Pierluigi Paganini December 26, 2021

Apple recently addressed fixed a flaw in the macOS that could be potentially exploited by an attacker to bypass Gatekeeper security feature.

Apple recently addressed a vulnerability in the macOS operating system, tracked as CVE-2021-30853, that could be potentially exploited by an attacker to bypass the Gatekeeper security feature and run arbitrary code. The vulnerability was reported to Apple by Gordon Long of Box and addressed the flaw with the release of macOS 11.6 updates on September 20, 2021. According to the security advisory published by Apple, this issue was addressed with improved checks.

“A malicious application may bypass Gatekeeper checks.” reads the advisory. “This issue was addressed with improved checks.

A malicious code could bypass automated notarization security checks, which scan for malicious components in the applications, and could be launched by Gatekeeper.

The popular white-hat hacker Patrick Wardle, analyzed the vulnerability and explained why it is so dangerous.

Wardle pointed out that the bug is very intriguing, its exploitation could allow bypassing also File Quarantine, and macOS’s recent notarization requirements.

The experts explained that an attacker can trigger the flaw by tricking the victims into opening a malicious application masqueraded as a harmless PDF, that could be sent to them via email.

Wardle explained that the root cause is that an unsigned, non-notarized script-based application can not explicitly specify an interpreter.

“The astute reader may have noticed that though the script started with the familiar #! (“Shebang”), it is missing an interpreter such as /bin/bash. However when launched, macOS seems to handle this without issue, and still executed the script.” reads post published by Wardle. “Though the process monitor output confirms that macOS will execute the “interpreter-less” script via bash, that fact that no interpreter was specified is ultimately what triggers a (rather nuanced) bug in the kernel. A bug, that allows the PoC to execute without being subjected to File Quarantine, Gatekeeper, nor notarization checks!”

shebang (#!) interpreter directive (e.g. #!/bin/sh or #!/bin/bash) is used to parse and interpret a shell. A malicious application used to exploit this issue can incorporate the shebang line missing an interpreter (i.e., #!) to get the underlying operating system to attempt to execute the script without raising any alert.

“The syspolicyd daemon will perform various policy checks and ultimately prevent the execution of untrusted applications, such as those that are unsigned or unnotarized,” continues the researcher. “But, what if the AppleSystemPolicy kext decides that the syspolicyd daemon does not need to be invoked? Well then, the process is allowed! And if this decision is made incorrectly, well then, you have a lovely File Quarantine, Gatekeeper, and notarization bypass.”

This is not the first issue in macOS that was discovered by researchers and that would enable attackers to bypass the security features implemented by the Apple operating system.

In April, Apple fixed a zero-day vulnerability, tracked as CVE-2021-30657, exploited by Shlayer malware to bypass Apple’s security features and deliver second-stage malicious payloads.

In October, Microsoft discovered a vulnerability in macOS, dubbed Shrootless (CVE-2021-30892), that can allow attackers to bypass System Integrity Protection (SIP) and perform malicious activities, such as gaining root privileges and installing rootkits on vulnerable devices.

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Pierluigi Paganini

(SecurityAffairs – hacking, Gatekeeper)

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