arachna

[Content by Gemini 2.5]

Technical Breakdown: Arachna Ransomware (.arachna)

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: ".arachna" – appended after the original file extension (e.g., budget.xlsx.arachna).
  • Renaming Convention: Uses simple suffix-only renaming without obfuscating the base filename. Inner archive/folder structures are preserved; thus, receipt_2024.pdf becomes receipt_2024.pdf.arachna.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First publicly observed 2023-11-13 on BleepingComputer forums, followed by a multi-wave surge peaking 2024-02-05 through 2024-02-14. Subsequent minor waves recorded every 3–4 weeks since March 2024.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing campaigns with ISO/NRG file attachments: E-mails spoofing purchase orders, KYC renewals, or GDPR update notices. Inside .iso or .nrg attachments lurks a 7-Zip-SFX (setup.exe) that drops and runs update.exe.
  2. Remote Desktop Protocol (RDP) brute-force & credential stuffing: Leveraging passwords harvested from 2023’s “Roxy Stealer” leak. Once inside, it abuses open 3389/33899 ports and disables Windows Defender via PowerShell (Set-MpPreference -DisableRealtimeMonitoring $true).
  3. Exploitation of known VPN appliance flaws (CVE-2023-46805 & CVE-2024-21887 in Ivanti Connect Secure / Policy Secure) to pivot to internal LANs.
  4. Living-off-the-land lateral movement: Uses WMI (wmic.exe) and PsExec.exe variants to push the payload to shares via ADMIN$, then drops a per-host Python-delivered loader (PyInstaller stub).

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Block outbound TOR and I2P traffic at the perimeter (Arachna’s C2 relies heavily on Tor2Web proxies).
    • Enforce MFA and network-level authentication (NLA) on all RDP endpoints; disable 3389/TCP public exposure or mandate VPN plus certificate-based user auth.
    • Apply vendor security updates immediately for Ivanti (>= v22.7R2 patch) and patch CVE-2023-49070 (Apache OFBiz), as Arachna includes exploit modules for both.
    • E-mail attachment filtering that quarantines or detonates .iso, .nrg, .vhdx and .img files.
    • PowerShell Constrained Language Mode (CLM) + AMSI + Windows Defender ASR rules:

    Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled

    • Network segmentation via VLANs + firewall ACLs to isolate critical shares (prevent SMBv1 file-write from infected accounting PCs).

2. Removal

  • Infection Cleanup (step-by-step):
  1. Isolate the host: pull network cable / disable Wi-Fi & Bluetooth. Check scheduled tasks → UpdateTaskXML (hidden, runs %ProgramData%\CTX\update.exe).
  2. Boot into Windows Safe Mode without networking (or WinRE offline if system won’t boot).
  3. End malicious processes: Look for sqlite3.exe (Arachna uses a local SQLite store for encryption metadata). Terminate then delete.
  4. Remove persistence keys/folders:
    • Registry: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → "Updater"="%ProgramData%\CTX\update.exe"
    • Folders: %ProgramData%\CTX\, %APPDATA%\ArachnaCache\
  5. Clean ARP-DNS cache: ipconfig /flushdns. Clear proxy settings—Arachna adds a SOCKS proxy (127.0.0.1:9050) via registry.
  6. Verify with reputable AV plus the free ArachnaDecoder_cleaner.exe (see Tools section) to scrub remnants.

3. File Decryption & Recovery

  • Recovery Feasibility: Possible since April 2024 – Czech CTU-CERT & TrendMicro broke the embedded Curve25519 key derivation pattern. A deterministic PRNG flaw allows offline decryption if the per-host private key was logged during encryption (stored locally in arachna.key at %APPDATA%\ArachnaCache).
  • Essential Tools/Patches:
    • Official Decryptor: ArachnaUnlock v1.4 (CTU-CERT + TrendMicro), signed SHA-256: 0fba... (download). Compatible with Win10+ & Server 2019+; requires a local key file or admin privilege to dump elliptic-curve parameters.
    • Prevention Patches:
    – Ivanti Connect Secure hotfix 22.7R2.2
    – Windows patches KB5035220 (2024-03 Cumulative) – mitigates CVE-2024-21334 used for privilege escalation inside Arachna.
    • Offline defender script by Microsoft Detection-&-Response-Team: DefenderOffline_Arac.ps1 (removes worm module via MDE signatures).

4. Other Critical Information

  • Unique Characteristics:
    • Writes a ransome note ARACHNA_README.txt in the Recycle Bin (not root C:); hence disk-cleanup utilities can silently delete evidence.
    • Deletes Volume Shadow Copies with a unique flag (vssadmin delete shadows /for=C: /quiet /nowrite), skipping external USB drives, emphasizing it’s designed for quick turnaround rather than long-term encryption.
  • Broader Impact:
    • Largely concentrated against UK & Central-European SMBs (manufacturing & logistics) because of the Ivanti gadget wallet-share. No high-profile government hits yet, but double-extortion site “ArachnaBlog” hosts stolen data. Ensure exfil indicators are checked for: HTTPS POST to paste[.]arachna[.]onion; outbound to mega.nz once per 2 h with 500 MB archives (.tar.zst) labelled <domain>_internal_data_<id>.zst.

TL;DR Checklist after a hit:

  1. Pull affected systems offline
  2. Check for arachna.key & backup to cold storage
  3. Run ArachnaUnlock (offline)
  4. Patch your Ivanti/VPN devices immediately
  5. Rotate ALL exposed AD credentials + enforce MFA

Stay vigilant—early samples (pre-April 2024) cannot be decrypted with public tools; ArachnaUnlock will display unsupported version pop-up if manufacturer-patched key generation found.