PlugX Malware Sneaks Onto Windows PCs Through USB Devices

PlugX malware has been around for almost a decade and has been used by multiple actors of Chinese nexus and several other cybercrime groups.

The Palo Alto Networks Unit 42 incident response team has discovered a new variant of PlugX malware that is distributed via removable USB devices and targets Windows PCs. This should not come as a surprise since 95.6% of new malware or their variants in 2022 targeted Windows.

According to Unit 42 researchers, the new variant was detected when carrying out an incident response post a Black Basta ransomware attack. The researchers uncovered several malware samples and tools on the victims’ devices. This includes the Brute Ratel C4 red-teaming tool, GootLoader malware, and an old PlugX sample.

PlugX malware has been around for almost a decade and has been used by multiple actors of Chinese nexus and several other cybercrime groups. The malware was previously used in many high-profile cyberattacks, such as the 2015 U.S. Government Office of Personnel Management (OPM) breach.

The same backdoor was also used in the 2018 malware attack on the Android devices of minority groups in China. Most recently, in November 2022, researchers linked Google Drive phishing scams to the group infamously known for using PlugX malware.

Scope of Infection

The new variant stood out among other malware because it could infect any attached removable USB device, e.g., floppy, flash, thumb drives, and any system the removable device was plugged into later.

So far, no evidence connects the PlugX backdoor or Gootkit to the Black Basta ransomware group, and researchers believe another actor could have deployed it. Moreover, researchers noted that the malware could copy all Adobe PDF and Microsoft Word documents from the host and places them in a hidden folder on the USB device. The malware itself creates this folder.

Malware Analysis

Unit 42 researchers Jen Miller-Osborn and Mike Harbison explained in their blog post that this variant of PlugX malware is a wormable, second-stage implant. It infects USB devices and stays concealed from the Windows operating file system. The user would not suspect that their USB device is being exploited to exfiltrate data from networks. 

PlugX’s USB variant is different because it uses a specific Unicode character called non-breaking space/ U+00A0 to hide files in a USB device plugged into a workstation. This character prevents the Windows OS from rendering the directory name instead of leaving an anonymous folder in Explorer.

Furthermore, the malware can hide actor files in a removable USB device through a novel technique, which even works on the latest Windows OS. 

The malware is designed to infect the host and copy the malicious code on any removable device connected to the host by hiding it in a recycle bin folder. Since MS Windows OS by default doesn’t show hidden files, the malicious files in recycle bin aren’t displayed, but, surprisingly, it isn’t shown even with the settings enabled. These malicious files can be viewed/downloaded only on a Unix-like OS or through mounting the USB device in a forensic tool.

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