Talal Haj Bakry and Tommy Mysk showed that a downloaded macOS app can have its main executable silently replaced after Gatekeeper’s initial check, letting attackers swap a trusted app with malware without triggering any alerts.
The researchers demonstrated that Gatekeeper only validates the code signature when the app is first launched. If an attacker can replace the executable file inside the app bundle after that initial validation — during a background update or through a man-in-the-middle attack on a download — the replacement runs without Gatekeeper raising a flag.
Apple’s notarization system also fails to catch the swap because the outer app bundle still carries the original, valid signature even after the executable inside has been replaced. The vulnerability affects any macOS user who downloads software from outside the App Store.
The researchers recommend that Apple implement runtime code integrity checks that periodically re-validate application executables rather than relying solely on the initial launch-time check.
