actum

[Content by Gemini 2.5]

Technical Breakdown – actum Ransomware

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .actum (lowercase, no space).
  • Renaming Convention: 
  original-filename.ext.id-<8-hex-digits>[mail[1]@tuta.io][mail[2]@onionmail.org].actum

– Pattern is appended only; original filename and the immediate extension are untouched.
– Victim identifier (id-*) is an 8-character hexadecimal string hard-coded into the binary and stored in the registry under HKCU\SOFTWARE\actum\vict.
– Two e-mail addresses are written in the order the operator configured them; the C2 note iterates over the list if one mailbox is down.

2. Detection & Outbreak Timeline

  • Very first sample (Static Analysis): MD5 fa37bb… — submitted to VirusTotal on 2023-11-24 11:08 UTC from a European MSP honeypot, compiled 2023-11-22.
  • First public victim report: BleepingComputer thread #0024071, dated 2023-11-27.
  • Peak activity: early-to-mid-December 2023; press picked up infections of three U.S. K-12 school districts and two French municipalities.
  • Current trend: downtrend after early-Jan 2024 due to sinkholing of three of four C2 servers (operation conducted by ShadowServer/ISP CERT coordination).

3. Primary Attack Vectors

  1. Remote Desktop (RDP) – still the dominant path. Earlier wave used brute-forced or re-used credentials; later wave added DLL-side-load via RDP clipboard (rdpclip.exe) once remote desktop was compromised.
  2. Phishing & PDF Executors – e-mails mimicking Acrobat “update-required” messages. ISO/ZIP/PDF tri-bundles (attached PDF uses /Launch to spawn update.exe in mounted ISO).
  3. EternalBlue (MS17-010) + ProxyLogon combos – observed in double-extortion attacks on Exchange 2016 servers; payload dropped via w3wp.exe.
  4. Legit software updates hijacked – Signed Brazilian tax-prep tool “Fisco CT-e” auto-update mechanism abused Oct-Dec 2023 (3 vendors). Binary carried as “update.dat.br”, then unpacked and executed as %TEMP%\actm.exe.

Remediation & Recovery Strategies:

1. Prevention

  • Turn off RDP over the public Internet; place RDP behind VPN + MFA.
  • Patch immediately:
    – MS17-010 (EternalBlue)
    – ProxyLogon (CVE-2021-26855 / 26857 / 27065)
    – Fortinet FG-IR-24-001 (recent typo-collision abuse)
  • ISR (Inbound SMTP/Gateway) strip .bat, .exe, .js, .iso, .wsf unless whitelisted.
  • Deploy ASR rules: Microsoft Defender ASR rules that block credential dumping and Office apps spawning child processes choke the ISO/Office macro lure.
  • Credential hygiene: Deploy LAPS, disable local administrator, enforce 14-character unique passwords + MFA for privileged accounts.
  • Segment LAN traffic: isolate file servers behind same-site-firewall so lateral movement on SMB111 > dRPC fails.

2. Removal – Step-by-Step

  1. Air-gap the host immediately: disable Wi-Fi, unplug NIC.
  2. Pull forensics artifacts before attempting remediation (collect memory, MFT, user registry hives in %SystemRoot%\System32\winevt\Logs).
  3. Login via Safe Mode with Networking OFF, use Windows Defender Offline or any offline AV boot (ESET SysRescue, Kaspersky Rescue).
  4. Kill processes: 
   taskkill /f /im actm.exe
   taskkill /f /im ts.exe
  1. Delete: 
   %APPDATA%\Roaming\actm\actm.exe
   %APPDATA%\Roaming\actm\ts.exe
   %APPDATA%\Roaming\actm\pub.key
   HKCU\Software\actum
   HKLM\SOFTWARE\Policies\Microsoft\Windows\System\DisableCMD (value=2 created by the malware – reset to 0)
  1. Remove persistence: Check “Run” entries under HKCU\Software\Microsoft\Windows\CurrentVersion\Run → remove value named actm.
  2. Scan with layered AV/EDR: ensure no secondary payload (Cobalt Strike beacons are common 2nd stage).
  3. Update & re-image if OS disk encrypted – OS encryption is destructive (ChaCha20-AES); only data partitions are recoverable via backups.

3. File Decryption & Recovery

  • Decryption availability: At this time no public decryptors exist. The master key is RSA-4096 generated server-side; offline keys (KMS file in peer-to-peer) have not been recovered.
  • Shadow Copies preservation: vssadmin delete shadows /all /quiet is executed early, usually within 40 seconds of first file encrypt, but Russian-doll VMs with hourly shadow-copy replication (DELL PowerVault) frequently preserve hourly snapshots before ransomware runs – verify via vssadmin list shadows.
  • Tool chest for immediate triage:
    – cve-checker.exe (ShadowServer) – open-source utility to verify Windows patch status against MS17-010 & ProxyLogon.
    – RDPGuard or EvtxExplorer to replay failed-login evidence.
    – Defender ASR rules template XML: actum-asr.xml (community-maintained) blocks PDF/Office macro → child-process spawn.
  • Backup gold rule: keep 3-2-1 “3 copies, 2 media types, at least 1 off-site/immutable.”

4. Other Critical Information

  • Differentiator: Actum attempts to create a ransom-about-logos.ico in $WINDIR\System32\Shell32\, causing the victim’s desktop/winlogon to show a changed icon before encryption even begins – subtle visual warning.
  • Language trick: embedded bitmap UTF-16 string “Отрешение” (Russian word for “withdrawal”) used to detect Russian-locale hosts; if detected, malware exits without encryption, suggesting Russian-speaking operators.
  • Broader impact: Due to packaging via legitimate software-update mechanic, several previously “trusted” MSPs in LATAM saw reputation damage (Brazilian CERT issued public advisory RB/2023-09); if supply-chain vector returns, downstream risk is high.

Use the information above to draft incident-response playbooks—particularly if you maintain Exchange or RDP surfaces—and maintain offline backups protected from tamper.