Why does my Mac say an app is damaged or from an unidentified developer?
Three different refusals with three different meanings: unidentified developer, damaged and cannot be opened, and blocked by security settings. What each one is actually telling you, and how to allow a genuine app without disabling the protection.
- Difficulty
- intermediate
- Time
- 25 min
- Read
- 6 min
- Safety
- warning
Short answer
"Unidentified developer" means the app is signed but not notarised by Apple, and you can allow it deliberately in System Settings, Privacy and Security, where an Open Anyway button appears for about an hour after the refusal. "Damaged and cannot be opened" usually means the same thing with an incomplete or unsigned app, and it is also exactly what a tampered download looks like — so re-download it from the developer's own site before working around it.
macOS checks every application the first time it runs. It verifies that the app is signed by a registered developer, that it has been submitted to Apple for automated malware scanning — notarisation — and that it has not been altered since. The check is triggered by a quarantine flag that gets attached to anything downloaded from the internet, which is why the same app runs freely when copied from a memory stick and refuses when downloaded. The wording of the refusal is unhelpfully varied, and the most alarming message, "damaged", is also the least literal: the file is almost never damaged in any ordinary sense.
Safety
Step by step
- Read which of the three messages you actually have."Cannot be opened because it is from an unidentified developer" means signed but not notarised. "Damaged and cannot be opened, move it to the Trash" means the signature is missing, broken or does not match. "Blocked to protect your Mac" is the newer wording for a notarisation failure. They differ in how much benefit of the doubt is warranted.
- Ask where the download came from before anything else.The developer's own site, or a link from it, is the only answer that justifies proceeding. A sponsored search result, a download aggregator, a torrent, or a link sent to you is not. Fake Mac software distributed through search advertising is a live and effective attack, and this dialogue is the thing standing in its way.
- Re-download from the developer if there is any doubt.An interrupted or partially downloaded file genuinely produces the damaged message. Downloading again from the official source costs minutes and resolves both the innocent case and any question about tampering.
- Allow a genuine app the supported way.Try to open it, let it be refused, then go to System Settings, Privacy and Security. An Open Anyway button appears there for that specific app. This is Apple's intended route, it applies to one application only, and it leaves the protection in place for everything else.
- Use the right-click route on older versions.On older macOS releases, right-clicking the app and choosing Open presents a dialogue with an Open button that the double-click version does not offer. Recent versions have moved this into the Privacy and Security panel instead.
- Understand the quarantine command before using it.The terminal command that strips the quarantine attribute makes the app run without any check. It is the standard advice on forums and it is a real decision, not a formality — you are personally vouching for that file in place of Apple's scan. Use it only for software you have verified, from a developer you know.
- Never disable Gatekeeper globally.The command that turns off the check for everything is posted constantly, and it stays off. That leaves every future download unchecked, including the ones you did not mean to make. If a page tells you to run it, treat that page's other advice with suspicion.
- Check for an updated version if the app is old.Notarisation requirements have tightened over successive macOS releases, and applications that have not been updated in years genuinely stop being accepted. An abandoned application refusing to run is information about the application.
Common mistakes
- Running the command that disables Gatekeeper for everything — It is the most commonly posted answer and the worst one. It is permanent until reversed, it applies to every future download, and it is exactly what malicious installer instructions ask people to do.
- Assuming "damaged" means the file is corrupt and nothing more — It is the same message macOS shows for a tampered application. Treating it as a technicality is how modified installers get run.
- Downloading Mac software from the first search result — Sponsored results impersonating well-known Mac applications are a persistent problem, and the fake versions are signed just well enough to get further than they should. Go to the developer's site directly.
If it doesn't work
"Damaged and cannot be opened" on an app from a known developer
Cause: Incomplete download, or an unsigned build — Fix: Download again from the developer's site. If the second copy behaves identically and the developer is genuine, check whether they publish an Apple silicon build — an old build for an unsupported architecture can produce this.
No Open Anyway button appears in Privacy and Security
Cause: The window is time-limited, or you are looking before triggering the refusal — Fix: Try to open the app first, then go to the settings panel within about an hour. The button relates to the most recent blocked launch.
An app that used to run now refuses after a macOS upgrade
Cause: Tightened requirements, or a version that is no longer supported — Fix: Check the developer's site for a current version. Major macOS upgrades have repeatedly removed support for older application formats and signing methods.
Refuses even after clearing quarantine
Cause: The signature is genuinely broken, or a security policy is in force — Fix: On a managed or work Mac, an administrator policy may prevent it entirely. On a personal Mac, a broken signature after modification is expected and is a reason to get a clean copy.
The app opens but then cannot access files, the camera or the disk
Cause: A separate permissions system, not Gatekeeper — Fix: That is macOS privacy protection. Grant access in Privacy and Security under the relevant category — Full Disk Access, Camera, Microphone, Files and Folders.
A downloaded installer says it is from an unidentified developer and asks for your password
Cause: Could be legitimate, could not be — Fix: An installer asking for an administrator password is asking for the keys to the machine. If you have any doubt at all about where it came from, cancel. That prompt is the last checkpoint.
Questions people ask
What does notarised actually mean?
The developer submitted the app to Apple, which ran an automated malware scan and issued a ticket that macOS checks at launch. It is not a review of quality or behaviour — it is a scan and an identity check. Unnotarised does not mean malicious, it means unverified.
Is software outside the App Store less safe?
Not inherently, and a great deal of the best Mac software is distributed directly. What matters is the developer's identity and where you got the file. Direct downloads from a known developer's own site with a valid signature are a normal and reasonable thing to run.
Why does the same app run from a memory stick but not a download?
The quarantine flag is attached by the browser or mail client that downloaded it, and it does not survive a copy from removable media in the same way. That is a quirk of how the flag is set rather than a security assessment, and it is not a recommended workaround.
Does any of this replace antivirus?
It reduces the need considerably, and macOS also runs its own malware scanner, XProtect, in the background. It is not a complete defence — it does not cover things you deliberately allow, browser-based attacks, or scams that persuade you to install something. The catalogue's guide on whether you still need antivirus covers the wider question.