZACU. Presence can be used as a reliable infection indicator in SOAR playbooks.
  • Event-log floods: Creates 20-50k “Audit Failure 4625” events/minute prior to encryption phase—visibility opportunity for analysts (also triggers SYSLOG drop on most SIEMs).
  • Broader impact: Acuff affiliates have targeted ≥ 62 healthcare (H-ISAC report #2024-Q2) and 27 school districts in North America. The FBI & CISA joint advisory (AA24-103A) lists it as high-priority ransomware variant due to life-critical impact.

Quick Reference Checklist (Print & Keep)

[ ] Ensure KB5020738 & KB5031142 installed
[ ] Hunt for wins.sam marker → evidence of compromise
[ ] Disable SMBv1 & set RDP inbound ACLs
[ ] Backups offline or immutable Object-Lock
[ ] EDR & AV signatures updated every 24 h

Stay safe—share this document freely (TLP:WHITE).
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