Apple macOS Gatekeeper flaw enables silent replacement of trusted apps

Security researchers Talal Haj Bakry and Tommy Mysk disclosed a macOS weakness that lets a user‑level attacker silently replace the main executable of any application downloaded from the web after it has been opened once. The technique uses a tar archive to delete the original app bundle and restore a modified version; because Gatekeeper caches the trust decision at first launch, it does not re‑verify the bundle, so no warning appears.

The issue affects apps installed outside the Mac App Store—such as Signal, Slack, Visual Studio Code, Brave and many others—and requires the attacker to already have the ability to run code under the logged‑in user, but no administrative privileges. Apple reviewed the report and concluded the behavior does not constitute a security vulnerability, closing the case without releasing a fix. Researchers stress that the flaw is a “once‑verified, always‑trusted” design flaw and recommend keeping macOS and applications updated, limiting the execution of untrusted code, and preferring App Store‑distributed software where possible.