Nemucod

Overview

TrojanDownloader:JS/Nemucod (or VBS/Nemucod) is one of the most prolific and historically significant malware downloaders. Primarily distributed via massive spam campaigns, Nemucod arrives as a tiny, heavily obfuscated JavaScript (`.js`) or VBScript (`.vbs`) file hidden inside a ZIP archive. Its sole purpose is to execute via the Windows Script Host (`wscript.exe`), silently connect to a compromised website, and download a catastrophic secondary payload—most famously, ransomware families like Locky, Cerber, or TeslaCrypt (`T1105`).

Understanding Nemucod (Script Downloaders)
To an end-user, they receive a fake invoice or shipping notification, extract the ZIP, double-click the seemingly harmless file, and see nothing happen. Minutes later, their files are encrypted. For a security team, Nemucod represents the danger of allowing script execution from user directories. It is an 'Initial Access Broker' that paves the way for total data loss.

Execution and Dropper Mechanics
The infection chain relies on social engineering (`T1566.001`) to trick the user into extracting and running the script. Because Windows associates `.js` and `.vbs` files with `wscript.exe` by default, double-clicking them executes the code (`T1059.007`). The script itself is heavily obfuscated (using string concatenation, random variables, and encoding) to evade static email filters (`T1027`). Once running, it uses COM objects (like `MSXML2.XMLHTTP`) to download the secondary payload (usually an `.exe` or a `.dll`) to the `%Temp%` folder, and then executes it.

Indicators of Compromise & Impact
The impact is almost always a severe secondary infection (Ransomware or Banking Trojans). Host-based IoCs include EDR alerts for `wscript.exe` or `cscript.exe` executing from within zip archives or `%Temp%` directories, and `wscript.exe` spawning unexpected child processes (like `cmd.exe` or dropping unknown executables). Network IoCs involve the `wscript.exe` process making outbound HTTP GET requests to compromised WordPress sites or unknown domains to fetch the payload.

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to the MITRE ATT&CK framework:

Tactical Mitigations

Based on the techniques used by this family, consider the following defensive strategies:

• T1059.007: Restrict execution of scripting languages such as PowerShell, VBScript, or Python to authorized administrators. Enforce Script Block Logging.
• T1105: Implement network intrusion detection systems (NIDS) and host-based firewalls to block unauthorized inbound or outbound file transfers.
• T1566.001: Scan email attachments for malicious macros, scripts, or suspicious archive files.

Generated Detections (Boilerplate)

These YARA and Sigma rules are auto-generated based on the family name and aliases. They must be heavily tuned before deployment in a production environment.

YARA Rule

rule MALWARE_WIN_NEMUCOD {
    meta:
        description = "Detects Nemucod (ransomware)"
        author = "SystemHelpdesk Boilerplate Generator"
        date = "2026-07-06"
    strings:
        $s1 = "nemucod" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

Sigma Rule

title: Suspicious Nemucod Activity
id: 2102339bdf9a56513e92109ea57834d5
status: experimental
description: Detects generic indicators of the nemucod malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*nemucod*"
    condition: selection
level: medium

Containment & Response Steps

Ordered checklist for responders. Adapt to your environment and engage professional support for active incidents.

1. Isolate the endpoint *immediately*; if Nemucod has executed, a ransomware payload is likely actively encrypting files.
2. Use EDR to trace the execution chain: locate the original spam email, the extracted script, and the domains it contacted.
3. Block the identified payload delivery domains at the corporate firewall to prevent other users who received the spam from downloading the ransomware.
4. Change the default Windows file association for `.js` and `.vbs` files to open with Notepad instead of `wscript.exe` via Group Policy (GPO).

What to Avoid

Common mistakes during response to this family that can destroy evidence, spread the infection, or worsen recovery.

1. Do not allow `wscript.exe` to execute scripts originating from `%Temp%` or `%Downloads%` directories without strict application control.
2. Avoid relying solely on email attachment scanning, as the obfuscated scripts are often designed to bypass static signature checks.

References & External Analysis

• Search "nemucod" on VirusTotal (External Analysis)

Frequently Asked Questions

Related Families (Category: ransomware)

Explore other malware families in the same category:

• Sbyinying
• Cmy3U
• Dungcoi
• Xkit
• Avkiller

Machine-readable

Get this profile as JSON: https://jordan123234-malware-families-explorer.static.hf.space/api/nemucod.json

Ecosystem & Interactive Environments

This profile is part of the Malware Families Catalog, a public dataset of 2,899 malware families. The catalog is also published across our ecosystem: Hugging Face, Kaggle, Zenodo, Replit, StackBlitz, CodeSandbox, and CodePen.