macOS: "App is damaged and can't be opened"

Hi, it's BlueByte. Before you drag that app to the Trash — it's almost certainly not damaged. macOS is refusing to run it because it came from the web unnotarized, and it phrases that as "damaged". Let's confirm that's what's happening, then, only if you trust the source, clear the flag that's blocking it.

What the message really means

Opening a downloaded app fails:

"App" is damaged and can't be opened. You should move it to the Trash.

The app is usually not damaged. macOS Gatekeeper is refusing to run it because it arrived from the internet and is not notarized, and recent macOS versions phrase that refusal as "damaged" rather than the older "unidentified developer" prompt. Because the wording is alarming, people trash working software — the actual issue is a security flag, not corruption.

Why Gatekeeper blocks it

When a file is downloaded, macOS tags it with the com.apple.quarantine extended attribute. Gatekeeper checks that tag on first launch, and if the app is unsigned or not notarized by Apple, it is blocked. There is a second, real possibility to rule out: the download was genuinely incomplete or corrupted, in which case clearing the tag will not help — and on Apple silicon, an Intel-only app that needs Rosetta produces a different but nearby prompt.

Confirm it's quarantine, not real corruption

Don't guess — check whether the quarantine attribute is actually present:

xattr -p com.apple.quarantine /Applications/App.app

If it prints a value, quarantine is the reason. Then confirm the download is intact — compare its size or checksum against the publisher's if they provide one. A size mismatch means re-download, not a Gatekeeper workaround.

Clear the flag — only for software you trust

1. Confirm you trust the source. This step removes a security check, so do it only for software from a site you trust.

2. Remove the quarantine attribute from the app:

xattr -dr com.apple.quarantine /Applications/App.app

-r clears it from everything inside the bundle. Replace the path with the app's actual location.

3. Open the app normally. If macOS still prompts, right-click the app → Open, then confirm — that records your approval for that specific app.

A real case: a GitHub release

You download an open-source tool from its official GitHub releases page, drag it to Applications, and macOS says it is "damaged". You run xattr -p com.apple.quarantine /Applications/Tool.app and it prints a quarantine value, confirming Gatekeeper, not corruption. Because you trust the GitHub release (and the checksum on the release page matches your download), you run xattr -dr com.apple.quarantine /Applications/Tool.app. The app opens on the next launch, and xattr -p now returns nothing.

Confirm the flag is gone

Launch the app; it should open without the warning. Re-running the xattr -p command should now return nothing, confirming the quarantine flag is gone — not just that the app happened to open once.

Keep it from recurring

Prefer notarized builds and the Mac App Store where possible, and when you do install from the web, keep to publishers you can verify. For software you distribute yourself, notarizing it with Apple removes this prompt for your users entirely.

How this differs from a Rosetta or notarization prompt

This is Gatekeeper, not an architecture problem. On Apple silicon, an Intel-only app instead prompts to install Rosetta — a different message. And "app can't be opened because Apple cannot check it for malicious software" is the milder, notarization-pending variant, cleared with the same right-click → Open. When it says "damaged", check for the quarantine attribute first.