Gootloader Malware Adopts 1,000-Part ZIP Archives for Evasion Tactics

Gootloader Malware Leverages Fragmented ZIP Archives for Stealth

Security researchers have identified a new evasion technique employed by the Gootloader malware, a first-stage payload commonly used to gain initial access to corporate networks. The malware now utilizes malformed ZIP archives split into up to 1,000 parts, a method designed to bypass traditional detection mechanisms.

Technical Details of the Evasion Technique

Gootloader, known for its use in SEO poisoning attacks, has historically relied on compressed archives to deliver malicious payloads. However, this latest iteration introduces a fragmented ZIP structure, where the archive is intentionally split into multiple smaller files. This approach exploits weaknesses in security tools that may fail to reconstruct or scan the full archive, allowing the malware to evade detection.

Key characteristics of the new method include:

• Concatenated ZIP archives: The payload is split into 1,000+ parts, each appearing as a separate file.
• Malformed structure: The archives are deliberately crafted to avoid standard validation checks.
• Stealthy execution: Once reassembled, the ZIP executes the Gootloader payload, which then proceeds with its infection chain.

Impact on Security Defenses

The use of fragmented archives poses significant challenges for endpoint protection platforms (EPP) and sandboxing solutions, which may not effectively analyze split files. Organizations relying on signature-based detection or static analysis could see reduced efficacy against this variant.

Additionally, the technique complicates incident response (IR) efforts, as security teams may struggle to identify and reconstruct the full attack chain. Given Gootloader’s role in initial access brokering (IAB), this evolution could lead to an increase in ransomware deployments and data exfiltration incidents.

Recommendations for Security Teams

To mitigate risks associated with this new Gootloader variant, security professionals should:

• Enhance archive scanning: Ensure security tools can reconstruct and analyze fragmented archives.
• Implement behavioral detection: Monitor for unusual file concatenation or reassembly activity.
• Update threat intelligence: Incorporate indicators of compromise (IOCs) related to Gootloader’s latest tactics.
• Educate employees: Reinforce awareness of SEO poisoning and malicious download risks.

As Gootloader continues to evolve, organizations must adapt their defenses to address file-based evasion techniques and multi-stage attack vectors.