Fixing the 'You can't open this app' macOS Gatekeeper error

Quick answer

Right-click the app in Finder and select Open, then click Open in the dialog. That single override works for most cases.

Why this happens

macOS Gatekeeper is your Mac's bouncer. It checks every app downloaded from the internet against a list of known developers and notarization status. When an app isn't notarized by Apple or comes from an unidentified developer, Gatekeeper slams the door with Error -128 and the message "You can't open this app because it is from an unidentified developer."

I see this daily with indie tools, older apps, or anything downloaded via a direct link rather than the Mac App Store. The first time it tripped me up was with a network diagnostic utility — I thought my download was corrupt. It wasn't. The file was fine; Gatekeeper just didn't trust it.

The error is annoying, but it's actually a safety feature. The real problem is that Apple's notarization process doesn't cover the long tail of freeware and open-source tools. So you're left with a few options: bypass Gatekeeper for that specific app, or worse, disable it entirely (don't).

Fix steps (in order)

1. Right-click the app in Finder (or hold Control and click). You'll see an Open option in the context menu — this is the magic door.
2. Click Open. A dialog appears: "Apple could not verify this app. Are you sure you want to open it?" Click Open again.
3. The app launches, and Gatekeeper records this exception. Next time you double-click, it opens normally — no extra steps.

That's it for 90% of cases. If the right-click doesn't work (some apps trigger a different block), try the System Settings route:

1. Quit the app if it's running.
2. Open System Settings (or System Preferences on older macOS) and go to Privacy & Security.
3. Scroll to the Security section. You'll see a note about the blocked app near the bottom.
4. Click Open Anyway, then enter your password or use Touch ID.
5. Launch the app as usual. You'll see the same confirmation dialog — click Open.

Alternative if the main fix fails

Sometimes the Open Anyway button is greyed out, or the app still refuses to launch. That usually means the app's quarantine attribute is stuck. You can clear it directly with a Terminal command. Don't worry — this is safe and targeted, not a global security hole.

spctl --add /Applications/YourApp.app

Replace YourApp.app with the actual app name. This adds a temporary exception. If it still won't open, inspect the quarantine attribute:

xattr -d com.apple.quarantine /Applications/YourApp.app

That removes the "downloaded from internet" flag, so Gatekeeper stops treating it as untrusted. Do this only for apps you trust — not for random downloads.

One more thing: check for a spatial issue where the app is inside a zip or DMG. If you didn't drag it to your Applications folder, macOS may be blocking it from a read-only mount. Copy the app to /Applications first, then repeat the right-click open.

Prevention tip

The best fix is to avoid the error altogether. Before you download any app, check if it's notarized. Right-click the DMG or app and select Get Info — under the General section, you'll see a line like "Origin: Notarized" or "Origin: Unidentified." If it's not notarized, the developer hasn't gone through Apple's automated validation. That's a red flag unless you know the source.

Also, always download from the developer's official site, not third-party mirrors. A legit app will usually have a Gatekeeper exception already built in. If it doesn't, the right-click open is your friend — use it sparingly.

Skip the temptation to run sudo spctl --master-disable to turn off Gatekeeper entirely. I've seen that advice online and it's terrible. It exposes your Mac to every unsigned piece of malware out there. The per-app override is safer and takes ten seconds.