User Documents Overwritten With Malicious Code in Recent Dridex Attacks on macOS

The cybercriminals behind the Dridex banking trojan have adopted a new tactic in recent attacks targeting macOS devices, overwriting the victim’s document files to deliver their malicious code, Trend Micro reports.

Active since at least 2012 and considered one of the most prevalent financial threats, Dridex survived a takedown attempt in 2015 and remained operational after receiving various updates. In 2019, the DHS warned of continuous Dridex attacks targeting financial institutions.

According to Trend Micro, a recently observed Dridex attack targeting macOS stood out because of a novel tactic employed to disguise the malicious Microsoft Word document used for malware delivery.

The attackers distribute a Mach-o executable file that is designed to search for .doc files in the current user directory and write malicious macro code to all of them – in plain hexadecimal dump, not in content.

“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,” Trend Micro notes.

The malicious embedded document, the cybersecurity firm explains, is not new, being first observed in the wild in 2015. The analyzed Mach-o file sample was first submitted to VirusTotal in 2019.

Analysis of the overwritten documents revealed the inclusion of an AutoOpen macro meant to call several functions with normal-looking names, but which were meant to perform nefarious actions.

According to Trend Micro, the payload delivered by the macro was an .exe file meant to fetch the Dridex loader. While the .exe file would not run on macOS, the analyzed variant might be in testing stages and could later be converted to fully work on macOS.

“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,” Trend Micro concludes.

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