Sillyfdc

Overview

Worm:Win32/Sillyfdc is a prolific and persistent worm designed to propagate rapidly across networks by infecting removable drives (USB flash drives). First emerging in the mid-2000s, variants of Sillyfdc remain highly prevalent in environments that rely heavily on physical media transfer, such as industrial control systems (ICS), educational institutions, and offline networks. Its primary mechanism relies on abusing the Windows Autorun/Autoplay functionality (`T1091`).

Understanding Sillyfdc (USB Worms)
To an end-user, the infection is often noticed when their USB drive suddenly contains `.lnk` (shortcut) files instead of their actual folders, or when the drive behaves erratically. For a security administrator, a Sillyfdc outbreak is a nightmare, capable of bridging air-gapped networks and causing massive, noisy file modifications across the enterprise.

Execution and Propagation Mechanics
Sillyfdc spreads via Replication Through Removable Media (`T1091`). When an infected USB drive is plugged into a Windows machine, the worm executes (historically via `autorun.inf`, or modernly by tricking the user into clicking a malicious shortcut). Once on the host, it establishes persistence by copying itself to `%SystemRoot%` or `%AppData%` and creating a Registry Run key (`T1547.001`). The worm then monitors the system for new USB drives. When a clean drive is inserted, Sillyfdc immediately copies its executable to the drive, hides all legitimate user folders on the drive (`T1564.001`), and creates malicious `.lnk` shortcuts with the exact names of the hidden folders. When a user on a *different* machine clicks the shortcut to access their files, they inadvertently execute the worm first, spreading the infection.

Indicators of Compromise & Impact
The impact is widespread infection, potential data loss (if the worm malfunctions or includes a destructive payload), and a massive drain on IT resources to clean the environment. Host-based IoCs include the presence of hidden folders on USB drives replaced by `.lnk` files pointing to a hidden executable (e.g., `cmd.exe /c start hidden_worm.exe`). EDR alerts will trigger on executables launching from the root directory of removable media or making anomalous modifications to `autorun.inf`.

MITRE ATT&CK Techniques

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

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_SILLYFDC {
    meta:
        description = "Detects Sillyfdc (advanced_threat)"
        author = "SystemHelpdesk Boilerplate Generator"
        date = "2026-07-06"
    strings:
        $s1 = "sillyfdc" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

Sigma Rule

title: Suspicious Sillyfdc Activity
id: 707308ea46e26c681a14296b7489306c
status: experimental
description: Detects generic indicators of the sillyfdc malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*sillyfdc*"
    condition: selection
level: medium

Containment & Response Steps

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

1. Implement an enterprise-wide Group Policy Object (GPO) to completely disable Autoplay/Autorun for all removable media.
2. Use EDR to isolate all infected machines and identify the specific executable name the worm is currently using.
3. Instruct users to enable 'Show hidden files and folders' in Windows Explorer to identify if their USB drives have been compromised.
4. Run a specialized script or AV tool across the fleet to delete the malicious `.lnk` files and unhide the legitimate user directories on USB drives.

What to Avoid

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

1. Do not click on folders on a USB drive if they display a 'shortcut' arrow icon; these are almost certainly malicious links created by the worm.
2. Avoid relying solely on host-based AV; if USB usage policies are not strictly enforced, the worm will continue to re-introduce itself.

References & External Analysis

• Search "sillyfdc" on VirusTotal (External Analysis)

Frequently Asked Questions

Related Families (Category: advanced_threat)

Explore other malware families in the same category:

• Rpcdcom
• Ytddownloader
• Uptodown
• Ytdownloader
• Enigmaprotector

Machine-readable

Get this profile as JSON: https://jordanricky1604-ship-it.github.io/malware-families-catalog/api/sillyfdc.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.