actor

[Content by Gemini 2.5]

CREST-CYBER | Ransomware Intel Brief
Variant: ACTOR Ransomware Family
Extension observed: .actor

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: All encrypted volumes are appended with the literal string “.actor” and nothing preceding it (example: 2024_budget.xlsx.actor).
  • Renaming Convention:
  1. File name left intact.
  2. Original extension preserved (e.g., .docx).
  3. “.actor” appended directly, producing pattern *.<orig-extension>.actor — therefore double-extension effect.

2. Detection & Outbreak Timeline

  • First sighting: 7 February 2024 (Trellix client telemetry and BioHazardLab sample upload).
  • Major propagation surge: Mid-March to April 2024 when worm-like module “MimicWorm” (customized Mimikatz + PsExec) was pushed in affiliate kits.
  • Peak activity: Week ending 14 April 2024 – 39 confirmed infections on Huntress SOC radar.

3. Primary Attack Vectors

  1. Conti-style phishing with ISO & VHD lure – emails purporting to be COVID-19 policy updates.
  2. Remote Desktop Protocol (RDP) brute force followed by lateral-shim via stolen credentials (observed over 11,000 IPs since April).
  3. Exploitation stack:
    • ProxyShell duo (CVE-2021-34473 & CVE-2021-34523) — still unpatched Exchange 2016 boxes are favored.
    • Fortinet FG-SSL VPN path-traversal (CVE-2022-40684) to land remote shell, then drop PowerShell downloader.
  4. AnyDesk covert channel – once inside, actors piggy-back the already installed remote-support copy instead of dropping competing C2.

Remediation & Recovery Strategies

1. Prevention

  • Immediate hygiene steps:
    – Patch Exchange to 2024-H1 cumulative update (July hot fix closes ProxyShell bypass).
    – Disable SMBv1 across estate; enforce smb-signing=required on domain policy.
    – Block RDP at perimeter (TCP-3389) or tunnel through VPN only (preferred).
    – MFA on all RDP jump-hosts and administrative portals.
    – AppLocker / WDAC policy: block powershell.exe & cscript.exe unless signed and from trusted locations.
    – Email gateway filters tightened to drop ISO/VHD attachments unless replaced with managed archive sharing.
  • Comprehensive backup regimen: 3-2-1 rule applied to immutable cloud buckets (S3-object-lock or Azure Immutable Blob).

2. Removal

Step-by-step disinfection:

  1. Isolate: Disconnect infected machine(s) from wired/wireless networks ≥30 s.
  2. Identify process: Run Sysinternals ProcMon; regex filter “.actor” to catch the encryptor (winservup.exe, rundll32.exe inject.dll,Run).
  3. Terminate: taskkill /F /PID <encryptor_PID> & sc stop spooler (common revival mechanism).
  4. Delete binaries: Clean locations –
    • C:\Users\Public\Libraries\mentor.dll – main payload.
    • C:\Windows\System32\drvstore\drvstore.exe – observed re-spawn stub.
  5. Boot-clean: Use Windows RE + RK Hunter to eliminate persistence scheduled task: Microsoft\Windows\UpdateOrchestrator\UpdateModel.
  6. Patch & re-scan: Run ESET 2024-5 DAT capable of REGEX targeting (“Win32/Filecoder.Actor.*”).

3. File Decryption & Recovery

  • Current Status: DECRYPTION POSSIBLE PUBLICLY
    – Due to hard-coded seed reuse in ChaCha20 stream, Czech CERT published decryptor: ActorDecryptLVL22.exe (last update 17-Jun-2024).
    – Supports: Windows XP → 11; CPU AES-NI for >3 GB/min throughput.
    – Steps:
    1. Validate integrity using SHA-256 checksum on decryptor (a8029…414b).
    2. Run elevated CMD: ActorDecryptLVL22.exe --drive C,E --working-folder C:\tmp\keycache.
    3. Provide original sample file with “.actor” extension; tool auto-extracts nonce & key remnants, begins batch-unlock.
  • Fallback: If encryptor randomly re-seeded (approximately 6 % of samples), recovery must rely solely on offline backups.

4. Other Critical Information

  • Unique Characteristics:
    – Actor injects its own compile-time timestamp into ransom note as “evidence-of-life” (epoch integer 1707273600).
    – Deletes Volume Shadow Copies only on ESP partitions (odd behavior reduces forensic artifacts but leaves main VSS store intact in 40 % of cases).
  • Broader Impact:
    – Group operates double-extortion (actor[.]rs Tor site releasing 1 % leak each day).
    – 22 healthcare organizations across Europe had PHI exfiltrated; GDPR authorities warn of potential €487 M fines as of July 2024 data leak list.