@bk.ru

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This document provides a comprehensive overview of the ransomware variant identified by the file extension @bk.ru, which is commonly associated with the STOP/Djvu ransomware family. This family is one of the most prolific and constantly evolving ransomware threats, primarily targeting individual users and small businesses.

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The exact file extension used by this variant is typically .bk.ru. It is appended to the original file name and extension.
  • Renaming Convention: STOP/Djvu ransomware variants, including the .bk.ru variant, follow a specific renaming pattern. When a file is encrypted, its original name is preserved, but the new extension is appended.
    • Example: A file named document.docx would be renamed to document.docx.bk.ru.
    • In some cases, a unique ID or a combination of the victim’s ID and the extension might be used, such as document.docx.id[unique_id].bk.ru, though the simpler .bk.ru suffix is common.
    • A ransom note, typically named _readme.txt, is dropped in every folder containing encrypted files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: The STOP/Djvu ransomware family first emerged in late 2017 and rapidly became one of the most active ransomware threats globally. The .bk.ru variant, like many others, is one of a continuous stream of new extensions released by the group, appearing as part of the ongoing evolution of the family throughout 2020-2023 and beyond. New variants are released almost daily, making it a persistent and evolving threat.

3. Primary Attack Vectors

STOP/Djvu ransomware, including the .bk.ru variant, primarily relies on social engineering and deceptive practices to infect systems.

  • Cracked Software/Pirated Content: This is the most prevalent infection vector. Users download “cracked” versions of popular software, key generators, or pirated media from unofficial websites (e.g., torrent sites, warez forums). The ransomware executable is often bundled with or disguised as part of these downloads.
  • Fake Software Updates: Malicious websites may push fake software updates (e.g., for Flash Player, Java, web browsers) that, when clicked, download and execute the ransomware.
  • Malicious Email Attachments/Links: Phishing campaigns, though less common for Djvu than for some enterprise-targeting ransomware, can still be a vector. These emails contain malicious attachments (e.g., seemingly legitimate documents with embedded macros) or links to compromised websites.
  • Adware/Bundled Software: The ransomware may be bundled with legitimate-looking, but ad-supported or low-quality free software, often downloaded from untrustworthy software download sites.
  • Drive-by Downloads: Visiting compromised websites can sometimes lead to an automatic download and execution of the ransomware, though this is less frequent.
  • No Network Propagation: Unlike some worms (e.g., WannaCry), STOP/Djvu ransomware typically does not self-propagate across networks (e.g., via SMB vulnerabilities like EternalBlue). Infection usually requires direct user interaction (e.g., executing a malicious file).

Remediation & Recovery Strategies:

1. Prevention

Proactive measures are crucial to mitigate the risk of STOP/Djvu ransomware infection:

  • Robust Backups: Implement a 3-2-1 backup strategy: at least three copies of your data, on two different media, with one copy offsite or offline (disconnected from the network). Test your backups regularly.
  • Antivirus/Endpoint Detection & Response (EDR): Use reputable, up-to-date antivirus or EDR solutions. Ensure real-time protection is active.
  • Software Updates & Patching: Keep your operating system, web browsers, and all software applications fully updated. Enable automatic updates where possible.
  • User Education: Train users to be wary of suspicious emails, unsolicited downloads, and unofficial software sources. Emphasize the risks of pirated software.
  • Strong Password Practices: Use strong, unique passwords for all accounts and enable Multi-Factor Authentication (MFA) whenever possible.
  • Disable Unnecessary Services: Disable RDP if not needed, or secure it with strong passwords and network-level authentication (NLA) if it must be exposed.
  • Application Whitelisting: Consider implementing application whitelisting policies to prevent unauthorized executables from running.
  • Firewall Rules: Configure firewalls to block outbound connections to known malicious IP addresses or C2 servers, though this can be difficult given the dynamic nature of ransomware infrastructure.

2. Removal

If a system is infected with the .bk.ru variant, follow these steps for removal:

  1. Isolate the Infected System: Immediately disconnect the infected computer from the network (unplug Ethernet cable, disable Wi-Fi). This prevents potential lateral movement or further communication with the attacker.
  2. Identify the Ransomware Process: Use Task Manager (Windows) or process monitoring tools to identify suspicious processes. STOP/Djvu executables often have random names (e.g., sadfgytr.exe, build.exe).
  3. Boot into Safe Mode: Restart the computer in Safe Mode with Networking. This often prevents the ransomware from fully executing and allows antivirus tools to run more effectively.
  4. Run a Full System Scan: Use a reputable antivirus or anti-malware solution (e.g., Malwarebytes, Windows Defender, ESET, Bitdefender) to perform a deep scan. Ensure the definitions are fully updated. Allow the tool to quarantine or remove all detected threats.
  5. Check for Persistence Mechanisms:
    • Registry: Check HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run and HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run for suspicious entries that launch the ransomware at startup.
    • Startup Folders: Check C:\Users\[Username]\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup and C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup.
    • Scheduled Tasks: Look for new, suspicious scheduled tasks that might re-launch the ransomware.
  6. Delete Ransomware Executables: After scanning, manually delete any remaining ransomware executables found.
  7. Remove Ransom Notes: Delete all _readme.txt files, though this is primarily for cleanliness and does not affect encryption.
  8. Change Passwords: Once the system is clean, change all passwords, especially for any accounts that might have been logged into on the infected machine.

3. File Decryption & Recovery

  • Recovery Feasibility: Decrypting files encrypted by the .bk.ru variant of STOP/Djvu ransomware is highly challenging and often not possible without the decryption key from the attackers.
    • Online Key vs. Offline Key: STOP/Djvu ransomware generates a unique encryption key for each victim.
      • Online Key: If the ransomware successfully connects to its command-and-control server, it generates an “online key” unique to that specific victim, which is then sent to the attackers. Decryption is almost impossible without paying the ransom or a leak of the master keys by law enforcement.
      • Offline Key: If the ransomware fails to connect to its C2 server (e.g., due to network issues, firewall, or the server being down), it uses a pre-generated “offline key” from an embedded list. While still unique to the victim for that particular session, these offline keys are part of a finite set. If security researchers (like Emsisoft) can obtain and analyze these offline keys, they can develop a decryptor.
    • Emsisoft Decryptor: The Emsisoft Decryptor for STOP/Djvu is the primary tool for potential decryption. It attempts to identify if your files were encrypted with an “offline key” that they have recovered.
      • How to use: Download the Emsisoft Decryptor, run it, and follow the instructions. It will scan your encrypted files and attempt to match them against known offline keys. It might require you to provide an encrypted file and its original, unencrypted version to help it determine the key.
    • No More Ransom Project: The No More Ransom website (nomoreransom.org) is a valuable resource. It hosts various decryptors developed by law enforcement and cybersecurity firms, including those for STOP/Djvu variants. You can upload an encrypted file and the ransom note to their Crypto Sheriff tool, which attempts to identify the ransomware family and point you to a potential decryptor.
  • Essential Tools/Patches:
    • Emsisoft Decryptor for STOP/Djvu: The most important tool for attempting decryption.
    • Reputable Antivirus/Anti-malware software: (e.g., Malwarebytes, Bitdefender, ESET, Kaspersky, Windows Defender) for removal.
    • Backup Solutions: For recovery from backups.
    • Operating System and Application Patches: To prevent initial infection.

4. Other Critical Information

  • Additional Precautions (Unique Characteristics of Djvu):
    • Offline Key Predominance: The prevalence of “offline keys” means that even if you pay, there’s no guarantee the attackers will have the correct online key for your specific infection if it happened offline. This is rare, but highlights the unreliability of paying.
    • Shadow Copy Deletion: STOP/Djvu often attempts to delete Volume Shadow Copies to prevent recovery using native Windows tools. This is done via vssadmin.exe delete shadows /all /quiet.
    • Hosts File Modification: It may add entries to the hosts file (C:\Windows\System32\drivers\etc\hosts) to block access to security-related websites (e.g., antivirus vendors, cybersecurity blogs) to hinder victims from seeking help or downloading tools. Restore the hosts file to its default state after cleaning.
    • Fake Error Messages: Sometimes displays fake error messages (e.g., “Windows cannot find the file specified”) to distract the user while encryption occurs in the background.
    • Persistence: As noted in the removal section, it often creates persistence mechanisms to ensure it runs on system reboot.
  • Broader Impact:
    • High Volume Threat: STOP/Djvu ransomware is one of the most widespread consumer-focused ransomware threats, affecting hundreds of thousands of individuals and small businesses globally. Its high volume makes it a significant problem.
    • Impact on Individuals: It primarily targets individual users, leading to the loss of personal documents, photos, and other irreplaceable data if backups are not in place and decryption is not possible.
    • Economic Impact: While individual ransom demands are relatively low (typically $490-$980), the sheer volume of infections contributes to a significant cumulative economic impact globally.
    • Evolutionary Pace: The rapid pace at which new variants (with new extensions and slightly modified code) are released makes it challenging for security researchers to develop universal decryptors quickly. This continuous evolution requires constant vigilance and updated security definitions.

By understanding the technical aspects and employing robust prevention and recovery strategies, individuals and organizations can significantly reduce their risk against the .bk.ru variant and the broader STOP/Djvu ransomware family.