Gootloader Malware Employs Massive Concatenated ZIP Archives for Evasion

Key Findings

• GootLoader malware is using a malformed ZIP archive with 500-1,000 concatenated ZIP files to evade detection

• The malicious ZIP file is designed to trigger parsing errors in many unarchiving tools, but can still be extracted by the default Windows unarchiver

• GootLoader employs "hashbusting" techniques by randomizing values in non-critical ZIP file fields to generate unique payloads for each victim

• The attack involves delivering the malicious ZIP as an XOR-encoded blob, which is decoded and repeatedly appended on the client-side to bypass security controls

Background

GootLoader is a JavaScript (JScript) malware loader that has been active since at least 2020. It typically spreads via search engine optimization (SEO) poisoning or malvertising, luring users to compromised WordPress sites hosting malicious ZIP archives.

Technical Details

• The malicious ZIP file is crafted by concatenating anywhere from 500 to 1,000 individual ZIP archives together

• This malformed structure is designed to trigger parsing errors in many unarchiving tools, preventing automated analysis

• However, the default Windows unarchiver can still successfully extract the payload, allowing victims to run the embedded JavaScript malware

• GootLoader also employs "hashbusting" techniques, randomizing values in non-critical ZIP file fields like disk number and number of disks

• This ensures that each downloaded ZIP file is unique, making it difficult to detect the malware using signature-based approaches

Impact and Mitigation

• The sophisticated obfuscation methods used by GootLoader make it challenging for security tools to detect and analyze the malware

• To mitigate the threat, organizations are advised to consider blocking "wscript.exe" and "cscript.exe" from executing downloaded content, and configure GPO to open JavaScript files in Notepad by default

Conclusion

The GootLoader actors continue to evolve their delivery tactics, leveraging a "ZIP bomb" technique to craft a malformed archive that can bypass many security controls. This highlights the need for organizations to stay vigilant and implement robust security measures to protect against such advanced malware threats.

Sources

• https://thehackernews.com/2026/01/gootloader-malware-uses-5001000.html

• https://www.bleepingcomputer.com/news/security/gootloader-now-uses-1-000-part-zip-archives-for-stealthy-delivery/

• https://cyberinsider.com/gootloader-malware-now-uses-zip-bomb-tactic-to-evade-detection/