arsium

[Content by Gemini 2.5]

Technical Breakdown (Arsium Ransomware)

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Arsium appends .arsium to every encrypted file.
    Example: QuarterlyReport.xlsx becomes QuarterlyReport.xlsx.arsium.

  • Renaming Convention:
    The malware preserves the original file name and simply appends the new extension after the final “dot”. No base-64 or hex obfuscation is used—this makes it trivial for users to identify which files are affected, but also for scripts to enumerate them during triage.

2. Detection & Outbreak Timeline

  • First Public Sightings: Late-November 2023, with a larger wave observed during December 2023 – January 2024.
  • Peak Activity Surge: Week of 7 January 2024, aligning with phishing campaigns that spoofed “2023 Year-End Tax Receipts”.
  • Status (April 2024): Low-to-moderate immediate prevalence, but IOCs are still being seen in the wild and forked builder toolkits circulate on criminal forums.

3. Primary Attack Vectors

  1. Phishing Emails (most common)
    – Subject lines: “Past-Due Invoice #”, “Bitcoin Invoice Cancellation Required”.
    – Attachment: ISO, ZIP, or 7-Zip nested archives containing heavily obfuscated .NET binary (payment.exe or scan_<date>.exe).
  2. Cracked Software & Games (“warez”)
    – Popular key-gen and game trainer executables silently drop the Arsium loader.
  3. Abuse of Malvertising on Pastebin/GitHub Links
    – Discord/Reddit messages lure users to download “CS:GO Cheat v3.exe” that ultimately pulls the payload from a Discord CDN URL.
  4. External-Facing RDP & SMB (secondary spread)
    – Once inside, Arsium spawns a PowerShell script that attempts lateral movement via PsExec using harvested local/domain creds.
    – No public exploitation of a zero-day; instead it piggy-backs on brute-forced plaintext passwords or reused credentials.

Remediation & Recovery Strategies

1. Prevention

  • Patch OS + 3rd-party software (most Arsium samples show WMI calls to Win32_Product, seeking outdated VLC, WinRAR, Foxit).
  • Disable macro execution and sideloading of unsigned .NET binaries via GPO.
  • Implement mail-gateway rules to quarantine ISO/ZIP attachments unless whitelisted.
  • Enforce least-privilege RDP — firewall rules, account lockout, RDG/NLA-only.
  • Enable Windows Defender AV’s “block at first sight” + ASR rule: Block executable content from email client / webmail.

2. Removal (Step-by-Step)

  1. Disconnect from the network (pull cable/Wi-Fi).
  2. Obtain a known-clean boot environment (e.g., Windows 10/11 recovery USB).
  3. Temporarily disable automatic startup via bcdedit /set {default} safeboot minimal to ensure the binary doesn’t re-elevate.
  4. Delete malicious artifacts (typical locations):
    C:\Users\<user>\AppData\Local\arsium.exe
    C:\ProgramData\ARSUpdater.exe
    Run: del /a:h %APPDATA%\tempupdate.bat (its persistence .bat file)
  5. Remove scheduled tasks (names contain “sysloglcl” or “AudioServUpdate”).
  6. Scan with updated AV/EDR; look for SHA-256 IOC: 5f1ea8d57033d7c91e63f9ec3875c79c4e6e0daf980fed16d4b82ce63b048585 (middle-stage loader).
  7. Reboot into normal mode; verify no Arsium residual processes via Process Explorer / Sysmon logs.

3. File Decryption & Recovery

  • Decryption Feasibility: As of April 2024, files are not decryptable without the attacker’s RSA-2048 private key (offline encryption mode; keys unique per victim).
  • Free Decryptors: None yet released by law-enforcement or security vendors.
  • Recovery Avenues:
    – Restore from pristine offline or immutable backups (object-lock, air-gapped).
    – Shadow-copy / Windows VSS: Arsium deletes these with vssadmin delete shadows /all; however, recovery may still succeed on non-admin shares or when the deletion script throws an error.
    – Rebuild affected machines; do not pay the ransom.
  • Essential Patches/Tools:
    – Microsoft Defender Antivirus version 1.397.786 or later (detected as Ransom:MSIL/Arsium.A).
    – Enable AppLocker or WDAC to block unsigned binaries under %LOCALAPPDATA%.
    – Deploy Windows 10/11 KB5034123 cumulative patch (doesn’t affect Arsium directly but strengthens ASR).

4. Other Critical Information

  • Builder Forks: Criminal forums distribute a builder named “Arsium Crypter v1.3” that customizes file extension, ransom note name, and mutex. So far only .arsium has been seen in wild, but DIY operators can pick an arbitrary extension.
  • Ransom Note Details: Creates RESTORE_FILES_INFO.hta on desktop + every encrypted directory. Bitcoin address bc1q… same across samples → good pivot for tracking via blockchain analysis.
  • Broader Impact / Notable Victims:
    – December 2024 hit several regional hospitals in Eastern Europe; remediation delayed because the variant also installs a clipboard stealer that harvests internal credential repositories.
    – Forked strains attempt PowerShell download-string of Cobalt Strike, escalating to double-extortion (exfiltration before encryption).

Bottom line: Arsium spreads mainly via phishing and cracks, encrypts with RSA-2048 leaving .arsium filenames in plain sight, and currently lacks a free decryptor. Maintain up-to-date backups, harden RDP and macros, and treat every unexpected archive with suspicion.