macOS Crypto-Miner Emulates Linux

New crypto-currency mining malware is targeting systems running macOS, and works by emulating Linux, Malwarebytes security researchers have discovered.

Detected as Bird Miner, the threat spreads via a cracked installer for the music production software Ableton Live, a tool for music composing, recording, mixing, and mastering. The cracked installer is available on a piracy website called VST Crack, and is over 2.6 GB in size. 

Bird Miner’s postinstall script was designed to copy installed files to new locations with randomized names. These files have a variety of functions, with three of them being launch daemons, charged with launching three different shell scripts.

Called Crax, one of the scripts is tasked with ensuring that the malware avoids analysis by checking if Activity Monitor is running and unloading the other processes if it is. If not, it performs a series of CPU usage checks and unloads everything if it’s pegging the CPU at more than 85%. 

Otherwise, it loads the daemons for the other two processes, each to run a different script. Nearly identical, each script loads a separate executable, Malwarebytes explains. 

The threat checks for Activity Monitor once again, and then launches an executable and passes a path to another file as a parameter. The executable is an old version of the open-source emulator Qemu, which allows users to run Linux executables on non-Linux systems. 

Here, Qemu is used to run the contents of an image file that contains a bootable Linux system (the Tiny Core Linux variant). A .tgz file in the image is used to load certain files at startup, including a script that contains commands to run when the Tiny Core system launches. 

In this case, as soon as the Tiny Core system boots up, the cryptominer launches without user login. The malware uses the XMRig miner, a tool that has been abused by various other malware families as well over the past several years. 

While Bird Miner attempts to evade detection by checking if Activity Monitor is running, and by obfuscating the miner code by hiding it inside Qemu images, it uses obvious launch daemons for persistence and shell scripts to kick everything off, which should seem suspicious. 

It’s also interesting that it runs via emulation, although it could have run as native code, giving it a smaller footprint and better performance. It also consumes more resources than necessary, given that it runs two separate miners. 

“The fact that Bird Miner was created this way likely indicates that the author probably is familiar with Linux, but is not particularly well-versed in macOS. Although this method does obfuscate the miner itself, which could help the malware evade detection, that benefit is countered by reliance on shell scripts and the heavy footprint of running not one but two miners simultaneously in emulation,” Malwarebytes notes. 

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