Gatekeeper Bypass Vulnerability

Microsoft has disclosed details of a now-patched security flaw in Apple macOS that could be exploited by an attacker to get around security protections imposed to prevent the execution of malicious applications.

The shortcoming, dubbed Achilles (CVE-2022-42821, CVSS score: 5.5), was addressed by the iPhone maker in macOS Ventura 13, Monterey 12.6.2, and Big Sur 11.7.2, describing it as a logic issue that could be weaponized by an app to circumvent Gatekeeper checks.

"Gatekeeper bypasses such as this could be leveraged as a vector for initial access by malware and other threats and could help increase the success rate of malicious campaigns and attacks on macOS," Jonathan Bar Or of the Microsoft 365 Defender Research Team said.

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Gatekeeper is a security mechanism designed to ensure that only trusted apps run on the operating system. This is enforced by means of an extended attribute called "com.apple.quarantine" that's assigned to files downloaded from the internet. It is analogous to the Mark of the Web (MotW) flag in Windows.

Thus when an unsuspecting user downloads a potentially harmful app that impersonates a piece of legitimate software, the Gatekeeper feature prevents the app from being run as it's not validly signed and notarized by Apple.

Even in instances where an app is approved by Apple, users are displayed a prompt when it's launched for the first time to seek their explicit consent.

Given the crucial role played by Gatekeeper in macOS, it's hard not to imagine the consequences of sidestepping the security barrier, which could effectively permit threat actors to deploy malware on the machines.

The Achilles vulnerability identified by Microsoft exploits a permission model called Access Control Lists (ACLs) to add extremely restrictive permissions to a downloaded file (i.e., "everyone deny write,writeattr,writeextattr,writesecurity,chown"), thereby blocking Safari from setting the quarantine extended attribute.

In a hypothetical attack scenario, an adversary could embrace the technique to craft a rogue app and host it on a server, which could then be delivered to a possible target via social engineering, malicious ads, or a watering hole.

The method also circumvents Apple's newly introduced Lockdown Mode in macOS Ventura – an opt-in restrictive setting to counter zero-click exploits – necessitating that users apply the latest updates to mitigate threats.

"Fake apps remain one of the top entry vectors on macOS, indicating Gatekeeper bypass techniques are an attractive and even a necessary capability for adversaries to leverage in attacks," Bar Or said.


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