491

[Content by Gemini 2.5]

As a cybersecurity expert specializing in ransomware, I’ve compiled a comprehensive resource on the ransomware variant identified by the file extension .491. This particular extension is primarily associated with a prolific and continuously evolving family of ransomware known as STOP/Djvu. Understanding this context is crucial for effective prevention and recovery.

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The exact file extension used by this ransomware variant is .491.
  • Renaming Convention: The ransomware typically appends the .491 extension to the end of encrypted files. The renaming pattern follows this structure:
    [original_filename].[original_extension].491
    For example, a file named document.docx would become document.docx.491, and photo.jpg would become photo.jpg.491.
    In addition to the encrypted files, the ransomware usually drops a ransom note. For STOP/Djvu variants, this note is typically named _readme.txt and is placed in every folder containing encrypted files, and often on the desktop.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: The STOP/Djvu ransomware family first emerged around late 2017 to early 2018. It has undergone continuous development, releasing new variants with different extensions (like .491) frequently. The .491 variant specifically would have appeared as one of the many iterations within this ongoing series of attacks. This family is known for its high volume of new variants, often released daily or weekly.

3. Primary Attack Vectors

The STOP/Djvu ransomware, including the .491 variant, primarily relies on social engineering and deceptive distribution methods rather than exploiting complex network vulnerabilities. Its main propagation mechanisms include:

  • Bundled Software / Software Crack Sites: This is the most prevalent vector. Users often download pirated software, “cracks,” key generators (keygens), software activators, or installers from untrustworthy websites. These downloads are often trojanized, containing the ransomware as a hidden payload.
  • Fake Software Updates: Malicious websites or pop-ups may trick users into downloading fake updates for legitimate software (e.g., Adobe Flash Player, Java, web browsers), which are actually ransomware installers.
  • Malvertising: Compromised legitimate advertising networks or malicious ads can redirect users to landing pages that automatically download the ransomware or prompt them to download a “required” update.
  • Deceptive Downloads: Files disguised as legitimate documents, installers, or media files on file-sharing sites.
  • Less Common: While not the primary method for STOP/Djvu, other general ransomware vectors like phishing emails (with malicious attachments or links) and RDP brute-force attacks can occasionally be observed. However, for STOP/Djvu, direct user interaction via dubious downloads is far more common.

Remediation & Recovery Strategies:

1. Prevention

Proactive measures are the most effective defense against the .491 variant and other ransomware:

  • Robust Backups: Implement a 3-2-1 backup strategy (3 copies of data, on 2 different media, with 1 copy off-site or air-gapped). Regularly test backup integrity. This is the ultimate safeguard.
  • Endpoint Security: Deploy and maintain reputable anti-malware and Endpoint Detection and Response (EDR) solutions on all systems. Ensure they are configured for real-time protection and regularly updated.
  • Software Updates & Patch Management: Keep operating systems, applications (especially web browsers, email clients, and media players), and security software fully patched. Many ransomware variants exploit known vulnerabilities.
  • User Education & Awareness: Train users to identify phishing attempts, suspicious links, and untrustworthy download sources. Emphasize the dangers of downloading pirated software or clicking on unsolicited attachments.
  • Network Segmentation: Isolate critical systems and data to limit lateral movement in case of an infection.
  • Principle of Least Privilege: Grant users and applications only the minimum necessary permissions to perform their tasks.
  • Disable Unnecessary Services: Turn off services like SMBv1, RDP (if not needed, or secure it with strong passwords, 2FA, and VPN access).
  • Application Whitelisting: Restrict the execution of unauthorized programs.

2. Removal

Effective removal of the .491 variant involves these steps:

  1. Isolate the Infected System: Immediately disconnect the infected computer from the network (unplug Ethernet cable, disable Wi-Fi) to prevent the ransomware from spreading to other devices.
  2. Identify and Terminate Processes: Use Task Manager or a more advanced process explorer (like Sysinternals Process Explorer) to identify and terminate any suspicious processes. The ransomware process might have a random-looking name or mimic legitimate system processes.
  3. Run a Full System Scan: Boot the system into Safe Mode with Networking (if necessary to download tools). Use a reputable, up-to-date anti-malware suite (e.g., Malwarebytes, Emsisoft Anti-Malware, HitmanPro) to perform a comprehensive scan and remove all detected malicious files, registry entries, and scheduled tasks created by the ransomware.
  4. Check for Persistence: Manually check common persistence locations such as:
    • HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
    • HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
    • Startup folders (shell:startup, shell:common startup)
    • Scheduled Tasks (taskschd.msc)
    • WMI event subscriptions
      Remove any suspicious entries linked to the ransomware.
  5. Address Information Stealer: A critical characteristic of the STOP/Djvu family is that it often bundles an information stealer (e.g., Vidar, Azorult, or RedLine Stealer) alongside the ransomware. After removing the ransomware, assume that your credentials, browser history, cryptocurrency wallets, and other sensitive data might have been compromised.
    • Change all critical passwords (email, banking, social media, online services) from an uninfected device.
    • Enable Two-Factor Authentication (2FA) wherever possible.
    • Monitor financial accounts for suspicious activity.

3. File Decryption & Recovery

  • Recovery Feasibility:
    • Offline Keys: For some victims, if the ransomware failed to establish a connection to its C2 server during encryption, it might use an “offline key.” If this specific offline key has been discovered and published by security researchers (e.g., by Emsisoft), then decryption is possible.
    • Online Keys: If the ransomware successfully connected to its C2 server, it uses a unique “online key” for each victim. Decryption in this scenario is generally not possible without obtaining the private decryption key from the attackers (which means paying the ransom, an option not recommended due to no guarantee of decryption and funding future attacks) or a significant breakthrough by security researchers.
    • No Universal Decryptor for All Variants: Due to the frequent release of new variants and unique keys, there is no single “magic bullet” decryptor that works for all STOP/Djvu infections.
  • Essential Tools/Patches:
    • Emsisoft Decryptor for STOP Djvu: This is the most reliable and frequently updated tool for STOP/Djvu decryption. Download it only from the official Emsisoft website. The decryptor attempts to identify the specific variant and then checks its database of known offline keys. It might be able to decrypt files encrypted with an offline key, or partially decrypt files if a known online key for an older variant happens to match.
    • Shadow Explorer: If System Restore or Volume Shadow Copy Service was enabled and the ransomware failed to delete shadow copies, you might be able to recover older versions of files using tools like Shadow Explorer. However, most modern ransomware variants, including STOP/Djvu, actively try to delete shadow copies using commands like vssadmin delete shadows /all /quiet.
    • Data Recovery Software: In some rare cases, if the ransomware deleted original files after encrypting them rather than encrypting in place, data recovery software might retrieve some unencrypted original files from unallocated space. Success rates are generally low.
    • Forensic Backups: If decryption is not immediately possible, create a forensic image of the encrypted drive. This preserves the encrypted data, allowing for potential future decryption if a key or tool becomes available.

4. Other Critical Information

  • Additional Precautions (Information Stealer): As mentioned, the persistent bundling of information stealers with STOP/Djvu ransomware is a significant differentiator. This means that beyond just encrypting files, your sensitive information (passwords, browser data, cryptocurrency wallets, system information) might have been exfiltrated. Prioritize changing all critical passwords from a clean system and monitor your financial accounts.
  • Broader Impact: The STOP/Djvu ransomware family, including the .491 variant, has a massive and widespread impact primarily on individual users and small businesses globally. Its high volume of variants, reliance on social engineering (targeting less technically savvy users), and distribution via pirated software sites make it one of the most common ransomware threats encountered by home users. While it typically doesn’t target large enterprises with sophisticated network attacks, its sheer volume means it collectively causes significant data loss and financial distress worldwide. It also highlights the ongoing challenge of combating malware distributed through illegitimate software channels.