GootLoader Malware Evades Detection Using Nested ZIP Archives

TL;DR

GootLoader is back, leveraging malformed ZIP archives to bypass security defenses and serve as an initial access broker for ransomware. This malware uses deliberately corrupted ZIP structures, making them difficult for security tools to unpack but easily accessible to victims. It also employs techniques like hashbusting and custom encoding to evade detection, highlighting the need for advanced threat mitigation strategies.

GootLoader's Evasive Techniques: A Deep Dive

GootLoader has resurfaced with advanced techniques to bypass modern security systems, acting as an initial access broker for ransomware attacks. This malware uses deceptive ZIP archives to compromise systems while evading detection. GootLoader is often distributed through SEO poisoning and malvertising, targeting users searching for business or legal document templates, leading them to compromised WordPress sites.

Malformed ZIP Archives: Evading Detection

The malware is delivered within ZIP archives that are deliberately malformed to confuse security tools. These archives contain 500-1,000 concatenated ZIP files, making it difficult for traditional security scanners to extract the contents. According to Expel, most unarchiving tools like 7-Zip and WinRAR fail to extract the contents, while the default Windows unarchiver opens it reliably. This ensures victims can execute the payload while defenders struggle to analyze it. The structure includes truncated sections and randomized values in critical fields, causing parsing errors. More on ZIP file structure#End_of_central_directory_record_(EOCD)).

A visual breakdown of a ZIP archive file’s structure (Source - Expel)

Image courtesy of Expel

Infection Mechanism and Persistence

Once the malicious ZIP file is opened, a JScript file embedded within executes automatically. The script establishes persistence by creating link files in the user’s Startup folder. These links point to a second JScript file stored in a random directory, ensuring the malware reactivates with every system restart. The JScript then spawns PowerShell with heavily obfuscated commands to download secondary payloads.

The well-formed ZIP archive loaded and parsed by a pattern in ImHex (Source - Expel)

Image courtesy of Expel

Evasion Techniques: Hashbusting and Encoding

GootLoader employs a technique called hashbusting, where every downloaded file contains unique characteristics. Each victim receives a completely different archive structure with randomized field values, making signature-based detection virtually impossible. The ZIP archive is delivered as an XOR-encoded blob, decoded and repeatedly appended on the client-side until it meets a set size. Details on XOR encoding. This bypasses security controls designed to detect ZIP file transmissions. The use of custom WOFF2 fonts with glyph substitution further obfuscates filenames.

Mitigation Strategies

To defend against GootLoader, organizations should implement several mitigation strategies:

Prevent JScript execution through Group Policy Objects (GPO) by reassociating .js files to Notepad instead of Windows Script Host.
Monitor for suspicious PowerShell process chains and detect NTFS shortname usage during script execution.
Scan for malformed ZIP structures using specialized YARA rules.
Restrict or block wscript and cscript if not needed.

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