The integrity of macOS relies heavily on Gatekeeper, the system designed to ensure that software from outside the App Store is notarized by Apple and remains unaltered. However, a new discovery by researchers Talal Haj Bakry and Tommy Mysk exposes a structural gap that allows this defense to be bypassed, effectively turning trusted applications into "evil twins."

The silent replacement mechanism

The vulnerability stems from a flaw in code signing rituals and post-installation checks. As detailed on Mysk's blog, once a user downloads and launches an app from the web for the first time, the main executable can be silently replaced by an attacker. Crucially, this process does not require elevated privileges, making the attack highly accessible.

When the application is relaunched, macOS fails to trigger any security warnings, executing attacker-controlled code under the guise of a previously approved and notarized app. This calls into question Apple's on-device security claims, as the integrity check appears to be bypassed after the initial validation.

A targeted ecosystem

This flaw emerges amidst a surge of sophisticated macOS threats. Recent reports highlight CrashStealer, an infostealer that mimics system crash reports to loot keychains and crypto wallets using notarized installers. Similarly, the ClickFix attack demonstrates how attackers are leveraging Terminal commands to mount malicious DMGs and install stealers silently.

Apple's stance

Despite the findings, Apple has reportedly shrugged off the issue. The company assessed the reported behavior as not requiring a security fix, suggesting it does not pose a significant risk. This stance is contentious among the security community, as it leaves a viable path for malware persistence on systems where users trust their installed software.

Global security implications

The discovery underscores a dangerous trend: over-reliance on Apple's notarization process. If software integrity is not verified upon every launch, the entire trust chain of macOS is weakened, enabling targeted attacks that replace essential productivity tools with malicious clones.