djvuu

[Content by Gemini 2.5]

STOP/DJVU Ransomware Variant Report (Extension: .djvuu)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The malware leaves each encrypted item with “.djvuu” appended to the original filename.
  • Renaming Convention: A typical affected file changes from document.pdf → document.pdf.djvuu.
    – The preceding filename and extension remain untouched; only the new extension is appended.
    – No alphanumeric IDs or e-mail addresses are interspersed in the name (unlike some other STOP variants).

2. Detection & Outbreak Timeline

  • First Public Sightings: Mid-2019, with a large spike around July-August 2019.
  • Ongoing Waves: The family continues to spawn new strains weekly; .djvuu represents one of many “offspring” extensions released inside the same payload framework.

3. Primary Attack Vectors

  • Malvertising & Pirated Software Bundles:
    – Fake Adobe Photoshop/Cracked-game installers distributed via torrents and ad-laden “free-software” sites drop the initial payload.
  • Spam / Spear-phishing:
    – ZIP or ISO attachments (“Invoice 2024-05-18.exe”, “DHLTracking.exe”) deliver the loader.
  • Software Vulnerability Exploitation:
    – Exploits for CVE-2017-0144 (EternalBlue) and CVE-2020-1472 (Zerologon) seen in laterally-moving infections (post-compromise after a human has run the dropper).
    – Brute-forced / compromised RDP credentials supply a secondary track.
  • VPS-as-a-Service Abuse:
    – Threat actors spin up disposable cloud servers to push updates (usually via HTTPS) once the ransomware phones home.

Remediation & Recovery Strategies

1. Prevention

  1. Robust E-mail & Web Filtering:
  • Block attachments containing archives (.zip, .7z, .iso) with double extensions or EXE content.
  1. User-Education:
  • Train staff to avoid pirated software, key-gens, and torrent downloads.
  1. Patch & Harden:
  • Apply cumulative Windows Updates to close the EternalBlue/SMBv1 channel.
  • Disable SMBv1 across the estate if not required.
  1. Mitigate RDP Exposure:
  • Restrict RDP to VPN users; enforce strong passwords + NLA (Network-Level Authentication) and lockouts via GPO.
  1. Advanced Defense:
  • Deploy behavior-based EDR (e.g., Microsoft Defender for Endpoint, SentinelOne, CrowdStrike) set to monitor for STOP’s typical file-overwrite + extension append pattern (seq. writes + CreateFileSystem + SetEndOfFile).

2. Removal

  1. Disconnect & Isolate:
    a. Power-off infected machines; disable Wi-Fi/Ethernet to stop propagation.
    b. Segregate the VLAN if lateral movement suspected.
  2. Document Forensics:
    a. Snapshot volatile memory (optional but valuable) via Magnet RAM Capture or Belkasoft.
    b. Photograph or note the ransom note (_readme.txt) locations.
  3. Clean Boot & Scan:
    – Boot from Windows Defender Offline USB / WinRE.
    – Run full scans with updated signatures of reputable engines (Bitdefender, Kaspersky Rescue Disk, Malwarebytes).
  4. Persistence Elimination:
    – Remove scheduled tasks named “Time Trigger Task” or string IDs like “endo“, “crona“, typically under:
    C:\Windows\System32\Tasks\ or registry “Run” keys:
    HKCU\Software\Microsoft\Windows\CurrentVersion\Run\SysHelper.
  5. Rebuild Clean Images:
    – For SOCs: Automate reload via MDT/SCCM – STOP/DJVU often bundles info-stealers (Vidar, RedLine). A clean reinstall is the safest route.

3. File Decryption & Recovery

  • Online vs Offline Key Check:
    – STOP/DJVU distinguishes between online and offline encryption keys. Files encrypted when the C2 is reachable (online) cannot be decrypted without the attackers’ RSA private key.
    – When the sample fails to reach C2 it falls back to an offline key hard-coded inside the binary; in such cases the same key is used for ALL victims of that campaign.
  • Decryptor Availability:
    – Use the free Emsisoft Decryptor for STOP Djvu:
    Download: https://www.emsisoft.com/ransomware-decryption-tools/stop-djvu
    – Run it on a sample file pair (original + encrypted) to check for offline key coverage.
    – If the tool states “Unfortunately, decryption is impossible,” the malware used an online key and no public workaround exists.
  • Alternative Recovery:
    – Shadow Copies (VSS) are usually deleted via vssadmin delete shadows /all as part of its arsenal; attempt recovery with ShadowExplorer in very recent infections.
    – File-system carving tools (PhotoRec / R-Studio) can rescue remnants of overwritten MP4, JPG, DOC files if Windows File History or backups exist on an external disk not mounted during infection.

4. Other Critical Information

  • Unique Characteristics:
    – Bundled Info-Stealer Payload: STOP/DJVU frequently delivers Vidar or AZORult, harvesting passwords, crypto wallets, and browser caches before encryption. Changing all credentials (local & cloud) post-infection is mandatory.
    – Regional Targeting: Certain download pages (keygens or cracks) attempt GeoIP filtering to present the DJVU installer to North American/European users preferentially.
  • Wider Industry Impact:
    – DJVU variants remain one of the most prevalent consumer ransomware families due to reliance on illicit software distribution. The low ransom (~490 USD rising to 980 USD) leads to frequent payments that fund further strain diversification (.aaqa, .bboo, .rotf, …).
    – Small businesses leveraging pirated Office or CAD tools in emerging markets suffer disproportionate downtime from this family.

Summary Action List for Incident Managers

  1. Prevent entry via email & piracy channels.
  2. If hit, immediately isolate and THEN reach for the Emsisoft decryptor to determine offline vs online encryption.
  3. Re-image systems; assume credential compromise, reset everything, and enable MFA.