macOS App Won't Open: Fix the 'Damaged' Error Fast

1. The Quarantine Flag (Most Common Cause)

When you download an app from outside the Mac App Store, macOS stamps it with an extended attribute called com.apple.quarantine. This tells Gatekeeper to check the app for notarization on first launch. If the notarization ticket is missing—maybe the developer hasn't updated it for your macOS version—the system shows a generic "app is damaged and can't be opened" error. It's rarely actual corruption.

The fix is to strip that quarantine flag manually. Open Terminal from Applications > Utilities and run:

xattr -cr /Applications/YourApp.app

Replace YourApp.app with the actual app name. The -c flag clears all extended attributes, and -r does it recursively for everything inside the app bundle. After that, try opening the app. If macOS still complains, move on to the next cause.

I've seen this work for everything from old Photoshop installers to niche indie tools. Why does this happen? Apple tightens notarization requirements with every major macOS release. An app that worked fine on Monterey might show “damaged” on Ventura because its notarization ticket is too old. The quarantine flag itself isn't the problem—it's the trigger for a check that the app fails.

2. Gatekeeper Override (Second Most Common)

Sometimes stripping the quarantine flag isn't enough. macOS might still block the app because of a Gatekeeper policy that's cached system-wide. This happens when you've tried opening the app before seeing the error—macOS remembers the rejection and won't let go.

Open System Settings > Privacy & Security (or System Preferences > Security & Privacy on older macOS). Scroll down to the Security section. You should see a message like: "YourApp was blocked from opening because it is not from an identified developer." Click the Open Anyway button next to it. You'll need to enter your admin password.

If that button isn't there, you waited too long—it disappears after about an hour. No worries. Open Terminal and run:

sudo spctl --master-disable

This disables Gatekeeper entirely. Then open the app once—it should launch. After it opens, re-enable Gatekeeper with:

sudo spctl --master-enable

Disabling Gatekeeper sounds extreme, but it's safe if you do it for the 10 seconds it takes to open the app. The reason it works: macOS caches the rejection decision in the kernel's security policy. Turning Gatekeeper off and on clears that cache.

3. Code Signature Mismatch (Less Common, More Annoying)

If neither of the above works, the app might genuinely have a broken code signature. This happens when you copy an app from an external drive or extract it from a corrupted archive. The signature doesn't match the app's actual contents.

Check the signature with Terminal:

codesign -dvvv /Applications/YourApp.app

Look for the Authority lines. If you see adhoc instead of a developer certificate, the signature is self-signed or missing. The only fix here is to re-download the app from a trusted source. I've also seen this when an app was modified by a utility like AppCleaner—it strips the signature while removing resources.

If it's a known good app and you're absolutely sure it's safe, you can forcibly re-sign it with your own ad-hoc signature:

codesign --force --deep --sign - /Applications/YourApp.app

The --deep flag signs all embedded frameworks and bundles. The - tells codesign to use an ad-hoc identity. This won't pass Gatekeeper's notarization check, but after you strip the quarantine flag (cause #1) and override Gatekeeper (cause #2), the app will launch. I use this only as a last resort for apps I've verified myself, like custom in-house tools.

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