Dridex Returns, Targets MacOS Using New Entry Method

Malware

The Dridex variant we analyzed targets MacOS platforms with a new technique to deliver documents embedded with malicious macros to users.

By: Armando Nathaniel Pedragoza January 05, 2023 Read time: ( words)

Normally, documents containing malicious macros enter a user’s system via email attachments posing as normal document files. However, while this might be the primary method of arrival, malicious actors have other ways of entering a victim’s system.

This blog entry primarily concerns Dridex, an online banking malware that has been active for years. The variant we analyzed has made its way into the MacOS platform and has adopted a new technique to deliver documents embedded with malicious macros to users without having to pretend to be invoices or other business-related files.

The Dridex sample we investigated arrived as a Mach-o executable file: a.out (which we detected as Trojan.MacOS.DRIDEX.MANP). The first submission for this in Virus Total (VT) dates to 2019, where it was tagged as malicious by security vendors with no specific detection names.

Figure 1. Mach-o regions which contain the header, load commands, and segments of the file

Figure 2. Detections of a.out from April 2019 to December 2022

The data segment of the sample contains the malicious embedded document and is used by the _payload_doc variable. The disassembly in Figure 5 shows that the malware performs a loop where the content of _payload_doc is copied until the counter reaches _payload_doc_len, the size of the malicious code. This is in preparation for the overwriting routine.

Figure 3. Disassembly of the __DATA__data segment

Figure 4. Disassembly of how the sample writes data onto the target files

Once the malicious code is ready, the cstring segment plays a role in overwriting the code to the target files. This segment contains the following bash script command:

for i in $(find ~ -name “*.doc”); do echo ‘%s’ | xxd -r -p > $i; done

First, the malware searches for files in the current user (~/User/{user name}) directory that use the .doc file extension using the find ~ -name “*.doc” command. It then traverses through each document file (i) using for loop, and then writes the malicious code via the echo ‘%s’ command (where %s is the malicious code from the data segment).

Adding xxd -r -p to the script means that the malicious code will be written in plain hexadecimal dump, and not the actual content. The > $i part of the script implies that the output will be printed on each document file.

Figure 5. Disassembly of the __DATA__cstring segment

The disassembly in Figure 6 shows the value of %s that will be written on the document files. The malicious code it overwrites has a D0CF file format signature as seen the image, which implies that it is a Microsoft document file.

Figure 6. Disassembly of the sample that shows the search and overwriting commands

The malicious embedded document was first detected in the wild in 2015 with the following information:

SHA256: 70c7bf63bfe1fb83420905db6e65946d721e171db219034a52b27116795ae53e
Filename: pmB3A6.doc
Detection name: W2KM_DRIDEX.SPB

Using oletools, a python package used to analyze OLE and Microsoft files, we observed that the affected .doc files now contain macros.

Figure 7. Text prompt showing that this document file contains macros

Figure 8. Macros contained in the overwritten .doc file that were extracted using oletools

Based on the extracted macros, the .doc file contains suspicious components. To elaborate, here are the VBA components of the overwritten documents:

ThisDocument is an object that includes the autoopen macro which calls the malicious functions. These functions use normal-looking names to pose as regular functions. For instance, CreatePicture and CreateColor are normally used to create image-related objects, but in this VBA project, they perform malicious tasks.

Figure 9. Code snippet from the autoopen macro

Module1 Creates an executable file in the temporary (TEMP) folder and then runs it. The malware uses string concatenation as a method for obfuscating the name of the executable file it creates.

Figure 10. Code snippet from Module1 that shows how the malware creates and executes an executable file

Module2 contains the routine that decrypts a set of strings, which is a URL, and then connects to it to retrieve a file using the GET command. The malware uses basic string encryption to hide the malicious URL it connects to. It calls the RuBik() function to perform the decryption routine.

Figure 11. Code snippet from Module2 showing the decryption routine to connect to the encrypted URL

Module3 writes the content of the file retrieved in Module2 to the executable file created in Module1.

Figure 12. Code snippet from Module3 showing where the malware writes to the executable file

In this section, we will analyze the payload dropped by the malware. Note that since it is an exe file, it will not run in a MacOS environment. It is possible that the variant we analyzed is still in the testing stages and has not yet been fully converted to work in MacOS-based machines.

When the document is opened and the macro is enabled, the malware connects to the URL decrypted in Module2 to retrieve a file (87i4g3d2d2.exe) using the GET command:

hxxp://pr-clanky[.]kvalitne[.]cz/65y3fd23d/87i4g3d2d2[.]exe

While the macro feature in Microsoft Word is disabled by default, the malware will overwrite all the document files for the current user, including the clean files. This makes it more difficult for the user to determine whether the file is malicious since it doesn’t come from an external source.

After connecting to the domain, the content of the portable executable (PE) file is written to trume1.exe (aa6873a6002e152669f54c80801ca7d500ee8c00d5a6a8c223203303b1cbaf50) as analyzed in Module1, 2, and 3. The file trume1.exe will then be executed.

Figure 13. Network activity from the sample that shows the details of the URL it connects to

Figure 14. Payload of the sample when the macro-enabled document is opened

The content of the dropped executable file is in an HTML format instead of a PE file format since the URL that it is trying to access is already down. The PE file that it tries to download is the Dridex loader.

Figure 15. Content of the executable file dropped by the malware

Conclusion

Dridex is not a new malware — it has been observed in the wild for years now. Despite its age, it continues to be used, and in fact has even seen many enhancements over the years. Its entry point into the user’s system has traditionally been through email attachments, but this blog entry illustrates that the malicious actors using Dridex are also trying to find new targets and more efficient methods of entry.

Currently, the impact on MacOS users for this Dridex variant is minimized since the payload is an exe file (and therefore not compatible with MacOS environments). However, it still overwrites document files which are now the carriers of Dridex’s malicious macros. Furthermore, it’s possible that the threat actors behind this variant will implement further modifications that will make it compatible with MacOS.

We encourage users to avoid being infected by attacks that use social engineering and malicious documents by refraining from clicking links or opening attachments and embedded documents in emails. Furthermore, organizations can consider using security technologies such as Trend Micro™ Worry-Free™ Business Security, which supports Mac and is ideal for small and medium-sized companies, and Trend Micro™ Apex One™, which is a powerful security solution for enterprise businesses.

Indicators of Compromise

The indicators of compromise for this entry can be found in this document.

MITRE Tools, Tactics, and Procedures

Tactic	ID	Name	Description
Discovery	T1083	File and Directory Discovery	Uses the find command to search for specific files within the file system and runs echo command to overwrite files
Execution	T1204.002	User Execution: Malicious File	Requires the victim to run the malware .out file.
Execution	T1027	Obfuscated Files or Information	Portions of files are encoded to hide the plain-text strings
Execution	T1059.005	Command and Scripting Interpreter: Visual Basic	Uses macros to execute payloads
Command and Control	T1071.001	Application Layer Protocol: Web Protocols	Uses HTTP GET requests to contact the command-and-control (C&C) server
Exfiltration	T1041	Exfiltration Over C2 Channel	Sends data to C&C server

You May Also Like

This Week in Security News – October 29, 2021 VP, Threat Intelligence

Examining Ransomware Payments From a Data-Science Lens

Pack it Secretly: Earth Preta’s Updated Stealthy Strategies