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A data-stealing USB Trojan that leaves no trace on a system includes a self-protection mechanism that makes it difficult to detect, copy and analyze.

The threat, called “USB Thief” by researchers at ESET, has been found on USB devices.

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One aspect that makes it stand out from other USB malware families is each copy is bound to a single USB drive, said ESET researchers in a post.

Unlike other USB threats that leverage autorun and shortcut files to get executed, USB Thief relies on the fact that users often store portable versions of Firefox, NotePad++, TrueCrypt and other popular applications on USB sticks.

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The malware injects itself into the execution chain of such applications by posing as a plugin or a DLL file, according to researchers at ESET. When a victim launches the targeted app from an infected USB drive, the Trojan also executes in the background.

The malware has six different component files — four executables and two configuration files. In order to ensure it cannot end up copied from a USB drive and reverse engineered, some USB Thief files also underwent encryption using an AES encryption key generated based on the USB device’s unique ID and certain disk properties. The names of the files executed during every stage of the infection routine are different for each instance as they end up generated based on file content and creation time.

This mechanism ensures the malware doesn’t work on devices other than the USB drive on which the attacker planted it, preventing researchers from analyzing the threat.

The first stage loader is responsible for executing the Trojan via portable applications and checking if the USB device from which it’s run is writeable so the stolen information can end up stored there.

The second stage loader verifies the name of the parent process to ensure it’s not executed in an analysis environment, while the third stage loader checks for the presence of antivirus software.

The final payload, which injects into a newly created process, can steal information from the infected device, including images, documents, the Windows registry tree, file lists from all available drives, and data collected by a Windows inventory utility called WinAudit. The stolen information ends up encrypted using elliptic curve cryptography and stored on the USB drive from which the infection started.

Since the malware executes from a USB device, the infection doesn’t leave any trace on the targeted machine. The threat is perfect for targeted attacks against air-gapped systems, ESET researchers said.

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