What is GuLoader malware

Just like the name suggests, GuLoader (sometimes also called CloudEyE and vbdropper) is a first-stage trojan designed to infect a system and drop a final payload. Typically other trojans or RATs. Once the malware makes its way into the victim's system, it attempts to establish a remote connection and download a malicious executable.

This malware is infamous for using advanced anti-detection and obfuscation techniques. It evades network detection, stops executing in virtual environments, and can slip past automatics security systems.

Researchers first observed GuLoader in December 2019, when it was used in a campaign delivering Remcos RAT. Throughout 2020, the trojan kept gaining popularity, at one point accounting for 25% of all packeted samples recorded by Check Point Research. Today, GuLoader remains a highly active threat. It often delivers NanoCore, Agent Tesla, LokiBot, and FormBook.

General description of GuLoader downloader

GuLoader is written in encrypted shellcode wrapped in a Visual Basic 6 (VB6) executable. Notably, it stores second-stage payloads in cloud drive services. Usually, in Google Drive or Microsoft OneDrive. This way, it can establish a connection and download the executable without raising any red flags. The payload is usually encrypted, allowing it to slip past the cloud host’s security measures.

This loader is infamous for its use of anti-analysis techniques:

1. The shellcode is heavily obfuscated and mixed with generous amounts of junk code.
2. It detects sandboxes with EnumWindows by counting the number of application windows on the screen and terminates with an error message.
3. It tampers with debugging software, causing crashes.
4. It uses process hollowing to inject malicious code into a benign process and evade detection by an antivirus.
5. To confuse analysts, its PE header contains only generic GetProcAddress instructions.

Like many downloaders, GuLoader is offered as a service. Prices start at $100 per month. It is distributed in the clearnet by a company with a domain name in the .eu zone. The website markets it under the name CloudEye, claiming that this is a security tool intended for protecting applications against cracking. However, the same site contains links to YouTube tutorials that clearly display how to use the software maliciously. They also show how to abuse cloud drives.

Researchers managed to link GuLoader to an Italian-based hacking group by analyzing emails left as contact details in old forum threads. One of the users behind the loader is known under the alias sonykuccio. He advertised a malware variant as far back as 2011 and offered paid services, claiming that he could make other malicious programs harder to detect. That is why GuLoader uses so many intricate anti-evasion techniques.

How to get more information from GuLoader malware

ANY.RUN helps researchers perform malware analysis of GuLoader and track its execution process in an interactive sandbox.

Figure 1: GuLoader text report generated by ANY.RUN

ANY.RUN allows users to save time during analysis and present crucial information extracted from malware immediately. Analysts may take a look inside GuLoader malware configuration 10 second after its process started.

Figure 2: GuLoader malware configuration

GuLoader execution process

The form of GuLoader's distribution changing over time, but its execution flow always stay pretty straightforward. Since the purpose of GuLoader is to download to the infected system main payload, after its start it check is it run inside virtual environment. When check passed, it starts connection and download payload. Once payload downloaded and starts execution, GuLoader stops.

But even if loader didn't connect to C2 during analysis, you always may look in extracted malware configuration to find out from where GuLoader is wants to receive payload!

Read a detailed analysis of GuLoader in our blog.

Distribution of GuLoader

The distribution method of GuLoader is very typical. The loader is usually delivered as an Office document attachment in spam email campaigns. When downloaded, it uses a macro to install the malicious program. Sometimes it is also delivered as an executable in a .rar archive.

During the pandemic, many campaigns exploited the fear surrounding Covid-19 by mentioning the virus. More recently, attackers have been using fake payment invoices. They will impersonate a bank and use social engineering to trick the victim into downloading an infected file to check “payment details.”

Conclusion

GuLoader is available as a service for a relatively low price, can be easily found in the clearnet, and comes with easy-to-follow instructions. No wonder, then, that creators claim they already have over 5000 clients. Thanks to the combination of advanced anti-evasion tricks and ease of use, we expect its popularity to continue to grow.

Thankfully, GuLoader is easily detectable in ANY.RUN sandbox. It only takes a few minutes to launch an interactive emulation and identify the threat.