Carbon Black’s security researchers recently discovered a new variant of the Shlayer malware that targets macOS versions ranging from 10.10.5 to 10.14.3.
The threat is being distributed through a large number of websites, fake or hijacked legitimate domains, and poses as an Adobe Flash update. Employing multiple levels of obfuscation, the malware can escalate privileges after infecting the target system.
The malicious payload is mainly delivered in the form of DMG files, most of which are signed with a legitimate Apple developer ID and perform the installation using legitimate system applications via bash. While most of the observed samples were DMG files, Carbon Black also found PKG, ISO, and ZIP payloads.
The security researchers noticed that, when the DMG is mounted and the installer executed, a .command script runs from a hidden directory, to decrypt a second script that contains and executes an additional script, representing the final step of the first stage of the infection.
After identifying the script location and performing a verification check, the first stage collects system information, including macOS version and UUID, generates a “Session GUID” using uuidgen, creates a custom URL using the harvested data, and then downloads the second stage payload.
The script attempts to download the password-protected payload (a ZIP file) using curl, and creates a directory in /tmp to store and unzip the archive.
Next, it makes the binary within the unzipped .app executable using chmod +x, runs the payload using specific arguments, and then performs a killall Terminal to kill the running script’s terminal window.
After the second stage payload has been executed, it attempts to escalate privileges with sudo using a technique invoking /usr/libexec/security_authtrampoline.
Once it has gained root privileges, the malware attempts to download additional software, and disables Gatekeeper to ensure the downloaded software can run without user intervention. Many of the downloaded applications are signed with a valid developer ID too, Carbon Black reveals.