Gootloader Malware Uses Malformed 1,000-Segment ZIP Files for Stealthy Payload Delivery

CyberDefender 3 mins0

Gootloader — a JavaScript-based malware loader used for initial access and follow-on payload delivery — is now using an unusual file format trick to evade detection: it packages malicious code inside a single ZIP file composed of up to 1,000 concatenated ZIP archives. This malformed archive structure is designed to break or confuse many common analysis and detection tools.

How the 1,000-Part ZIP Trick Works

  • Concatenation of many ZIPs: The malware creator constructs a massive file by chaining 500–1,000 standard ZIP archives end-to-end. This isn’t how typical ZIP files are structured.
  • Malformed structure: The file intentionally has an incorrect End of Central Directory record and inconsistent metadata, causing many unpackers (especially 7-Zip, WinRAR, scanning tools) to fail or crash, while Windows Explorer’s built-in ZIP handling still successfully extracts the payload.
  • Anti-analysis: Many security tools rely on proper ZIP formatting to inspect file contents. By breaking ZIP indexing but leaving enough intact that Windows can still extract the malicious JScript inside, Gootloader avoids detection and deep inspection.

Why This Matters

  • Initial access and subsequent payloads: Gootloader itself isn’t usually the end payload; it provides an entry point for other threats like ransomware, Cobalt Strike or other malware loaders.
  • Unique obfuscation: Beyond the multi-part ZIP trick, the operators also randomize parts of the archive and generate unique samples on every download to evade signature-based defenses.
  • Threat actor evolution: This change follows Gootloader’s return after a pause earlier in 2025, where researchers also observed other evasion techniques being introduced.

Defense Recommendations (General Best Practices)

While specific mitigation advice for this exact ZIP technique hasn’t been widely published yet, security researchers typically recommend:

  • Don’t open unexpected archives — especially if they appear to be ZIP files with unusual filenames or from untrusted sources.
  • Use robust analysis tooling — tools capable of deeper behavioural inspection rather than just structural parsing.
  • Enhance endpoint inspection — modern EDR and XDR solutions can catch execution behaviour even if archive parsing fails.

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