Pierluigi Paganini January 30, 2023

The threat actors behind the GOOTLOADER malware continues to improve their code by adding new components and implementing new obfuscation techniques.

Mandiant researchers reported that the UNC2565 group behind the GOOTLOADER malware (aka Gootkit) continues to improve their code by adding new components and implementing new obfuscation techniques.

Gootkit runs on an access-a-as-a-service model, it is used by different groups to drop additional malicious payloads on the compromised systems. Gootkit has been known to use fileless techniques to deliver threats such as the SunCrypt, and REvil (Sodinokibi) ransomware, Kronos trojans, and Cobalt Strike.

In the past, Gootkit distributed malware masquerading as freeware installers and it used legal documents to trick users into downloading these files. 

The attack chain starts with a user searching for specific information in a search engine. Attackers use black SEO technique to display a website compromised by Gootkit operators among the results.

Upon visiting the website, the victim will notice that it is presented as an online forum directly answering his query. This forum hosted a ZIP archive that contains the malicious .js file, which is used to establish persistence and drop a Cobalt Strike binary in the memory of the infected system.

In November 2022, Mandiant researchers spotted a new variant of GOOTLOADER, tracked as GOOTLOADER.POWERSHELL, that used a new infection chain. The new variant writes a second .JS file to disk and creates a scheduled task to execute it. Below is the attack chain of this new variant:

1. The user visits an UNC2565-compromised site (usually related to business documents) and downloads a malicious ZIP archive.
2. The malicious ZIP file is saved to the user’s Downloads folder.
3. The user opens the ZIP file and clicks the .JS file inside. This is a trojanized JavaScript library containing an obfuscated JScript file, which will ultimately execute GOOTLOADER.POWERSHELL. Recently observed trojanized JavaScript libraries include jQuery, Chroma.js, and Underscore.js.
4. The JS file is launched using WScript.exe.
5. The WScript.exe process creates an inflated file with a .LOG extension to C:\Users\%USERNAME%\AppData\Roaming\<RANDOM_DIRECTORY>\<HARD_CODED_FILE_NAME>. The dropper writes more obfuscated JScript code followed by a padding of random characters to increase the file size.
6. The .LOG file is renamed with a .JS file extension.
7. The dropper creates a scheduled task that executes the new JScript file. The scheduled task is executed immediately after creation but also serves as a persistence mechanism to run the second JScript file at the next logon.
8. WScript.exe and CScript.exe launch a PowerShell process that reaches out to 10 hard coded domains.
   • Victim information collected includes environment variables, Windows OS version, filenames, and running processes. This information is Gzip compressed, Base64 encoded, and sent to the command and control (C2) server in the Cookie header.
9. The C2 returns a payload, which is executed using the Invoke-Expression PowerShell cmdlet. This leads to the download of two payloads into registry keys: FONELAUNCH and a secondary payload to be executed by FONELAUNCH (mirroring steps 6 through 10 of the previous infection chain).

Mandiant researchers observed the UNC2565 group adopting three obfuscation techniques, such as hiding the code within altered versions of legitimate JavaScript libraries such as jQuery, Chroma.js, and Underscore.js, in an attempt to evade detection.

“Beginning in November 2022, Managed Defense observed a new obfuscation variant, tracked as variant 3, with modified infection that is more complex than the previous variants. This new variant contains additional string variables that are used in a second deobfuscation stage.” reads the report published by Mandiant. “This new variant has been observed trojanizing several legitimate JavaScript libraries, including jQuery, Chroma.js, and Underscore.js”

Upon successful execution of the GOOTLOADER file, additional payloads are downloaded, including FONELAUNCH and Cobalt Strike BEACON or SNOWCONE that will be stored in the registry. Then the payloads are executed via PowerShell in the later stages.

FONELAUNCH is a .NET-based launcher, it loads an encoded payload from the registry into memory. Since May 2021, the researchers have observed threat actors using three different variants of FONELAUNCH that differ in their loading mechanism:

• FONELAUNCH.FAX reads and decodes data from the HKCU\SOFTWARE\Microsoft\Fax\%USERNAME% registry key. The returned content is expected to be a .NET assembly, which is loaded at runtime into memory.
• FONELAUNCH.PHONE mainly reads and decodes data placed in a specific registry key. The returned data is expected to be a DLL, which is loaded via a publicly available DynamicDllLoader project.
• FONELAUNCH.DIALTONE reads and decodes data from the HKCU\SOFTWARE\Microsoft\%USERNAME% registry key. The returned content is expected to be a PE file, which is injected into a separate process and executed.

The report published by Mandiant also includes Indicators of Compromise (IoCs) and YARA rules associated with these threats.

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Pierluigi Paganini

(SecurityAffairs – hacking, GOOTLOADER)