bttu

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STOP/Djvu Ransomware (.bttu variant) – Technical & Recovery Guide

Last updated: 2024-06-09

Technical Breakdown

1. File Extension & Renaming Patterns

  • Exact Extension: .bttu
  • Renaming Convention: Original name → <base filename>.<original extension>.bttu
    Example: Report_Q2_2024.xlsx becomes Report_Q2_2024.xlsx.bttu

2. Detection & Outbreak Timeline

  • First Public Sightings: Mid-September 2022 (late wave #262 of the STOP/Djvu family)
  • Peak Activity: October 2022–February 2023, with continual low-volume flare-ups ever since.
  • Geographic Spread: Initially concentrated in North-America and Western-Europe, now truly global due to cracked-software distribution channels used by the threat actors.

3. Primary Attack Vectors

Typical propagation chain:

| Vector | Details | Mitigation Reference |
|——–|———|———————-|
| Crack / Keygen sites & torrents | Dropping fake Adobe, Office, and game activators. Bundled Setup.exe writes the dropper %TEMP%\SysHelper.dll. | Block/alert user downloads from high-risk TLDs via Web-proxy policy. |
| Exploited advertising networks (“malvertising”) | Redirects users to fake update pages pushing the same payload. | Use DNS-filtering (Quad9, Cisco Umbrella, or NextDNS) |
| RDP/SSH brute-force (secondary) | Once a system is already compromised by the malware above, attackers occasionally pivot via SysAdmin tools to nearby hosts; however the initial infection never comes via RDP. | Enforce NLA, 15+ char unique passwords, lockout thresholds on 3389/tcp. |
| SMBv1 disabled deadline (no EternalBlue) – STOP/Djvu does not self-spread laterally via SMB. | — | — |

Payload characteristics (excerpt):

SHA-256: 4f92a3f7f2c0b38a... (STOP sample “helper.exe”)
Dropped files:
  %LocalAppData%\<random>\<random>.exe         – Main trojan
  %SystemDrive%\SystemID\PersonalID.txt        – Victim ID
  C:\_readme.txt                               – Ransom note
Registry persistence:
  HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run  “SysHelper” = …
Network callbacks:
  https[:]//temisaurus[.]com/statistics/get.php  C2 beacons
  In rare cases Tor mirrors: helprestore[.]onion

Remediation & Recovery Strategies

1. Prevention

  1. Application allow-listing / WDAC / AppLocker – block unsigned binaries in user-writable paths.
  2. Never install pirated software; cracked tools are still the #1 distribution channel.
  3. Disable macros, enable Office “Protected View” and warn on VBA execution.
  4. Keep Windows fully patched; install Windows Security (Defender) with Cloud-delivered protection ON.
  5. Enforce least-privilege, disable local admin rights for daily-use accounts.
  6. Offline & cloud backups (3-2-1 rule), with write-once (immutable) snapshots for at least 30 days.
  7. SMTP filtering, attachment sandboxing, and user phishing simulation campaigns.

2. Removal (Step-by-Step)

⚠️ Do not pay the ransom. There is a free decryptor (see §3). Isolate the host first.

  1. Physical or network isolation – disconnect Ethernet / Wi-Fi.
  2. Boot into Safe Mode with Networking (so the decryptor can phone home for keys).
  3. Run a reputable AV boot scan:
  • Microsoft Defender Offline
  • Malwarebytes 4.x
  • ESET SysRescue Live
    – Quarantine any files in %LocalAppData% and C:\ProgramData.
  1. Delete the scheduled task “Time Trigger Task” created by the malware.
  • schtasks /delete /tn "Time Trigger Task" /f (admin CMD)
  1. Remove registry persistence:
  • reg delete "HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /v "SysHelper" /f
  1. Reboot into normal mode. Verify the malware service is gone (sc query SysHelper should fail).

3. File Decryption & Recovery

  • Free Decryptor Availability: YES – the .bttu variant is supported by the Emsisoft STOP/Djvu decryptor (v1.0.0.31+).
    Download directly from: https://decrypter.emsisoft.com/stopdjvu
  • Pre-requisites:
  1. You need a pair of original + encrypted versions of any file >150 KB.
  2. Internet access required for key lookup (the tool queries Emsisoft’s key server).
  3. Run the tool as Administrator, point it at the drive root, and let it brute-force when necessary.
  • Chance of Success:
  • Online key (t1 prefix in PersonalID.txt): ~15 % – only known when Emsisoft has seized the criminals’ key database.
  • Offline key (t2 prefix or static ID shared by many victims): 100 % – the decryptor will download the combined key automatically.
  • If decryptor cannot find the key:
  • Keep the encrypted files.
  • Use shadow copies (vssadmin list shadows) or archival backups as fallback.
  • Occasionally re-run the decryptor – new keys are added weekly.

4. Other Critical Information

  • Unique Characteristics of .bttu:

  • Uses Salsa20 encryption for data + RSA for key wrapping; each file has its own 256-bit Salsa key encrypted by the victim’s unique RSA public key.

  • Drops multiple ransom note copies (____readme.txt, *readme.txt, and a .hta pop-up every restart).

  • Bundles AZORult infostealer strain: expect credential dumps. A full password change + MFA refresh is mandatory post-infection.

  • Broader Impact & Notable Facts:

  • STOP/Djvu accounts for over 75 % of consumer ransomware submissions to ID-Ransomware since 2022.

  • The revenue model focuses on volume over size: ransoms start at US$490 if paid within 72 hours, doubling afterward.

  • Law-enforcement seizures in early-2024 captured some TA servers, yielding hundreds of RSA private keys – reason an increasing share of .bttu, .bbbr, .bhtw and other late-2022 variants can be decrypted today.

Stay vigilant, maintain backups, and share this guide to help others avoid or recover from the .bttu ransomware swiftly.