*[email protected]*.combo

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This document provides a comprehensive analysis of the ransomware variant identified by the file extension *[email protected]*.combo, a known variant of the notorious STOP/Djvu ransomware family.

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files encrypted by this ransomware variant will typically have a double extension. The first part is the original file extension, and the second part is the ransomware’s specific extension, which includes the contact email. For example, a file named document.docx would be renamed to document.docx.combo.
    • Example: report.pdf becomes report.pdf.combo
    • Example: photo.jpg becomes photo.jpg.combo
  • Renaming Convention: The ransomware appends .combo to the end of the encrypted file’s name. The [email protected] part is primarily used in the ransom note as the contact email, although some older Djvu variants might have integrated similar email patterns directly into the file extension (e.g., [email protected]). For the .combo variant, the .combo suffix is the primary identifier on the file itself.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Variants using the [email protected] contact email, particularly those with the .combo extension, are part of the continuously evolving STOP/Djvu ransomware family. The .combo extension itself emerged in late 2023 and continued into 2024, indicating it’s a relatively recent iteration designed to evade detection and previous decryption efforts. The STOP/Djvu family has been active since at least 2018, with new extensions and contact emails appearing almost weekly.

3. Primary Attack Vectors

The *[email protected]*.combo variant, like other STOP/Djvu ransomware variants, primarily relies on social engineering and deceptive distribution methods:

  • Software Bundles & Pirated Software: One of the most common vectors is through deceptive software bundles, often downloaded from untrustworthy websites. This includes:
    • Cracked Software: Illegally downloaded or “cracked” versions of popular software (e.g., Adobe products, Microsoft Office, games, video editors).
    • Software Activators/Keygens: Tools designed to bypass software licensing.
    • Fake Updates: Malicious installers disguised as legitimate software updates (e.g., Flash Player updates, browser updates).
  • Malicious Email Attachments: While less common for Djvu than for some other ransomware families, spear-phishing campaigns can be used. These emails contain infected attachments (e.g., seemingly legitimate documents with malicious macros, or ZIP archives containing executables).
  • Adware & Malvertising: Redirection to malicious sites or automatic downloads via compromised ad networks.
  • Remote Desktop Protocol (RDP) Exploits: While less typical for this specific family compared to enterprise-targeted ransomware, poorly secured RDP endpoints can be brute-forced or exploited to gain initial access, after which the ransomware payload is manually executed.
  • Exploitation of Vulnerabilities: Unlike some high-profile ransomware (e.g., WannaCry’s use of EternalBlue), STOP/Djvu variants generally do not self-propagate through network vulnerabilities like SMBv1 or EternalBlue. Their primary method is direct user execution of the malicious payload.

Remediation & Recovery Strategies:

1. Prevention

  • Regular Data Backups: Implement a robust 3-2-1 backup strategy (3 copies of data, on 2 different media, with 1 copy off-site/offline). This is the most critical prevention and recovery measure. Test backups regularly.
  • Robust Antivirus/Anti-malware Software: Install and maintain reputable endpoint protection (AV/EDR). Ensure it’s always up-to-date and performs regular scans.
  • System and Software Updates: Keep your operating system, web browsers, and all installed software fully patched. Many ransomware attacks exploit known vulnerabilities.
  • Strong User Account Control (UAC): Do not disable UAC, as it can prompt you before malicious software makes significant changes.
  • Firewall Configuration: Enable and properly configure your firewall to block unauthorized incoming and outgoing connections.
  • Email Security: Use email filtering solutions to detect and block malicious attachments and links. Be extremely cautious with unsolicited emails.
  • User Education: Train users about phishing, suspicious attachments, and the dangers of downloading software from unofficial sources. Emphasize the risks of cracked software.
  • Disable RDP if Not Needed: If RDP is essential, secure it with strong, complex passwords, multi-factor authentication (MFA), and restrict access via firewall rules (e.g., only from trusted IPs).
  • Application Whitelisting: Consider implementing application whitelisting to prevent unauthorized executables from running.

2. Removal

  • Isolate the Infected System: Immediately disconnect the infected computer from the network (unplug Ethernet cable, disable Wi-Fi) to prevent further spread.
  • Boot into Safe Mode: Restart the computer and boot into Safe Mode with Networking. This often prevents the ransomware from fully loading its malicious components.
  • Scan and Remove Malware:
    1. Download and run a full scan with a reputable anti-malware tool (e.g., Malwarebytes, ESET, Bitdefender, SpyHunter). Ensure the tool’s definitions are up to date.
    2. Perform a deep scan.
    3. Allow the anti-malware software to quarantine or remove all detected threats.
  • Check Startup Items and Task Scheduler: Manually check and remove any suspicious entries that allow the ransomware to run at startup.
    • Windows: Use msconfig (for startup items) and taskschd.msc (Task Scheduler).
  • Delete Suspicious Files: Look for suspicious files in common locations like C:\Users\[Username]\AppData\Local, C:\ProgramData, and C:\Windows\Temp. Be cautious not to delete critical system files.
  • Clean Host File: Ransomware often modifies the hosts file (C:\Windows\System32\drivers\etc\hosts) to block access to security-related websites. Open it with Notepad and remove any suspicious entries.
  • Change All Passwords: After cleaning the system, change all passwords, especially for online accounts, as the ransomware might have harvested credentials.

3. File Decryption & Recovery

  • Recovery Feasibility:
    • Generally Difficult for .combo variants: Newer STOP/Djvu variants, including .combo, predominantly use online keys for encryption. This means a unique encryption key is generated for each victim and transmitted to the attacker’s server. Without this specific key, which the attackers hold, decryption is currently impossible through public tools.
    • Offline Keys: In rare cases, if the ransomware failed to connect to its command-and-control server during encryption, it might use an offline key. If an offline key was used, there is a slim possibility that a decryptor could eventually be developed if enough offline keys are collected from various victims and a pattern can be identified.
    • Do NOT Pay the Ransom: Paying the ransom does not guarantee decryption and funds criminal activities. There is no assurance that the attackers will provide a working key or decryptor.
  • Methods or Tools Available (Limited for .combo):
    • Emsisoft Decryptor for STOP/Djvu: Emsisoft, in collaboration with malware researcher Michael Gillespie, provides a free decryptor for STOP/Djvu ransomware. This is the primary tool to attempt decryption.
      • How it works: You submit encrypted files and their original, unencrypted versions (if available). The tool then attempts to identify the encryption key (especially for offline keys).
      • Important Note: As of early 2024, the .combo variant mostly uses online keys, meaning the Emsisoft decryptor cannot decrypt files encrypted with online keys unless the specific key is known. Keep checking Emsisoft’s website for updates.
    • Shadow Volume Copies (VSS): Many ransomware variants attempt to delete Shadow Volume Copies to prevent recovery. Try using tools like vssadmin (command line) or third-party tools like ShadowExplorer to see if any previous versions of files survived.
      • Command: vssadmin delete shadows /all /quiet (this is what ransomware runs, do not run this yourself!). To list them: vssadmin list shadows.
    • File Recovery Software: Tools like PhotoRec, Recuva, or Disk Drill might be able to recover some older, deleted, unencrypted versions of files, but this is less likely for files directly encrypted by the ransomware.
  • Essential Tools/Patches:
    • Emsisoft Decryptor for STOP/Djvu: The go-to tool for this ransomware family.
    • Reputable Anti-malware Software: (e.g., Malwarebytes, ESET, Bitdefender) for removal.
    • Backup Solutions: For future prevention and recovery (e.g., Veeam, Acronis, cloud backups).
    • Windows Security Updates: Crucial for patching vulnerabilities.
    • Strong Password Manager: For securing new credentials.

4. Other Critical Information

  • Additional Precautions:
    • Ransom Note: The ransomware typically drops a _readme.txt file in every folder containing encrypted files, providing instructions for payment and contact information (e.g., [email protected]). Do not trust any links or instructions provided in this note.
    • Hosts File Modification: This variant commonly modifies the hosts file to block access to security and antivirus websites, preventing victims from seeking help or downloading security tools. Always check and clean the hosts file.
    • Information Stealer Module: Newer STOP/Djvu variants often come bundled with information-stealing malware (e.g., RedLine Stealer, Vidar Stealer, Amadey). This means your credentials, cryptocurrency wallets, browser data, and other sensitive information might have been exfiltrated before encryption. This makes password changes even more critical.
    • Fake Decryptors: Be wary of fake decryptors advertised online. Only use tools from reputable cybersecurity vendors.
  • Broader Impact:
    • Data Loss: The primary impact is the loss of access to critical data, which can be permanent if decryption is not possible and backups are unavailable.
    • Financial Cost: This includes potential ransom payment (not recommended), costs associated with IT recovery, and lost productivity.
    • Operational Disruption: Business operations can be severely disrupted, leading to downtime and loss of revenue.
    • Reputational Damage: For organizations, a ransomware attack can damage trust with customers and partners.
    • Privacy Breach: Due to the common bundling with info-stealers, the attack could also constitute a data breach, requiring additional compliance and notification steps.

Combating *[email protected]*.combo and similar ransomware variants requires a proactive and multi-layered security approach, with emphasis on robust backups and user education.