GootLoader malware utilizes malformed ZIP files consisting of hundreds of concatenated archives to bypass security tools while remaining functional on Windows systems. This initial access tool is frequently employed by ransomware groups to compromise environments before deploying secondary payloads like Cobalt Strike or Rhysida.
GootLoader operates on an access-as-a-service model where the threat group UNC2565 provides initial entry for various cybercriminal affiliates. Over the years, it has evolved from distributing banking trojans to serving as a primary delivery mechanism for high-profile ransomware such as REvil and SunCrypt. By masquerading as legitimate documents or freeware installers, the malware tricks users into initiating the infection process on their workstations.
The malware has recently resurfaced with advanced evasion techniques specifically designed to frustrate automated analysis and security software. The primary innovation involves a first-stage loader that uses a highly unusual ZIP file structure. This malformed archive is intentionally broken in a way that prevents common third-party tools like 7-Zip or WinRAR from opening it, yet it remains perfectly accessible to the native Windows unarchiver used by most victims.
Technically, the delivery file is created by gluing together between 500 and 1,000 individual ZIP archives into a single file. Because the ZIP format is read from the end of the file, Windows can still identify the directory and extract the malicious JScript hidden inside. This concatenation method, combined with damaged and randomized metadata, effectively hides the payload from many signature-based scanners and automated sandboxes that cannot parse the complex structure.
To further increase its success rate, each GootLoader ZIP file is unique and generated at the moment of download. This prevents security teams from using static file hashes or fingerprints to block the threat across multiple systems. By building the file from encoded data directly on the victim's system, the actors also avoid detection by network-level monitors that look for known malicious file patterns.
The resilience of GootLoader highlights a shift in the threat landscape where actors prioritize anti-analysis techniques over simple obfuscation. Because the malware evades traditional file-based defenses, security professionals must rely on behavioral monitoring to catch the script when it finally executes. This ongoing development ensures that GootLoader remains a significant component of the ransomware ecosystem, accounting for a notable percentage of successful security bypasses.
Source: GootLoader Uses Malformed ZIP Files to Bypass Security Controls