Opening Chromium on macOS when it says it cannot be opened
Why macOS blocks third-party Chromium builds on first launch, and the correct way to open one without disabling Gatekeeper entirely.
You download a Chromium build, double-click it, and macOS refuses:
"Chromium" cannot be opened because Apple cannot check it for malicious software.
Or, on a build that is not signed at all:
"Chromium" is damaged and can't be opened. You should move it to the Bin.
Nothing is damaged. This is Gatekeeper, and it is doing what it is supposed to do.
Why it happens
Apple notarisation requires a paid Apple Developer account and a submission process for every build. Volunteer Chromium maintainers publishing free builds generally do not do this, so their applications arrive un-notarised and macOS blocks them by default.
The "damaged" wording is misleading and has confused people for years. It usually just means the quarantine attribute is set on an application with no valid signature.
The correct way to open it
Right-click the application and choose Open, then click Open in the dialog that appears.
That is the entire fix. Right-clicking uses a different code path from double-clicking: it offers you an override, where double-clicking simply refuses. You only need to do it once per application.
If Open does not appear in the dialog, go to System Settings, Privacy & Security, scroll down, and there will be a message about the blocked application with an Open Anyway button.
If it still refuses
On recent macOS versions, particularly for downloaded .dmg and .zip files, you may need to clear the
quarantine attribute directly:
xattr -dr com.apple.quarantine /Applications/Chromium.app
That removes the flag macOS attaches to downloaded files. Run it on the application you actually intend to open, and understand what you are doing: you are telling macOS you vouch for this specific application.
Verify the checksum first. This step removes a protection, so it should only ever follow confirming the file matches what the maintainer published. See verify your download.
What not to do
Do not disable Gatekeeper globally with spctl --master-disable. It turns the protection off for everything
you will ever download, to solve a problem that has a per-application answer. Older guides still recommend it;
they are wrong.
Which builds have this problem
All the third-party ones, to some degree. Thorium, ungoogled-chromium and Marmaduke's builds are all published un-notarised.
The official Chromium snapshots from Google's own build servers are not notarised either, and behave the same way.