GootLoader Malware Exploits Windows ZIP Handling with 1,000-Part Nested Archives to Evade Detection

Executive Summary

GootLoader is a highly adaptive malware loader that has recently advanced its evasion capabilities by distributing malicious payloads within 500–1,000 concatenated ZIP archives. This sophisticated technique is engineered to bypass modern security solutions and significantly hinder manual analysis, representing a critical threat to organizations across multiple sectors. The loader is primarily used as an initial access vector, often facilitating the deployment of ransomware, Cobalt Strike, and other high-impact secondary payloads. The unique use of malformed, deeply nested ZIP files, combined with advanced obfuscation and hashbusting, makes GootLoader one of the most challenging threats to detect and remediate in the current threat landscape.

Threat Actor Profile

The operators behind GootLoader are a financially motivated cybercriminal group, frequently associated with the Gootkit group. While there is no confirmed nation-state attribution, the group is known for its opportunistic targeting of legal, healthcare, financial, technology, manufacturing, education, and government sectors. Their campaigns have a global reach, with a particular focus on North America, Europe, and Australia. The group leverages advanced social engineering, SEO poisoning, and malvertising to maximize infection rates, and is recognized for rapidly evolving its tactics, techniques, and procedures (TTPs) to evade detection and maximize impact.

Technical Analysis of Malware/TTPs

GootLoader’s infection chain begins with SEO poisoning and malvertising, which direct users searching for business or legal document templates to compromised WordPress sites. These sites host ZIP files named to match the victim’s search query, increasing the likelihood of download and execution. The ZIP files themselves are highly engineered: each contains 500–1,000 concatenated ZIP archives, with the final malicious payload—a heavily obfuscated JavaScript file—deeply nested within.

The evasion strategy relies on several technical innovations. The ZIP archives are malformed, with truncated End of Central Directory (EOCD) records and randomized non-critical fields such as disk number and number of disks. This causes most unarchiving tools, including WinRAR and 7-Zip, to fail when parsing the files. Only the default Windows Explorer ZIP handler can reliably extract the payload, which is a deliberate abuse of Windows’ permissive ZIP parsing. Each archive is unique due to randomized fields and the number of concatenated files, rendering hash-based detection ineffective. The JavaScript payload is also hashbusted, further complicating detection.

Recent campaigns have introduced additional obfuscation layers, such as custom WOFF2 fonts with glyph substitution to disguise filenames, and delivery of ZIP payloads as XOR-encoded blobs that are decoded and appended client-side. This approach bypasses many network security controls and frustrates both automated and manual analysis. Upon execution, the JavaScript payload runs via wscript.exe from a temporary directory, establishes persistence by creating a .lnk shortcut in the Startup folder, and may execute a secondary JavaScript file via cscript.exe to spawn PowerShell for further exploitation and command-and-control (C2) communication.

Exploitation in the Wild

Active exploitation of this technique has been observed since late 2023, with a notable resurgence in late 2025 as new obfuscation and delivery methods were introduced. GootLoader campaigns have targeted a broad range of sectors, including legal, healthcare, financial, technology, manufacturing, education, and government organizations. The loader is frequently used as an initial access vector for high-profile ransomware and post-exploitation frameworks such as Cobalt Strike. Security vendors and threat intelligence platforms, including Red Canary and Expel, have reported that most endpoint protection platforms (EPP) and email gateways do not scan deeply nested or malformed ZIP archives, allowing the initial infection vector to bypass defenses. Each victim receives a unique ZIP file, making signature-based detection and hash sharing largely ineffective.

Victimology and Targeting

GootLoader’s targeting is broad and opportunistic, with a focus on organizations in North America, Europe, and Australia. The most frequently targeted sectors are legal, healthcare, financial, technology, manufacturing, education, and government. The group’s use of SEO poisoning and malvertising ensures that victims are often actively seeking business or legal templates, increasing the likelihood of successful infection. The loader is commonly used to deliver ransomware, Cobalt Strike, Gootkit, Osiris, and Sodinokibi payloads, making it a critical threat to organizations of all sizes and industries.

Mitigation and Countermeasures

Organizations should take a multi-layered approach to defend against GootLoader. Security tools must be configured to scan deeply nested and malformed ZIP archives, and policies should be implemented to block the execution of wscript.exe and cscript.exe from user directories unless explicitly required. Group Policy Objects (GPO) can be used to associate .js files with Notepad by default, preventing accidental execution. User awareness training is essential, emphasizing the risks of downloading files from unfamiliar or search-result-linked websites. Threat hunting teams should monitor for unusual archive extraction activity, script execution from user directories, and the creation of .lnk files in Startup folders. Incident response procedures should include isolating affected systems, performing forensic analysis for persistence mechanisms and C2 activity, and removing malicious scheduled tasks and registry keys such as HKCU\SOFTWARE\Microsoft\Phone\%USERNAME%, HKCU\SOFTWARE\Microsoft\Personalization\%USERNAME%, and HKCU\SOFTWARE\Microsoft\Fax\%USERNAME%.

Detection logic can be enhanced by monitoring for wscript.exe or cscript.exe processes executing JavaScript files from user temp directories, and for command lines containing MS-DOS short names (e.g., ~1.js). Outbound connections to known C2 infrastructure and downloads from compromised WordPress sites (often via /wp-comments-post.php) should be closely monitored and blocked where possible.

References

• The Hacker News: GootLoader Malware Uses 500–1,000 Concatenated ZIP Archives to Evade Detection

• Red Canary Threat Detection Report: Gootloader

• MITRE ATT&CK Framework

• Expel Security Research

• BleepingComputer: Gootloader now uses 1,000-part ZIP archives for stealthy delivery

• SCWorld: How Gootloader uses malformed ZIP archives to evade detection

• CyberInsider: Gootloader malware now uses “ZIP bomb” tactic to evade detection

About Rescana

Rescana is a leader in third-party risk management (TPRM), providing organizations with a comprehensive platform to assess, monitor, and mitigate cyber risks across their supply chain. Our platform leverages advanced threat intelligence, automation, and continuous monitoring to help organizations stay ahead of emerging threats and ensure the resilience of their digital ecosystem. For more information or to discuss how Rescana can support your organization’s cybersecurity posture, we are happy to answer questions at ops@rescana.com.