Netsky

Overview

Worm:Win32/Netsky is a historically monumental, prolific mass-mailing computer worm that emerged in February 2004, causing tens of millions of dollars in global network congestion and computer disruptions. Authored by 17-year-old German high school student Sven Jaschan (who simultaneously authored the devastating Sasser worm), Netsky spread across the internet with unprecedented speed. The worm harvests email addresses from cached files across victim hard drives (.html, .eml, .txt, .doc) and utilizes its own built-in SMTP engine to send deceptive emails with forged sender addresses and enticing subject lines (such as 'Re: Details', 'Your password', or 'Delivery status'). Netsky also spreads across local network shares and peer-to-peer (P2P) file-sharing directories (such as Kazaa and eMule) using deceptive executable filenames. Notably, Jaschan programmed Netsky to wage a digital turf war against competing malware families Bagle and Mydoom, actively seeking out and removing Bagle and Mydoom registry entries and files from infected computers. In May 2004, German police in Lower Saxony arrested Jaschan following an international tip facilitated by Microsoft's Anti-Virus Reward Program ($250,000 bounty), leading to Jaschan's conviction for computer sabotage.

Known aliases

Threat reports may refer to this family under multiple names:

• Netsky
• Worm.Win32.Netsky
• W32/Netsky
• Win32.Netsky
• Moodown

Threat Actor Attribution

Attribution is probabilistic, not a science. Each link below states its confidence level and the evidence behind it.

• Sven Jaschan (Author, Germany) High confidence
  Basis:

Family Pedigree & Lineage

Malware authors regularly fork and iterate upon existing codebases. Observed upstream lineage and downstream variants for this family include:

Known Derivatives & Downstream Variants: Netsky.A · Netsky.B · Netsky.C · Netsky.D (Peak Outbreak) · Netsky.P · Netsky.Z

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:

• T1059.003: Restrict execution of Windows Command Shell (cmd.exe) and block unauthorized batch scripts.
• T1071.003: Monitor network traffic for anomalous application layer protocols like non-standard HTTP/S patterns or unexpected DNS requests.
• T1566.002: Inspect email links for known malicious domains and use link-rewriting services for time-of-click analysis.

MITRE D3FEND™ Defensive Countermeasures

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

Exploited Vulnerabilities (CVEs)

This malware family is documented weaponizing the following common vulnerabilities and exposures:

• CVE-2001-0154 — In-depth exploitation analysis and catalog intelligence (NIST NVD ↗).
• CVE-2004-0115 — In-depth exploitation analysis and catalog intelligence (NIST NVD ↗).

Command & Control (C2) and Network Profile

How this family talks to its operators. C2 traffic can often identify a family even when its code and configuration are hard to analyse. Ports and patterns are historical and change between campaigns, so use them as hunting leads rather than block-only rules.

Indicators of Compromise

Public indicators drawn from CISA advisories and vendor reporting. These are historical and intended for retrospective threat hunting; current campaigns may use different infrastructure. SystemHelpDesk adheres to responsible disclosure and does not host live samples.

File hashes (SHA-256, sample)

Cryptographic hashes serve as reference indicators for triage and detection rules. In accordance with responsible threat intelligence disclosure, this catalog indexes reference metadata and does not host or distribute live malware binaries.

• 3333333333333333333333333333333333333333333333333333333333333333
• 4444444444444444444444444444444444444444444444444444444444444444

Delivery vectors

• mass_mailing_smtp
• p2p_file_sharing

Persistence mechanisms

• HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
• Win.ini run=

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_NETSKY_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Netsky"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "netsky" ascii wide nocase
        $s2 = "netsky" ascii wide nocase
        $s3 = "worm.win32.netsky" ascii wide nocase
        $s4 = "w32/netsky" ascii wide nocase
        $s5 = "win32.netsky" ascii wide nocase
        $s6 = "moodown" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Netsky (Worm) Execution & Triage
id: e3cc925b-5edd-5997-b251-534eaf8e953a
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Netsky malware family. Detects suspicious execution from temporary user
  directories with hidden window flags and background transfer utilities.
references:
- https://malware-families-catalog.systemhelpdesk.com/families/netsky
- 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.worm
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1566_002
- attack.t1114_001
- attack.t1021_002
- attack.t1071_003
- attack.t1560_001
logsource:
  category: process_creation
  product: windows
detection:
  selection_process:
    Image|endswith:
    - \cmd.exe
    - \powershell.exe
    - \cscript.exe
    - \wscript.exe
    CommandLine|contains:
    - AppData\Local\Temp
    - -WindowStyle Hidden
    - /e:jscript
    - bitsadmin /transfer
  condition: selection_process
falsepositives:
- Legitimate enterprise administrative scripts during approved change windows
- Authorized IT software deployment utilities and backup management agents
level: medium

rule MALWARE_WIN_NETSKY_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Netsky 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 = "netsky" 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. Block outbound direct port 25 SMTP traffic from user endpoints to stop worm propagation.
2. Deploy email gateway filters inspecting executable attachments with double extensions (.pif, .scr, .exe).
3. Disable open writable network shares across corporate subnets.
4. Apply security patch MS04-011 for LSASS vulnerability CVE-2004-0115.

What to Avoid

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

1. Do not allow direct outbound port 25 access from client subnets without relaying through trusted mail servers.
2. Avoid opening unsolicited email attachments ending in .pif, .scr, or .cmd.
3. Do not operate unauthenticated file-sharing services across untrusted networks.

References & External Analysis

• Microsoft DCU: Sven Jaschan Anti-Virus Reward Case Summary
• German Federal Criminal Police Office (BKA) Netsky Investigation
• NIST National Vulnerability Database CVE-2004-0115
• Search "netsky" on VirusTotal (External Analysis)
• Search "netsky" on Any.Run (Sandbox Sandbox)
• Search "netsky" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Worm)

Explore other documented threats in the same operational category:

• Allaple
• Chir
• Skypespam
• Chindo
• Cambot
• Vbs_Worm_Generator
• Brambul
• Pluto

Machine-readable

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