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 our technical analysis hubs and the official 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.

MITRE D3FEND™ Defensive Countermeasures

Defensive techniques mapped to counter the offensive tradecraft observed in this family (via the MITRE D3FEND matrix):

Tiered Detection Rules (Cisco CAIRN Model)

Modern threat detection requires multi-layered rule engineering. We classify detections into three operational tiers based on signal fidelity:

rule MALWARE_WIN_NEMUCOD_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Nemucod"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "nemucod" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Nemucod (Ransomware) Execution & Triage
id: a9396799-fa2d-53ed-b2c6-56f03d701486
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Nemucod malware family. Detects volume shadow copy deletion, boot status
  tampering, and recovery inhibition characteristic of modern ransomware operations.
references:
- https://malware-families-catalog.systemhelpdesk.com/families/nemucod
- https://malware-families-catalog.systemhelpdesk.com/defensive/ransomware-protection
- https://malware-families-catalog.systemhelpdesk.com/
author: Ricky Jordan, Lead Threat Intelligence Analyst, SystemHelpDesk MSP
date: '2026-10-14'
modified: '2026-10-14'
tags:
- attack.ransomware
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1105
- attack.t1566_001
- attack.t1059_007
- attack.t1027
logsource:
  category: process_creation
  product: windows
detection:
  selection_process:
    Image|endswith:
    - \vssadmin.exe
    - \wmic.exe
    - \wbadmin.exe
    - \bcdedit.exe
    CommandLine|contains:
    - delete shadows
    - shadowcopy delete
    - resize shadowstorage
    - recoveryenabled no
    - ignoreallfailures
  condition: selection_process
falsepositives:
- Legitimate enterprise administrative scripts during approved change windows
- Authorized IT software deployment utilities and backup management agents
level: critical

rule MALWARE_WIN_NEMUCOD_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Nemucod tradecraft"
        tier = "T3 - Attribution Fingerprint"
        confidence = "High (Confirmed Family Signature)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        // Core family identifier & heuristic payload signatures
        $attrib_str = "nemucod" ascii wide nocase
        $pe_magic = "MZ" ascii
        $net_marker = "http" ascii nocase
    condition:
        uint16(0) == 0x5a4d and ($attrib_str and $net_marker) and filesize < 15MB
}

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)
• Search "nemucod" on Any.Run (Sandbox Sandbox)
• Search "nemucod" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Ransomware)

Explore other documented threats in the same operational category:

• Ransim
• Bridex
• Xxycff
• Hptrickbot
• Penzievs
• Sfdld
• Shufab
• Ctqc

Machine-readable

Get this profile as JSON: https://malware-families-catalog.systemhelpdesk.com/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.