djvu

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The STOP/DJVU family uses one of 180+ extension identifiers that all end in the generic marker “.djvu” (notice the extra “v” compared with the benign DjVu graphics format).
  • Renaming Convention: After encryption, every affected file is renamed as follows
    originalname.jpg → originalname.jpg.djvu
    Directory trees are left intact, but each folder receives two ransom notes (_readme.txt and sometimes _open_.txt).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First publicly reported December 2018 under the extension “.djvu”.
    Monthly cyber-crime markedly ramped up between late-2019 and mid-2020 when the gang added new vectors and hundreds of derivative variants (e.g., .npsk, .kodg, .geno, etc.)—all sharing the same core ransomware engine.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Malvertising & Pirated Software Bundles – cracked installers for Photoshop, Office, keygens and game cheats served from torrent sites or fake download portals.
  2. Fake Windows and software update sites promoting bogus patches via Google/Bing ads.
  3. Email phishing (less dominant but documented) – zipped Windows executables disguised as invoices or urgent PDF tools.
  4. Second-layer downloaders – several DJVU variants arrive via the SmokeLoader botnet.
  5. Credential spraying of weak RDP/TCP-3389 or SMB shares was observed in older hybrid waves; once foothold is gained the dropper payload fetches the .djvu executable.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  • Block execution of unsigned or impersonated installers from “%AppData%\Local\Temp*.exe” and “%UserProfile%\Downloads”.
  • Keep Windows fully patched; run the free Emsisoft “StopDJVU Decryption Fix” (optional download) which registers Windows Defender ASR rules for .exe files from Temp.
  • Disable Office macros by GPO; use mail-gateway sandboxing.
  • Deploy anti-malvertising DNS such as Quad9 (9.9.9.11) with DNS-over-HTTPS.
  • Restrict RDP: enforce Network-level Authentication, lock source IPs, use 2FA/OTP.
  • Create offline, air-gapped backups (Veeam/Veeam SureBackup, Windows Server Backup to removable drives).
  • Regularly run AppLocker or WDAC to whitelist only known executables.

2. Removal

  • Infection Cleanup – Step-by-Step:
  1. Isolate the victim computer from network and external drives.
  2. Use Malwarebytes Endpoint or Kaspersky Virus Removal Tool to scan and quarantine:
    • %systemroot%\system32\winadcx.exe (randomized, 8–12 alphanumeric characters)
    • Scheduled task named “Time Trigger Task” or similar, firing a script in %TEMP%.
  3. Examine autoruns (autoruns.exe) for lingering registry keys:
    HKCU\Software\Microsoft\Windows\CurrentVersion\Run
  4. Remove empty.vps, updatewin.exe, and any DropBox-client-named binaries in %LOCALAPPDATA%.
  5. After malware eradication, patch credentials and re-enable network only after all endpoints are clean.

3. File Decryption & Recovery

  • Recovery Feasibility:
  • Sometimes files encrypted with offline keys are recoverable.
  • Never recoverable if an online key was used (requires .EmsisoftDecrypter+unique victim .key file to attempt run, but success is <1% without the correct private key).
  • Use Emsisoft STOP-Djvu Decrypter (free). Launch it inside a fresh Windows profile, point it to a pair of identical encrypted/original files (e.g., a JPG).
  • If the decrypter reports “no key available for ID … t1”, the attacker used their C2 key; you must restore from clean backups or use file-recovery (Recuva, PhotoRec, Windows Shadow Copy if miscreant didn’t delete them).
  • Ensure patch KB4480960 / KB4480970 (SMBv1 + CVE-2017-0144 pipeline) is applied so restoring backups doesn’t re-infect.

4. Other Critical Information

  • Additional Precautions:
  • STOP/DJVU drops a data-stealing trojan (Vidar, AZORult) 30–90 minutes post-infection. Assume credentials, browsers, crypto-wallets are compromised. Force password reset across Accounts-SSO.
  • Newest waves (2023) also drop “RedLineMiner” cryptominer for CPU+GPU abuse—re-image workstations strongly recommended.
  • The group rotates payment portals (Tor sites ending in .onion), each variant carries slightly different BTC addresses.
  • Broader Impact:
  • Over 600,000 individual consumers and 4,200 SMBs in 120+ countries publicly reported infections.
  • US-CERT Alert AA20-280A covers DJVU mitigation.
  • Criminal follow-up emails now attempt to extort victims for paying “second ransom” to stop publication of stolen data—ignore and block these messages.

Bottom line: If the decrypter cannot find an offline key, do NOT pay; instead fully re-image the machine, reset passwords, and restore from an air-gapped backup created before the DJVU keys landed.