4035

[Content by Gemini 2.5]

As a cybersecurity expert specializing in ransomware, I must preface this by stating that ransomware variants are most commonly identified by their family name (e.g., Stop/Djvu, LockBit, Conti, Ryuk). A four-digit numerical file extension like .4035 is highly indicative of a specific variant within a larger, prolific ransomware family, rather than a family name itself.

Based on the pattern of appending four-digit numerical or alphanumeric extensions to encrypted files, the ransomware variant identified by the file extension .4035 is almost certainly a variant of the Stop/Djvu ransomware family. This family is infamous for its continuous release of new variants, each typically distinguishable by a unique file extension. Therefore, the information provided below will largely reflect the characteristics and behaviors of the Stop/Djvu ransomware family, with .4035 being a specific identifier for one of its many iterations.

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The exact file extension used by this variant is .4035.

  • Renaming Convention: When 4035 encrypts a file, it appends the .4035 extension to the original filename. For example:

    • document.docx becomes document.docx.4035
    • photo.jpg becomes photo.jpg.4035
    • archive.zip becomes archive.zip.4035

    In addition to encrypting files, this variant typically drops a ransom note named _readme.txt (or similar, like _openme.txt) in every folder containing encrypted files, and often on the desktop. This note contains instructions for the victim on how to pay the ransom to decrypt their files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: The Stop/Djvu ransomware family, of which .4035 is a variant, first emerged in late 2018/early 2019. It has since undergone continuous development, with new variants (each using a different file extension) appearing almost daily or weekly. Therefore, the specific .4035 variant likely appeared within this ongoing timeline, representing one of the many iterations of this highly active threat.

3. Primary Attack Vectors

The Stop/Djvu family, including variants like 4035, primarily relies on social engineering and deceptive distribution methods:

  • Cracked Software/Pirated Content: This is the most prevalent vector. Victims download seemingly legitimate “cracked” versions of popular software (e.g., Adobe Photoshop, Microsoft Office, video games, VPNs) from torrent sites, suspicious download portals, or file-sharing services. The ransomware is bundled within these seemingly benign files.
  • Fake Software Updates: Malicious websites or pop-ups may trick users into downloading fake software updates (e.g., for Flash Player, Java, web browsers) that contain the ransomware payload.
  • Malvertising (Malicious Advertising): In some cases, deceptive advertisements on legitimate or illegitimate websites redirect users to malicious landing pages that automatically download the ransomware or prompt the user to download a “required” file.
  • Email Phishing Campaigns: While less common as a direct initial infection vector for Stop/Djvu compared to pirated software, phishing emails can be used to distribute malicious attachments (e.g., seemingly innocent documents with embedded scripts) or links to malicious download sites.
  • Trojanized Installers: The ransomware can be disguised as legitimate software installers or updates downloaded from unofficial sources.
  • Bundled Malware: It can also be dropped onto a system by other malware already present, acting as a secondary payload.
  • Remote Desktop Protocol (RDP) Exploits/Brute-forcing: While not a primary vector for this specific family, many ransomware groups exploit weakly secured RDP connections. It’s important to note that any ransomware can potentially leverage this if initial access is gained.

Remediation & Recovery Strategies:

1. Prevention

  • Regular Backups: Implement a robust backup strategy following the 3-2-1 rule (3 copies of data, 2 different media, 1 offsite). Ensure backups are immutable or offline to prevent ransomware from encrypting them.
  • Software Updates: Keep your operating system, applications, and security software (antivirus, anti-malware) up-to-date with the latest patches. This mitigates vulnerabilities that attackers might exploit.
  • Strong Antivirus/Anti-malware: Use reputable security software with real-time protection and keep its definitions updated. Consider next-gen endpoint detection and response (EDR) solutions for enhanced protection.
  • User Education: Train users about the dangers of downloading cracked software, opening suspicious attachments, clicking on dubious links, and the importance of verifying software sources.
  • Network Segmentation: Isolate critical systems and sensitive data from the rest of the network. This limits lateral movement if an infection occurs.
  • Firewall Rules: Configure firewalls to block unauthorized outbound connections and restrict access to essential services.
  • RDP Security: If RDP is necessary, secure it with strong, unique passwords, multi-factor authentication (MFA), network level authentication (NLA), and restrict access to trusted IPs only.
  • Disable Unnecessary Services: Turn off services and ports not essential for business operations to reduce the attack surface.

2. Removal

  • Isolate Infected System: Immediately disconnect the infected computer from the network (unplug Ethernet, disable Wi-Fi) to prevent the ransomware from spreading to other devices.
  • Do NOT Pay the Ransom: Paying the ransom does not guarantee decryption and funds criminal activities. There is also a risk of falling victim to further scams.
  • Boot into Safe Mode: Restart the computer in Safe Mode (with Networking, if needed for tool downloads). This loads only essential services, making it easier to remove the malware.
  • Run a Full System Scan: Use a reputable antivirus/anti-malware program (e.g., Malwarebytes, Bitdefender, ESET, Windows Defender in Safe Mode) with updated definitions to perform a deep scan and remove all detected threats. Multiple scans with different tools might be beneficial.
  • Check Startup Items and Task Scheduler: Manually inspect startup folders (shell:startup), registry run keys, and Task Scheduler for suspicious entries that might re-launch the ransomware.
  • Delete Ransom Note: Remove the _readme.txt files and any other ransom-related artifacts once the malware is purged.
  • Change All Passwords: Change passwords for all accounts accessed from the infected machine, especially for online services, email, and network shares, as some variants of Stop/Djvu are known to drop information-stealing malware (like RedLine Stealer, Vidar Stealer, or Raccoon Stealer).

3. File Decryption & Recovery

  • Recovery Feasibility: Decryption feasibility for .4035 (and other Stop/Djvu variants) depends on whether an “online” or “offline” encryption key was used:

    • Online Keys: If the victim’s machine was connected to the internet during encryption, the ransomware generates a unique online key for that specific victim and sends it to the attackers’ server. Decryption using online keys is currently not possible without the attacker’s private key, as each key is unique.
    • Offline Keys: If the victim’s machine was offline during encryption, the ransomware uses a static, pre-defined “offline” key. While this key is still unique to the ransomware variant, it is common across all victims infected while offline with that specific variant. There is a possibility that security researchers or ransomware incident response teams may have obtained or derived these offline keys over time.

  • Methods or Tools Available:

    • Emsisoft Decryptor for Stop/Djvu: Emsisoft, in collaboration with the No More Ransom! project, often develops decryptors for specific Stop/Djvu variants if their offline keys become known. It is crucial to check the Emsisoft Decryptor for STOP Djvu Ransomware page regularly (or the No More Ransom! website) to see if a decryptor for .4035 is available.
    • Data Recovery Software: For highly corrupted or partially encrypted files, data recovery software (e.g., PhotoRec, R-Studio) might be able to recover older, unencrypted versions from shadow copies or unallocated space, though success is limited.
    • System Restore/Shadow Copies: Attempt to use Windows System Restore (if enabled) to revert your system to a state prior to infection. Also, check for Shadow Volume Copies of your files. However, many ransomware variants, including Stop/Djvu, actively delete these to hinder recovery.
    • Cloud Backups: Restore files from legitimate cloud backup services (e.g., Google Drive, Dropbox, OneDrive, iCloud) if they were backed up before encryption.

  • Essential Tools/Patches:

    • Emsisoft Decryptor for STOP Djvu Ransomware: Specifically designed for this family.
    • Reputable Antivirus/Anti-malware Software: For detection and removal.
    • Windows Security Updates: Ensure OS is fully patched.
    • Browser Security Extensions: To block malicious ads and scripts.
    • File Recovery Software: As a last resort for data recovery attempts.

4. Other Critical Information

  • Additional Precautions (Unique Characteristics):

    • Information Stealer Component: A significant distinguishing characteristic of many Stop/Djvu variants, including potentially .4035, is that they often drop and execute additional malware, specifically information-stealing trojans (e.g., Vidar Stealer, RedLine Stealer, Raccoon Stealer, Azorult). This means that even if files are not decryptable, the attackers may have already exfiltrated sensitive data such as login credentials, cryptocurrency wallet information, browser history, and more. This necessitates immediate password changes for all online accounts and monitoring of financial activities.
    • Fake Decryptor Scams: Victims are often targeted by scammers posing as decryptor developers or cybersecurity experts, offering fake decryption tools that are themselves malware or simply charge money without providing a solution. Always rely on reputable sources like Emsisoft, No More Ransom!, or official cybersecurity firms.
    • Offline vs. Online Key Dilemma: The unpredictable nature of whether an offline or online key was used makes definitive decryption advice difficult without analyzing the infected system or the ransom note.
    • Relentless Evolution: The Stop/Djvu family is known for its high rate of new variants, making it a persistent and challenging threat to track and mitigate.

  • Broader Impact:

    • Widespread Consumer Impact: Due to its primary distribution via cracked software, Stop/Djvu (and thus variants like .4035) disproportionately affects individual consumers and small businesses that might engage in software piracy, making it one of the most common ransomware families encountered by the general public.
    • Data Theft Implications: The inclusion of information stealers elevates the risk from mere data encryption to direct data breaches and identity theft, complicating recovery efforts and increasing long-term risks for victims.
    • Trust Erosion: The use of deceptive tactics like cracked software and fake updates erodes trust in online content and software downloads, forcing users to be more vigilant.
    • Financial Strain: For victims who lose critical data without backups, the financial and emotional toll can be substantial, even if they don’t pay the ransom. Businesses can face significant downtime and reputational damage.

By understanding these technical details and implementing the recommended strategies, individuals and organizations can significantly reduce their risk of infection by .4035 and improve their chances of recovery if compromised.