Source: https://mactoolbox.app/glossary/gatekeeper

# What is Gatekeeper on Mac?

By Nikhil Barik · Published November 3, 2026

## Quick answer

Gatekeeper is the macOS feature that checks apps you download before they run. It looks at the developer signature and notarization and may block apps that fail.

## What Gatekeeper is

When you open an app that came from the internet, Gatekeeper checks whether it is signed and notarized, or from the App Store. If not, macOS warns you or refuses to open it.

## Why it matters on a Mac

It protects you from tampered or unknown software, but it can also block harmless apps from small developers, which is when you see messages such as the app cannot be opened.

## How to check or change it on macOS

- Open System Settings > Privacy & Security. After a blocked app you may see an Open Anyway button there.
- Control-click an app and choose Open to get a dialog with an Open button.
- In Terminal, spctl --assess -vv /path/to/App.app shows Gatekeeper's verdict.

## How Toolbox handles it

Fix App Is Damaged shows the signature and Gatekeeper check for an app you drop on it, then lets you clear the quarantine flag if you trust the source.

## Frequently asked questions

**Can I turn Gatekeeper off?**

It can be relaxed, but turning it off removes a useful protection. Allow single apps instead.

**Does Gatekeeper scan for viruses?**

It checks signatures and notarization. It is not a full antivirus.

**Why does it only check downloaded apps?**

It relies on the quarantine flag that browsers and similar apps add to downloads.

## Tools mentioned

- [**Fix “App Is Damaged”**: Check who made an app that won’t open, then clear the download quarantine so macOS lets it run.](https://mactoolbox.app/app-is-damaged-cant-be-opened-mac)

## Keep reading

- [Fix “App Is Damaged”](https://mactoolbox.app/app-is-damaged-cant-be-opened-mac)
- [Notarization](https://mactoolbox.app/glossary/notarization)
- [Quarantine attribute](https://mactoolbox.app/glossary/quarantine-attribute)