Have you ever heard of a ZIP file that seems impossible to analyze but actually hides an insidious danger? This is the case with Gootloader, a malware that made its comeback in 2025 with an ingenious strategy. Intrigued by how a simple file can challenge security experts? Discover how Gootloader manages to outsmart protection systems while continuing its role in the ransomware universe.
The 3 must-know facts
- Gootloader uses a malformed ZIP archive to bypass analysis tools while allowing the execution of the malicious script.
- The ZIP file consists of several hundred archives, making its analysis complex for security software.
- Solutions exist to prevent the execution of malicious scripts, such as associating .js files with a text editor.
A strategic return of Gootloader in 2025
Gootloader, known since 2020 for its key role in the initial intrusion of systems, remained discreet for several months before returning to the forefront at the end of 2025. Its new approach relies on the use of a deliberately malformed ZIP archive, designed to complicate the task of security tools. This technique allows transporting a malicious script without hindering its execution.
A ZIP file complex to analyze
The ZIP file distributed by Gootloader contains a unique JScript script but has an astonishingly large size, sometimes exceeding 70 MB. This apparent inconsistency is explained by the concatenation of 500 to 1,000 archives, creating a complex structure difficult for security software to analyze. However, this complexity does not prevent targeted victims from opening and executing the malicious script.
Unarchiving tools and security solutions often attempt to reconstruct a coherent structure by analyzing more fields, but Gootloader operators alter these structures to complicate automatic analysis.
Preventing infection: simple but effective strategies
To limit the risk of infection, it is advisable to associate .js and .jse files with a text editor, thus preventing their direct execution. Blocking wscript.exe and cscript.exe for downloaded files is another effective preventive measure. These actions help stop the infection at the moment the script is launched, even if the ZIP archive is opened.
Gootloader: a historical player in the ransomware economy
Since its appearance in 2020, Gootloader has played an essential role in the ransomware ecosystem. As an initial intrusion tool, it paves the way for other actors specialized in encryption and extortion operations. Its ability to adapt and innovate, as shown by its recent malformed ZIP archive strategy, demonstrates the increasing sophistication of cyber threats faced by modern security systems.
