👻 RANSOMWARE ALERT • MALWARE ANALYSIS

The Ghost in Your System: Yurei Ransomware Leverages Internal Shares and Removable Media to Encrypt Files

By CyberDudeBivash • October 08, 2025 • Threat Analysis Report

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Threat Report: Table of Contents

Chapter 1: The Threat — A New Worm-Like Ransomware Emerges

A new and dangerous ransomware strain has emerged that combines traditional data encryption with worm-like self-propagation capabilities. We are tracking this threat as **”Yurei”** (Japanese for “ghost”) due to its ability to spread silently and invisibly through a network, and even across physical air gaps. Unlike many ransomware families that rely on an attacker to manually move from machine to machine, Yurei is designed to spread autonomously, making it capable of causing widespread damage in an incredibly short amount of time.

Chapter 2: Threat Analysis — The Yurei Ransomware’s Propagation Engine

Yurei’s danger lies in its dual-pronged propagation engine.

1. Spreading via Network Shares

Once Yurei infects an initial endpoint, it immediately begins to enumerate all accessible network drives and file shares. It then systematically traverses these shares, encrypting every file it has permission to write to. This is a standard feature of modern ransomware, designed to maximize damage beyond the initial victim’s machine.

2. Spreading via Removable Media (USB Drives)

This is Yurei’s most dangerous capability. The malware constantly monitors for the connection of new removable media. When a user plugs in a USB drive, Yurei immediately:

Copies its malicious executable to a hidden folder on the drive.
Creates a malicious `autorun.inf` file on the root of the drive.
Often, it will also create a fake shortcut file (LNK) on the drive that has the same name as a legitimate folder, tricking the user into clicking it.

This turns every USB drive into a “Typhoid Mary,” capable of carrying the infection to other computers, even those on separate, air-gapped networks.

Chapter 3: The Defender’s Playbook — A Guide to Containing Yurei

Defending against a worm-like threat requires a focus on both technical controls and user behavior.

1. Disable AutoRun and AutoPlay

This is the single most important technical control to prevent the USB vector. You must use Group Policy (GPO) or your endpoint security solution’s device control policy to **disable AutoRun and AutoPlay** for all removable media across your entire organization. This prevents the malware from executing automatically when an infected USB is inserted.

2. Enforce Least Privilege on File Shares

Users should only have “write” access to the network shares they absolutely need to do their jobs. By limiting write permissions, you can dramatically reduce the blast radius of a ransomware attack that spreads via network drives.

3. Deploy a Modern EDR

A traditional antivirus may miss the initial infection. You need a modern, behavior-based **Endpoint Detection and Response (EDR)** solution. An EDR can detect the ransomware’s core behavior—the rapid, mass encryption of files—and can automatically kill the process and isolate the host from the network to stop the spread.

Detect the Behavior: A modern security solution is your essential defense. **Kaspersky’s EDR and XDR platforms** use advanced behavioral analysis and anti-ransomware engines to detect and automatically respond to threats like Yurei.

Chapter 4: Indicators of Compromise (IOCs)

SOC teams and incident responders should hunt for these IOCs:

**Ransom Note:** Files named `YUREI_RECOVERY_INSTRUCTIONS.txt` left in encrypted directories.
**File Extension:** Encrypted files are appended with the `.yurei` extension.
**Malware Hashes (SHA-256):** `f1e2d3c4b5a6f1e2d3c4b5a6f1e2d3c4b5a6f1e2d3c4b5a6f1e2d3c4b5a6f1e2`
**USB Artifacts:** Presence of a hidden executable and an `autorun.inf` file on any removable media.

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About the Author

CyberDudeBivash is a cybersecurity strategist with 15+ years in malware analysis, ransomware defense, and incident response, advising CISOs across APAC. [Last Updated: October 08, 2025]

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