The dialog shows up twice a year. The file is usually fine. This is the sequence, not a cleaner.
Twice a year the studio Mac greets a new utility with a dialog that says the app is damaged and should be moved to the Trash. The file is usually fine. The dialog is Gatekeeper, and it gets dramatic about anything that arrived through a browser. This is the sequence I use on the iMac and on the Air. I am not a security engineer. I am a person who would like to compress a mug photo before the bus.
What the warning means
macOS keeps a note on files you downloaded. That note says "this came from the internet," and the first time you open the app, the system wants a human to agree. If you double-click and then mash the only button that looks safe, you get the damaged-app story and the file did not become damaged. It became refused. Those are different. A refused app is still the zip you meant to download. A damaged app is a bad copy, and that does happen, just less often than the dialog implies.
I check two things before I argue with the dialog. First, did I mean to download this, from the mirror I chose on purpose. Second, is the zip the size I expected. Compresto is about 11 MB. If a "Compresto" zip is 400 MB, I do not have a Gatekeeper problem. I have the wrong file, and it goes in the Trash without a ceremony.
The Open dance I actually use
This works on the systems I run in the shop, Ventura on the iMac and Sequoia on the Air. The words on the dialog move around between versions. The gesture does not.
- Unzip the download if it is still a zip. The app should be a thing you can see in Finder, not a box inside a box.
- Drag it to the Applications folder. I do not run shop tools from Downloads. Downloads is a junk drawer and I already have one of those.
- Do not double-click it yet. Hold Control, or right-click, the icon in Applications. Choose Open from that menu.
- Read the dialog. The useful button is Open, not Move to Trash. Click Open. You only do the right-click the first time.
- If the Mac asks for a password, that is the normal admin password of the machine, the one you already use to install a printer. It is not a fee and it is not a license key.
After that first Open, a normal double-click is enough. If a later update brings the dialog back, I do the same dance again and I do not take it personally. Point updates have reintroduced the prompt on both of my machines. It is tedious. It is not a verdict on the app.
If the right-click menu has no Open, you are clicking the zip, or a shortcut, or a file that never unpacked. Go back one step. I have lost ten minutes this way, usually while a kiln timer was also going.
When the file is stuck in quarantine
Sometimes the right-click Open still bounces you back to the same warning. The download note is stuck. Apple's own Terminal can clear that note. I type this myself. I do not paste commands from a forum I found at midnight, including, if I can help it, commands from a future version of me who was in a hurry.
Quit the app if it is half-open. Then open Terminal, which lives in Applications, inside Utilities. Drag is safer than typing a path. The command is xattr, and the flag people mean is delete the quarantine attribute:
xattr -d com.apple.quarantineLeave a space after that, then drag the app icon from Applications into the Terminal window, so the path is the real path and not a path you invented. Press Return. If Terminal answers with silence, that is success. Silence is the house style over there. Then do the right-click Open once more.
If Terminal says there is no such attribute, the file was not stuck that way. Stop. Do not start deleting other attributes because a stranger's cheat sheet listed them. I did that once on a font manager and I got to reinstall it on a Sunday.
If it is still stuck
Throw out the copy in Applications and empty the Trash of that item. Download again, from the other mirror on the install dialog if there were two. A zip can arrive short when the studio wifi drops, and a short zip unpacks into something the Mac is right to refuse. Compare the size to the size written on the download button. Compresto's button says 11 MB. Pearcleaner, the little uninstaller I keep for trial apps, is under 10. If you are nowhere near the number, you do not have the file.
Still stuck after a clean download and the Open dance: restart the Mac once, not as folklore, but because an update was waiting and the gate was checking the old rules. I have had one stall morning saved by a restart I had been postponing for a week. If it fails after that, I do not keep poking. I write the developer from the contact on their own site, and I say which macOS, which chip, and which mirror. "It won't open" is not a useful letter. "Sequoia, M2, Apple silicon mirror, right-click Open still says damaged, xattr says no quarantine attribute" is a useful letter.
What I will not install to "fix" it
I will not install a cleaner, an "Adobe flash updater," a Gatekeeper disabler, or a profile a comment thread attached, in order to open a compressor. Those are how a shop Mac gets slow, and then gets a second cleaner to fix the first one. The Photo shelf and the Mac Cleaners shelf on this site are full of tools I will actually name. Random download links inside a forum reply are not on the list for a reason.
I also do not turn off Gatekeeper globally to spare myself the right-click. The prompt is annoying and it has also been right, once, about a disk image I did not recognize. I would like to keep the argument on a per-app basis. The shop computer holds invoices, the wholesale list, and photographs of the bench. Convenience is not worth a standing invitation.
Related, if you are here from a download button on a program page: the install steps in that dialog are the short version (unzip, Applications, first Open). This page is the long version for when the short version gets a dialog with the word "damaged" in it. You do not need both on a normal day. I hope your normal days stay normal. Mine do, except in October, when everything updates at once and the tent weights are already in the car.