490

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The ransomware variant identified by the file extension .490 is a specific iteration belonging to the prolific STOP/Djvu ransomware family. This family is notorious for its widespread infections, continuous development of new variants, and often challenging decryption prospects.

Here’s a detailed breakdown and guide for the .490 variant of STOP/Djvu ransomware:

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The exact file extension used by this variant is .490.
  • Renaming Convention: Upon successful encryption, the ransomware appends the .490 extension to every encrypted file. The typical renaming pattern follows the structure:
    [original_filename].[original_extension].490
    For example:
    • document.docx would become document.docx.490
    • photo.jpg would become photo.jpg.490
      The ransomware also drops a ransom note file, typically named _readme.txt, in every folder containing encrypted files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: The STOP/Djvu ransomware family has been active since late 2017/early 2018. New variants, including those using numeric or alphanumeric extensions like .490, are released regularly, sometimes daily. The .490 variant would have appeared as part of this continuous release cycle. The family remains one of the most prevalent ransomware threats, particularly targeting individual users.

3. Primary Attack Vectors

STOP/Djvu ransomware primarily relies on social engineering and deceptive tactics to infect systems, often targeting users seeking pirated or “free” software.

  • Propagation Mechanisms:
    • Cracked Software & Illicit Downloads: This is the most common vector. Users download cracked versions of popular software (e.g., games, productivity suites, video editors, operating system activators) from torrent sites, warez forums, or untrustworthy download sites. The ransomware is often bundled within these executables or installers.
    • Fake Software Updates: Malicious websites or pop-ups may trick users into downloading what appears to be a legitimate software update (e.g., Flash Player, Java, browser updates) but is actually the ransomware dropper.
    • Malicious Advertisements (Malvertising): Compromised ad networks or websites serving malicious ads can redirect users to landing pages that automatically download the ransomware or prompt them to download a “required” plugin.
    • Email Phishing: While less common than for enterprise-level ransomware, basic phishing emails with malicious attachments (e.g., seemingly legitimate documents with embedded macros) can also be used.
    • Remote Desktop Protocol (RDP) Exploits: While not a primary vector for this specific family due to its focus on individual users, weak RDP credentials or unpatched RDP vulnerabilities could theoretically be exploited if the attacker shifts tactics, though this is rare for Djvu.

Remediation & Recovery Strategies:

1. Prevention

Proactive measures are the most effective defense against STOP/Djvu and similar ransomware.

  • Regular Data Backups: Implement a robust 3-2-1 backup strategy (3 copies of data, 2 different media types, 1 offsite). Ensure backups are immutable or offline to prevent ransomware from encrypting them.
  • Software Updates & Patching: Keep your operating system, web browsers, antivirus software, and all installed applications fully updated. This closes security vulnerabilities that ransomware might exploit.
  • Reputable Antivirus/Anti-Malware: Use a comprehensive, up-to-date cybersecurity suite with real-time protection, behavioral analysis, and exploit prevention.
  • Strong Password Practices & MFA: Use strong, unique passwords for all accounts and enable Multi-Factor Authentication (MFA) wherever possible.
  • Email & Browser Hygiene: Be extremely cautious with unsolicited emails, suspicious attachments, and links. Avoid visiting dubious websites, especially those offering “free” or cracked software.
  • User Account Control (UAC): Ensure UAC is enabled on Windows to prompt for administrative privileges, which can sometimes alert users to malicious activity.
  • Disable Autorun: Disable the Autorun feature in Windows to prevent malware from automatically executing when external drives are connected.

2. Removal

If an infection occurs, immediate and careful steps are required.

  • Isolate the Infected System: Immediately disconnect the infected computer from the network (unplug Ethernet cable, disable Wi-Fi). This prevents the ransomware from spreading to other devices on the network.
  • Identify the Ransomware: The presence of .490 extensions and the _readme.txt ransom note confirms the Djvu/STOP variant.
  • Run a Full System Scan: Boot the system into Safe Mode with Networking (if necessary to download tools) or use a bootable anti-malware rescue disk. Perform a full system scan using a reputable anti-malware tool (e.g., Malwarebytes, Emsisoft, Bitdefender, SpyHunter).
  • Remove Detected Threats: Allow the anti-malware software to quarantine and remove all detected ransomware components, associated malware, and persistence mechanisms.
  • Check for Persistence: Manually check common persistence locations (e.g., Startup folders, Registry Run keys, Scheduled Tasks) for any remnants if you are an advanced user.
  • Change All Passwords: Once the system is confirmed clean, change all passwords for accounts accessed from the infected computer.

3. File Decryption & Recovery

The feasibility of decrypting .490 files without paying the ransom depends critically on whether an “online” or “offline” encryption key was used.

  • Recovery Feasibility:
    • Online Keys (Most Common): If the ransomware successfully communicated with its Command & Control (C2) server, it will generate a unique “online” encryption key for your system. In this scenario, decryption is currently not possible without the private key held by the attackers. Public decryptors cannot recover these files.
    • Offline Keys (Less Common): If the ransomware failed to connect to its C2 server (e.g., due to network issues, C2 server being down), it uses a hardcoded “offline” encryption key. For these cases, there is hope.
    • Emsisoft Decryptor for STOP/Djvu: Emsisoft, in collaboration with Michael Gillespie (MalwareHunterTeam), provides a free decryptor for STOP/Djvu ransomware.
      • How it works: This decryptor collects specific data from your encrypted files and the ransom note (_readme.txt) to determine if an offline key was used and if it’s a known key. If a match is found, it can decrypt your files.
      • Important Note: Even if an offline key was used, it must be a previously discovered offline key for the decryptor to work. As new offline keys are occasionally discovered, it’s worth trying the decryptor periodically, even if it doesn’t work initially.
  • Essential Tools/Patches:
    • Emsisoft Decryptor for STOP/Djvu: This is the primary tool to attempt decryption. Download it only from the official Emsisoft website or trusted cybersecurity resources.
    • Reputable Anti-Malware Software: For prevention and removal (e.g., Malwarebytes, Bitdefender, ESET, Kaspersky).
    • Operating System Patches & Updates: Ensure your Windows operating system is fully updated to address known vulnerabilities.
    • Data Recovery Software: In some rare cases, if the original files were simply copied and the originals deleted (instead of being directly overwritten and encrypted), data recovery software might recover older versions, but this is highly unlikely for Djvu.
  • No Decryption (Online Key): If the decryptor fails and indicates an online key was used, unfortunately, there is currently no way to decrypt your files without the attackers’ private key. Paying the ransom is strongly discouraged as it fuels criminal activity and offers no guarantee of decryption. Focus on restoring from backups.

4. Other Critical Information

  • Unique Characteristics:
    • Online vs. Offline ID: The most distinguishing feature of STOP/Djvu is the reliance on online vs. offline encryption IDs. The ransom note often contains a “Personal ID” that can help determine this. If the ID ends in “t1” (e.g., XXXXXXXXXXXXt1), it generally indicates an online key. If it doesn’t end in “t1” (e.g., XXXXXXXXXXXX), it might be an offline key.
    • Constant Variant Creation: The sheer volume of new variants (each often with a new extension) makes it challenging for security researchers to develop decryptors for every single iteration, especially for online keys.
    • Info Stealer Bundling: Many STOP/Djvu infections are bundled with information-stealing malware (e.g., RedLine Stealer, Vidar, Azorult, Amadey). This means your passwords, cryptocurrency wallet details, browser history, and other sensitive data may have been exfiltrated before encryption.
    • _readme.txt Ransom Note: This specific file name is a hallmark of the STOP/Djvu family.
  • Broader Impact:
    • Individual User Focus: Unlike many other ransomware gangs that target enterprises, STOP/Djvu heavily preys on individual users, making it a persistent and widespread threat in the consumer space.
    • Psychological Impact: The loss of personal photos, documents, and memories due to the often irreversible encryption can have a significant emotional toll on victims.
    • Economic Impact: While individual ransoms are smaller (typically a few hundred to a thousand dollars), the cumulative amount collected by this prolific family is substantial, fueling further cybercrime.
    • Risk of Further Compromise: The bundling with info-stealers means that even if files are recovered, victims need to assume their personal data has been compromised and take immediate action to secure accounts (password changes, credit monitoring).

By understanding these technical details and implementing robust prevention and recovery strategies, individuals can significantly reduce their risk and mitigate the impact of the .490 variant and other STOP/Djvu ransomware infections.