New MacOS Malware Discovered

A wave of malware targeting MacOS over the past month has raised the profile of the operating system once advertised as much safer than Windows. The newest attack code for the Mac includes three pieces of malware found in June — a zero-day exploit, a package that includes sophisticated anti-detection and obfuscation routines, and a family of malware that uses the Safari browser as an attack surface.

The zero-day exploit, dubbed OSX/Linker by researchers at Intego who discovered it, takes advantage of a vulnerability in MacOS Gatekeeper — the MacOS function that enforces code-signing and has the ability to limit program execution to properly sign code from trusted publishers.

The MacOS X GateKeeper Bypass vuln used in OSX/Linker was first discovered in February 2019 by independent researcher Filippo Cavallarin, who says that he notified Apple of the finding. After a 90-day disclosure deadline passed, Cavallarin publicly disclosed the vulnerability on May 24.

The vulnerability itself is in the way GateKeeper treats files on the local network, which don't receive the same locked-down scrutiny as files from the Internet. A cleverly formatted program can pretend that a file on a server sitting anywhere is on the local network, and should therefore be trusted.

Joshua Long, chief security analyst at Intego, says that OSX/Linker is the first evidence he knows of malicious actors trying to take advantage of the vulnerability, which is still unpatched by Apple as of this posting. "The sample that we found actually appeared to be just a proof-of-concept at the time that it was uploaded to VirusTotal," Long says. "It was able to write something to to a text file on the victim's computer."

The second piece of malware found by Intego researchers takes a long-used technique and adds "stealth" to the mix: the so-called OSX/CrescentCore is the name given to a new generation of fake Adobe Flash Player malware that adds significant obfuscation to its capabilities. "This is another bit of malware that is exploiting people's fear of having outdated software that might allow their computer to become infected," Long says.

The malware first checks to see whether it's being run inside a VM. If so, it won't complete installation. Similar checks are then done for common anti-malware software and reverse-engineering tools.

"This was actually found in the wild," Long says. "Unlike OSX/Linker, which seemed to have been a proof-of-concept, based on the nature of it, it's possible that this could have been used."

Intego researchers also discovered another exploit, OSX/NewTab, which injects new tabs into the Safari browser — tabs that can contain additional loaders and malware packages.

The three malware packages found by Intego in June are just the latest examples of increasing activity in MacOS malware.  A zero-day exploit in Firefox, described in detail by researcher Patrick Wardle in a three-part series, now can spread a variety of MacOS malware types. So far, the malware families dropped via the exploit have been backdoor spyware programs that can log keystrokes and take screenshots of the victim's computer.

Meanwhile, in June, a Mac-based cryptominer named LoudMiner or Bird Miner arrived on the scene. Described in detail by researchers Michel Malik of ESET and Thomas Reed of Malwarebytes Labs, LoudMiner is notable for creating a small Linux instance running in a virtual machine and then running the cryptomining software on the Linux platform. Attackers are using weaponized music applications as carriers for the new cryptominers.

Changing Malware Fortunes

The new MacOS malware variants indicate that the OS is becoming worth cybercriminals' time to develop malware for the platform, Long says. And that additional attention has an unexpected consequence: "It's clear to me that Windows, at this point, could easily be described as a safer platform than MacOS," he says, overturning a Macintosh reputation for security that was once so well-established that Apple once ran ads touting safety.

That's because, Long says, MacOS has become more of a target, and newer versions of Windows come with big security improvements. "Microsoft has had to do a lot of things to improve their image," Long says. "They've built in to their operating system a much better, more robust anti-malware capability than we've seen on MacOS."

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