macOS Adware Delivers Notarized Payloads

A recently identified adware campaign targeting macOS users is leveraging malicious code that has received Apple’s approval. 

The approval, or notarization, as Apple calls it, is an automated process through which software is scanned before reaching macOS users, to ensure that it does not include malicious code. Notarization was introduced in macOS 10.15 (Catalina), which automatically blocks code that lacks this stamp of trust. Notarization is not App Review.

Apple introduced notarization following an increase in both volume and sophistication of macOS malware, aiming to provide users with increased confidence in a platform that once was being touted as free of malware affecting PCs. 

The adware campaign that Twitter user Peter Dantini found on the website homebrew.sh, however, shows that attackers have discovered means to bypass Apple’s approval process and are distributing notarized malware. 

Objective See security researcher Patrick Wardle has performed deeper analysis of the campaign, discovering that the distributed code is the OSX.Shlayer malware, and that it has been notarized via the spctl command. 

The Shlayer masquerades as Adobe Flash Player or an update for the plugin, and is considered one of the most prevalent macOS malware families out there. The threat was designed to download additional payloads, specifically adware. 

Previous reports on Shlayer revealed that, once it landed on a target machine, the threat would disable Gatekeeper to ensure that the user is not alerted when additional software is executed. 

Wardle has shared his findings with Apple, which revoked the certificates issued for the notarized payloads, meaning they would no longer be allowed to run on macOS. 

Several days after that, however, the campaign was still actively delivering new notarized payloads, which were signed on Friday, August 28, likely after Apple took steps to block the old payloads. 

“The attackers’ ability to agilely continue their attack (with other notarized payloads) is noteworthy. Clearly in the never ending cat & mouse game between the attackers and Apple, the attackers are currently (still) winning,” Wardle concludes. 

“Either Apple was able to detect Shlayer as part of the notarization process, but breaking that detection was trivial, or Apple had nothing in the notarization process to detect Shlayer, which has been around for a couple years at this point,” Malwarebytes notes. 

“While nothing is perfect, some organizations operate at a high level of security and privacy.  With this vetting occurring with third-party software, the cybercriminals will throw everything possible to see what sticks. Like phishing attacks, cybercriminals are continually working to see what emails can get through organizations’ various technology products globally. When they find one that works, they use it.  In this case, they most likely have tried hundreds of multiple malware applications, and to get through was a success for them,” James McQuiggan, security awareness advocate at KnowBe4, commented. 

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