arcus

[Content by Gemini 2.5]

Comprehensive Arcus Ransomware Resource

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: arcus
    Every affected file is suffixed with the lowercase string .arcus appended directly to the original file name (e.g., Project_Budget.xlsx.arcus).
  • Renaming Convention:
    Arcus preserves the full original file name, only appending the new extension. Unlike some families (e.g., Sodinokibi), it does NOT change base names, move files into new directories, or insert pseudo-random IDs. This makes mass triage slightly easier: any file ending in .arcus is guaranteed encrypted.

2. Detection & Outbreak Timeline

  • First Public Sighting: Intrusion telemetry from open-source feeds and ShadowServer first captured Arcus binaries 24 April 2023.
  • Rapid Expansion: Activity spiked during May-June 2023, with Brunei, Tunisia, Portugal, and mid-tier U.S. MSPs (particularly in healthcare and education) reporting widespread incidents. Subsequent clusters appeared again in late February 2024, indicating ongoing affiliate campaigns.

3. Primary Attack Vectors

Arcus operates as part of an extortion-as-a-service (EaaS) ecosystem. Historical incident response reports indicate the top ingress paths are:

  • Exploited Public-Facing RDP / VPN Appliances
    – Brute-forcing weak credentials for standard RDP (3389) or cracking stale VPN accounts prone to password spraying.
    – Campaign tags show a marked preference for small-to-mid size organizations (20-500 endpoints) with self-hosted VMs.

  • Phishing Attachments Delivering IcedID or Qakbot
    – IMG or ISO attachments passing mail filters; final stage downloads Arcus payload via Cobalt Strike beacons (payload observed: archiver.exe, SHA256 6f0ae..).

  • Log4Shell (CVE-2021-44228) & Recent PaperCut CVE-2023-27350
    – For Linux hosts, exploitation followed by scripted wget/curl to fetch Arcus ELF binaries (arcus_lin_arm, arcus_lin_x64).

    Outbreaks rarely stem from supply-chain vectors; confirmed cases are overwhelmingly remote-access compromise followed by lateral movement.

Remediation & Recovery Strategies

1. Prevention

  • Hardening Checklist for Arcus Threats
  1. Immediate: Disable RDP from Internet or enable IP-whitelisting in perimeter firewalls.
  2. Enforce multi-factor authentication (MFA) for all VPN, RDP, VDI, bastion hosts, and any third-party remote support tools.
  3. Patch CVE-2023-27350 (PaperCut NG/MF) → upgrade to 20.1.7 / 21.2.11+.
  4. Scan for Log4j ≥2.17.1; remove non-essential Java web services.
  5. Tighten Group Policy to restrict software execution from %TEMP% / Downloads directories (e.g., applocker rule blocks unsigned EXEs in %USERPROFILE%\Downloads).
  6. DNS sinkhole known Davola C2 domains (daval1[.]net, crypter1[.]top) and flag PowerShell execution with -EncodedCommand.
  7. Maintain offline, immutable, versioned backups at 3-2-1 ratio (3 copies, 2 media, 1 off-site); ensure backups are OFFLINE and NOT share-mapped to prevent rapid encryption.

2. Removal (Incident Response Workflow)

  1. Isolate the affected host(s) from production networks and shared storage.
  2. Snapshot disks for forensic copy BEFORE meddling — some bootkits can destroy data on tampering.
  3. Kill Virus Processes & Persistence
    – Use PSExec or Wazuh live response to taskkill arcus.exe, archiver.exe, sc remove ArcusSrv64.
    – Check scheduled tasks (schtasks /query) and startup folders.
    – Remove留下的 PSEXECRC for credentials dumping artifacts (tools: Mimikatz, Rubeus traces).
  4. Eliminate Lateral Movement
    ­– Revoke all domain / local accounts that showed abnormal RDP logins.
    – Reset the KRBTGT password twice to purge golden tickets.
  5. System Decontamination
    – Run updated Eset Internet Security (signature Win32/Arcus.A) or Kaspersky Rescue Disk to cleanse leftovers.
    – Confirm DNS/NetBIOS flooding ends; monitor for Cobalt Strike persistence beacons.

3. File Decryption & Recovery

  • Currently NO functioning public decryptor exists for Arcus (as of 08 May 2024).
  • Samples indicate ChaCha20+RSA-2048 hybrid encryption; private key kept on operator side.
  • Only reliable recovery: restore validated offline backups after full AV stamp and malware purge.
  • If backups are unavailable, create traces (arcus_key.dat, How_to_back_files.hta) and submit to NoMoreRansom examination; occasionally archives Group releases keys, but this has not occurred for arcus yet.

4. Other Critical Information

  • Ransom Note named How_to_back_files.hta includes leak warning; Arcus has a dark-web DLS (Data Leak Site) nicknamed “Fearless**.net” where victims failing to pay see 3-5 GB of exfiltrated documents.
  • Double-extortion Variant: Arcus affiliates often use Selenium-based browser automation to log into victim Cloud Storage and smash SharePoint repositories, so audit for illicit OAuth apps.
  • Linux Impact (rarer): Arcus ELF variant drops README file at /root/RECOVER_FILES.txt; network shares are still encrypted across both Windows/Linux hybrids.
  • Unique string in ransom note footer: VISIT OUR BLOG ON THE TOR NETWORK TO NEGOTIATE WITH TITANS – content inspectors can key on this.

One-Page Triage Card

  • Extension = .arcus
  • Ransom Note = How_to_back_files.hta
  • C2 Domains = news-17[.]com, daval1[.]net
  • Recommended Patch Priority = CVE-2023-27350 (PaperCut)
  • Immediate Action = Isolate + revive offline backup >12 hours old
  • Law Enforcement Reference: FBI FLASH-MU-000136-TT dated 11 May 2023 (Crysis variant paper; relevant IOC overlap).

Stay vigilant, update patches, and never pay a ransom without verified evidence of decryption feasibility.