What is Darktortilla crypter?

DarkTortilla is a crypter that has been utilized since 2015 to deliver some of the most popular RATs, such as NanoCore, AsyncRat, and AgentTesla, as well as information stealers like RedLine. It is equipped with obfuscation and anti-analysis functionality.

DarkTortilla is a multi-stage crypter. To deploy on the target host and start operating, it relies on a loader and a .DLL core processor. It can run its harmful payload entirely in the computer's memory (RAM). This means it does not need to save any files to the hard drive, making it more difficult for traditional security software to detect.

The crypter can make use of social engineering by displaying fake messages to users that look like real software errors or updates. This tricks victims into thinking that is is a safe and legitimate program. By doing this, the malware can continue to operate without raising suspicion

DarkTortilla crypter execution process

Let's upload a sample of DarkTortilla to ANY.RUN sandbox to see how it operates.

The infection begins when the victim unknowingly runs the initial loader, which is often concealed within an archive or a malicious document. This loader is responsible for retrieving the .NET-based DLL (core processor). The DLL might be embedded within the loader's resources or downloaded from external sources like Pastebin.

DarkTortilla threat report generated by ANY.RUN

Once the initial loader is executed, it decodes and loads the core processor. This core component performs several tasks based on its configuration, including:

• Displaying Fake Messages: It shows fake message boxes to deceive users, making them believe the crypter is a real program.
• Evading Detection: It performs checks to detect if it is running in a virtual machine or a sandbox environment, which are commonly used by security researchers.
• Establishing Persistence: It ensures that the malware stays on the system by modifying system files or using techniques like moving its execution to the Windows %TEMP% directory. This makes it difficult to remove the malware completely. It can achieve persistence by modifying user .LNK files' target path to point to its executable. DarkTortilla ensures that it can execute again even after a system reboot. This further complicates the removal process and helps the malware remain active on the infected system.

The core processor then injects the main malicious payload into the system. This payload can be various types of malware, such as Remote Access Trojans (RATs) or information stealers.

Process graph generated by ANY.RUN allows us to see the main process of AsyncRAT injection through DarkTortilla

In our case, the payload is AsyncRAT. The sandbox session lets us see how the injection process is done in memory.

DarkTortilla crypter delivery methods

DarkTortilla spreads using different methods, yet the two main ones include:

• Malicious attachments: The attackers send emails that look like they come from trusted sources. These emails usually come with attachments in the form of archives (like .zip or .tar). These files often hide the initial loader that starts the infection process.
• Fake websites: Another way DarkTortilla can be delivered to the victim's machine is through phishing websites. Users are usually asked to download a file, which then turns out to be a loader.

Conclusion

DarkTortilla’s advanced encryption methods, in-memory execution, and anti-analysis capabilities make it particularly challenging to detect and mitigate. To avoid malware infection by DarkTortilla, it’s important to focus on a combination of security practices, including using a malware sandbox to proactively analyze any suspicious email, file, or link.

The ANY.RUN sandbox provides valuable tools for researchers to analyze and understand threats like DarkTortilla. By using it, security professionals can expose malware and phishing threats in seconds.

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