{
  "family": "silly",
  "sample_count": 1,
  "category": "ransomware",
  "description": "<h3>Executive Summary</h3>\nSilly (often referred to as SillyFDC or Worm.Silly) is a prolific family of legacy worms that primarily spread via removable media, most notably USB flash drives. While considered a \"classic\" threat, Silly remains highly prevalent in industrial control systems (ICS), operational technology (OT) environments, and air-gapped networks where USB drives are frequently used to transfer data.\n\n<h3>Propagation and Execution Lifecycle</h3>\nSilly's primary objective is rapid, silent propagation across local networks and physical media.\n\nIts execution lifecycle relies heavily on abusing Windows features:\n<ul>\n<li><strong>USB AutoRun Exploitation:</strong> When an infected USB drive is inserted into a Windows machine, the worm relies on the `AutoRun.inf` feature (or exploits vulnerabilities in how Windows handles shortcut `.lnk` files) to automatically execute its payload without any user interaction.</li>\n<li><strong>System Modification and Hiding:</strong> Once active on the host, Silly copies itself to the Windows System directory and modifies the Registry to ensure it starts upon reboot. Crucially, it alters Windows Explorer settings to hide file extensions and hidden system files, ensuring the user cannot see the malicious executables on the USB drive.</li>\n<li><strong>Continuous Infection Cycle:</strong> The worm continuously monitors the system for new removable drives. When a clean USB drive is inserted, Silly immediately copies itself to the drive, creates a malicious `AutoRun.inf` file, and waits for the drive to be plugged into a new victim machine.</li>\n</ul>\n\n<h3>Threat Assessment</h3>\nWhile Silly rarely contains highly destructive payloads (like ransomware), its presence is a massive operational disruption. It can rapidly saturate networks, corrupt files on USB drives, and serve as a \"dropper\" for more severe malware. Furthermore, its ability to easily bridge \"air-gapped\" networks makes it a significant threat in critical infrastructure environments.\n\n<h3>Eradication and Incident Response</h3>\n<ul>\n<li><strong>Disable AutoRun/AutoPlay:</strong> The most critical defense against Silly is enforcing strict Group Policy Objects (GPOs) that entirely disable AutoRun and AutoPlay functionality across all drives in the enterprise.</li>\n<li><strong>USB Device Control:</strong> Implement endpoint security policies that restrict or completely block the use of unauthorized USB storage devices.</li>\n<li><strong>Registry Remediation:</strong> Removing the executable is insufficient. Incident response must include repairing the registry keys altered by the worm to restore the visibility of hidden files and folders, allowing users to verify their drives are clean.</li>\n</ul>",
  "cta": "Published by the SystemHelpdesk team.",
  "aliases": [
    "Worm.SillyFDC",
    "W32/Silly",
    "AutoRun.Silly"
  ],
  "enrichment_level": "insufficient_information",
  "faq": [],
  "faq_count": 0,
  "mitre_attack": [
    "T1091",
    "T1547.001",
    "T1564.001"
  ],
  "cisa_advisory": null,
  "last_updated": "2026-07-01T16:59:16Z",
  "type": "Worm",
  "target_industries": [
    "Global / Opportunistic"
  ],
  "motivation": "Opportunistic",
  "threat_actors": [
    "Unknown / Cybercriminal"
  ],
  "target_geographies": [
    "Global"
  ]
}