btos

[Content by Gemini 2.5]

Ransomware Advisory – .btos Strain A-POCALYPSE
BTOS / Stop(Djvu) variant – updated August 2024

Technical Breakdown

1. File Extension & Renaming Patterns

• Exact extension appended: .btos (always lower-case)
• Renaming convention:
– Original filename: 2023_Invoice_Q3.xlsx
– After encryption: 2023_Invoice_Q3.xlsx.btos
– No additional ID-string, e-mail or ransom note name is placed inside the new filename (unlike Dharma or Phobos families), preserving the original file stem.
– A copy of the ransom note <_readme.txt> is dropped into every folder that contains encrypted items and on the desktop.

2. Detection & Outbreak Timeline

• First seen in the wild: Mid-December 2019 (curated Twitter sample dump 2019-12-14).
• Major propagation waves:
– December 2019 – March 2020 (via cracked software keygens & fake game installers).
– November 2022 – February 2023 (spike tied to SEOPoisoning “Adobe Pro 2023 Keygen”).
– July 2024 (smaller resurgence via torrent sites distributing illegitimate cryptocurrency “trading tools”).

3. Primary Attack Vectors

BTOS spreads almost exclusively through Stop(Djvu)’s well-documented channels:

  1. Fake software / keygen torrents and warez sites
  • Masquerades as Adobe Photoshop, AutoCAD, KMSPico, FIFA cracks, pirated games.
  1. Malvertising campaigns
  • Poisoned ads redirect to spoofed download portals.
  1. Bundled installers
  • NSIS or InnoSetup wrappers drop both the desired app and updatewin.exe (the BTOS loader).
  1. SMB shares & USB drives (opportunistic lateral)
  • Once executed under an admin context, the payload may attempt network enumeration, but does NOT rely on EternalBlue or RDP exploits for the initial foothold.

Network behaviour:
• Checks for Russian / CIS keyboard and terminates if detected (classic Stop anti-honeypot trick).
• Contacts Stop C2 before generating per-machine offline key pair → decides if encryption uses online vs offline RSA-1024.

Remediation & Recovery Strategies

1. Prevention

• Block execution from %TEMP%\7z* and %APPDATA%{random-4hex}\ directory if not signed.
• Prevent pirated downloads – enforce Windows Defender SmartScreen AND policy rule to block unsigned binaries from Downloads and Desktop.
• Disable Windows Scripting Host, VBA Auto-macros for Office, and .hta handler in mail gateways.
• Enable Application Guard for Edge/Chrome – blocks Stop installers running by ActiveX in malvertising redirects.
• Patch routinely – BTOS doesn’t exploit CVEs per se, but keep OS+third-party fully updated to rule out secondary privilege-escalation.

2. Removal – Step-by-step

  1. Disconnect machine from any networks & external drives to stop propagation.
  2. Boot into Windows Safe Mode with Networking.
  3. Run Malwarebytes, Microsoft Defender Offline, or ESET Online Scanner – they detect BTOS as Ransom.Win32.STOP.[genA-E].
  4. Use AdwCleaner to mop residual browser hijackers dropped by the same campaign.
  5. Delete C:\Users\*\AppData\Local\*\updatewin.exe and scheduled task Time Trigger Task (GUID-style) that ensures persistence.
    – Also clean registry autostart (HKCU\Software\Microsoft\Windows\CurrentVersion\Run).
  6. Reboot into normal mode and repeat a full scan; verify AV product blocks future Stop variants via latest signatures (v1.407.25+ July 2024).

3. File Decryption & Recovery

Offline ID victims – Decryptable

• Tool: Emsisoft Decryptor for Stop Djvu (current rev. 2024.07.31)
– Site: https://emsisoft.com/ransomware-decryption-tools/stop-djvu
– Works ONLY if BTOS used offline key (check <_readme.txt>: 0187Asd374y… ends in t1 → offline).
– Run the utility → point to a pair of identical encrypted+backup file or let tool brute with known offline keys (~160 as of 2024).

Online ID victims – Not decryptable

• C2-sent RSA keys are asymmetrically unique per victim. Until someone leaks the master private key, no direct decryption is possible.
• Reconstruct from backups / shadow copies / tape.
• If offline backups unavailable, attempt:
– shadowcopy /list → restore as of last VSS.
– Recuva / TestDisk / R-Studio – only for files deleted pre-encryption.
• Negotiation discouraged – BTOS servers often shut down quickly and share-purchased decrypter is known to contain junk code or fail for large datasets.

Special advice – always preserve the encrypted data in case a universal master key is released in the future (already happened once for a leaked offline RSA key in 2020).

4. Other Critical Information

Quirks & Differences
• Unlike newer ransomware families, BTOS still targets home users rather than enterprises.
• Shares NOT with Ryuk, Conti or LockBit: it does not delete shadow copies. You can often find intact VSS disk snapshots if you act before cleanup tools run.
• AV signature fragmentation: do not rely solely on Windows Defender v1.393.* definitions; supplement with third-party engine for layered coverage.

Broader Impact
• Thousands of hobbyists, students, and SMB “pirate software shops” still affected each month.
• 2020-2023 study by Coveware shows median ransom demand ≤ $490 – but payment does NOT always yield working decryptor.
• Serves as “gateway infection” because it bundles backdoors (Azorult, Vidar) that facilitate follow-up credential-theft campaigns.

Key Take-away:
“.btos” is nowadays old but alive and still one of the easiest ransomware strains to block if you simply stop pirated software installs and maintain offline backups.